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BGP option 'route limit' is marked as obsolete. 'import limit' should be used instead.
2013-08-16 02:20:05 +08:00
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<H2><A NAME="s6">6.</A> <A HREF="bird.html#toc6">Protocols</A></H2>
<H2><A NAME="ss6.1">6.1</A> <A HREF="bird.html#toc6.1">BGP</A>
</H2>
<P>The Border Gateway Protocol is the routing protocol used for backbone
level routing in the today's Internet. Contrary to the other protocols, its convergence
doesn't rely on all routers following the same rules for route selection,
making it possible to implement any routing policy at any router in the
network, the only restriction being that if a router advertises a route,
it must accept and forward packets according to it.
<P>
<P>BGP works in terms of autonomous systems (often abbreviated as
AS). Each AS is a part of the network with common management and
common routing policy. It is identified by a unique 16-bit number
(ASN). Routers within each AS usually exchange AS-internal routing
information with each other using an interior gateway protocol (IGP,
such as OSPF or RIP). Boundary routers at the border of
the AS communicate global (inter-AS) network reachability information with
their neighbors in the neighboring AS'es via exterior BGP (eBGP) and
redistribute received information to other routers in the AS via
interior BGP (iBGP).
<P>
<P>Each BGP router sends to its neighbors updates of the parts of its
routing table it wishes to export along with complete path information
(a list of AS'es the packet will travel through if it uses the particular
route) in order to avoid routing loops.
<P>
<P>BIRD supports all requirements of the BGP4 standard as defined in
RFC 4271
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt</A>
It also supports the community attributes
(RFC 1997
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt</A>),
capability negotiation
(RFC 3392
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt">ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt</A>),
MD5 password authentication
(RFC 2385
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt</A>),
extended communities
(RFC 4360
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4360.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4360.txt</A>),
route reflectors
(RFC 4456
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt</A>),
multiprotocol extensions
(RFC 4760
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt</A>),
4B AS numbers
(RFC 4893
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4893.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4893.txt</A>),
and 4B AS numbers in extended communities
(RFC 5668
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc5668.txt">ftp://ftp.rfc-editor.org/in-notes/rfc5668.txt</A>).
<P>
<P>For IPv6, it uses the standard multiprotocol extensions defined in
RFC 2283
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt</A>
including changes described in the
latest draft
<A HREF="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt">ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt</A>
and applied to IPv6 according to
RFC 2545
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt</A>.
<P>
<H3>Route selection rules</H3>
<P>BGP doesn't have any simple metric, so the rules for selection of an optimal
route among multiple BGP routes with the same preference are a bit more complex
and they are implemented according to the following algorithm. It starts the first
rule, if there are more "best" routes, then it uses the second rule to choose
among them and so on.
<P>
<UL>
<LI>Prefer route with the highest Local Preference attribute.</LI>
<LI>Prefer route with the shortest AS path.</LI>
<LI>Prefer IGP origin over EGP and EGP origin over incomplete.</LI>
<LI>Prefer the lowest value of the Multiple Exit Discriminator.</LI>
<LI>Prefer routes received via eBGP over ones received via iBGP.</LI>
<LI>Prefer routes with lower internal distance to a boundary router.</LI>
<LI>Prefer the route with the lowest value of router ID of the
advertising router.</LI>
</UL>
<P>
<H3>IGP routing table</H3>
<P>BGP is mainly concerned with global network reachability and with
routes to other autonomous systems. When such routes are redistributed
to routers in the AS via BGP, they contain IP addresses of a boundary
routers (in route attribute NEXT_HOP). BGP depends on existing IGP
routing table with AS-internal routes to determine immediate next hops
for routes and to know their internal distances to boundary routers
for the purpose of BGP route selection. In BIRD, there is usually
one routing table used for both IGP routes and BGP routes.
<P>
<H3>Configuration</H3>
<P>Each instance of the BGP corresponds to one neighboring router.
This allows to set routing policy and all the other parameters differently
for each neighbor using the following configuration parameters:
<P>
<DL>
<DT><CODE>local [<I>ip</I>] as <I>number</I></CODE><DD><P>Define which AS we
are part of. (Note that contrary to other IP routers, BIRD is
able to act as a router located in multiple AS'es
simultaneously, but in such cases you need to tweak the BGP
paths manually in the filters to get consistent behavior.)
Optional <CODE>ip</CODE> argument specifies a source address,
equivalent to the <CODE>source address</CODE> option (see below).
This parameter is mandatory.
<P>
<DT><CODE>neighbor <I>ip</I> as <I>number</I></CODE><DD><P>Define neighboring router
this instance will be talking to and what AS it's located in. Unless
you use the <CODE>multihop</CODE> clause, it must be directly connected to one
of your router's interfaces. In case the neighbor is in the same AS
as we are, we automatically switch to iBGP. This parameter is mandatory.
<P>
<DT><CODE>multihop [<I>number</I>]</CODE><DD><P>Configure multihop BGP
session to a neighbor that isn't directly connected.
Accurately, this option should be used if the configured
neighbor IP address does not match with any local network
subnets. Such IP address have to be reachable through system
routing table. For multihop BGP it is recommended to
explicitly configure <CODE>source address</CODE> to have it
stable. Optional <CODE>number</CODE> argument can be used to specify
the number of hops (used for TTL). Note that the number of
networks (edges) in a path is counted, i.e. if two BGP
speakers are separated by one router, the number of hops is
2. Default: switched off.
<P>
<DT><CODE>source address <I>ip</I></CODE><DD><P>Define local address we
should use for next hop calculation and as a source address
for the BGP session. Default: the address of the local
end of the interface our neighbor is connected to.
<P>
<DT><CODE>next hop self</CODE><DD><P>Avoid calculation of the Next Hop
attribute and always advertise our own source address as a
next hop. This needs to be used only occasionally to
circumvent misconfigurations of other routers. Default:
disabled.
<P>
<DT><CODE>next hop keep</CODE><DD><P>Forward the received Next Hop
attribute even in situations where the local address should be
used instead, like when the route is sent to an interface with
a different subnet. Default: disabled.
<P>
<DT><CODE>missing lladdr self|drop|ignore</CODE><DD><P>Next Hop attribute
in BGP-IPv6 sometimes contains just the global IPv6 address,
but sometimes it has to contain both global and link-local
IPv6 addresses. This option specifies what to do if BIRD have
to send both addresses but does not know link-local address.
This situation might happen when routes from other protocols
are exported to BGP, or when improper updates are received
from BGP peers. <CODE>self</CODE> means that BIRD advertises its own
local address instead. <CODE>drop</CODE> means that BIRD skips that
prefixes and logs error. <CODE>ignore</CODE> means that BIRD ignores
the problem and sends just the global address (and therefore
forms improper BGP update). Default: <CODE>self</CODE>, unless BIRD
is configured as a route server (option <CODE>rs client</CODE>), in
that case default is <CODE>ignore</CODE>, because route servers usually
do not forward packets themselves.
<P>
<DT><CODE>gateway direct|recursive</CODE><DD><P>For received routes, their
<CODE>gw</CODE> (immediate next hop) attribute is computed from
received <CODE>bgp_next_hop</CODE> attribute. This option specifies
how it is computed. Direct mode means that the IP address from
<CODE>bgp_next_hop</CODE> is used if it is directly reachable,
otherwise the neighbor IP address is used. Recursive mode
means that the gateway is computed by an IGP routing table
lookup for the IP address from <CODE>bgp_next_hop</CODE>. Recursive
mode is the behavior specified by the BGP standard. Direct
mode is simpler, does not require any routes in a routing
table, and was used in older versions of BIRD, but does not
handle well nontrivial iBGP setups and multihop. Recursive
mode is incompatible with
<A HREF="bird-2.html#dsc-sorted">sorted tables</A>. Default: <CODE>direct</CODE> for singlehop eBGP,
<CODE>recursive</CODE> otherwise.
<P>
<DT><CODE>igp table <I>name</I></CODE><DD><P>Specifies a table that is used
as an IGP routing table. Default: the same as the table BGP is
connected to.
<P>
<DT><CODE>ttl security <I>switch</I></CODE><DD><P>Use GTSM (RFC 5082 - the
generalized TTL security mechanism). GTSM protects against
spoofed packets by ignoring received packets with a smaller
than expected TTL. To work properly, GTSM have to be enabled
on both sides of a BGP session. If both <CODE>ttl security</CODE> and
<CODE>multihop</CODE> options are enabled, <CODE>multihop</CODE> option should
specify proper hop value to compute expected TTL. Kernel
support required: Linux: 2.6.34+ (IPv4), 2.6.35+ (IPv6), BSD:
since long ago, IPv4 only. Note that full (ICMP protection,
for example) RFC 5082 support is provided by Linux
only. Default: disabled.
<P>
<DT><CODE>password <I>string</I></CODE><DD><P>Use this password for MD5 authentication
of BGP sessions. Default: no authentication. Password has to be set by
external utility (e.g. setkey(8)) on BSD systems.
<P>
<DT><CODE>passive <I>switch</I></CODE><DD><P>Standard BGP behavior is both
initiating outgoing connections and accepting incoming
connections. In passive mode, outgoing connections are not
initiated. Default: off.
<P>
<DT><CODE>rr client</CODE><DD><P>Be a route reflector and treat the neighbor as
a route reflection client. Default: disabled.
<P>
<DT><CODE>rr cluster id <I>IPv4 address</I></CODE><DD><P>Route reflectors use cluster id
to avoid route reflection loops. When there is one route reflector in a cluster
it usually uses its router id as a cluster id, but when there are more route
reflectors in a cluster, these need to be configured (using this option) to
use a common cluster id. Clients in a cluster need not know their cluster
id and this option is not allowed for them. Default: the same as router id.
<P>
<DT><CODE>rs client</CODE><DD><P>Be a route server and treat the neighbor
as a route server client. A route server is used as a
replacement for full mesh EBGP routing in Internet exchange
points in a similar way to route reflectors used in IBGP routing.
BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation,
which behaves like plain EBGP but reduces modifications to advertised route
attributes to be transparent (for example does not prepend its AS number to
AS PATH attribute and keeps MED attribute). Default: disabled.
<P>
<DT><CODE>secondary <I>switch</I></CODE><DD><P>Usually, if an import filter
rejects a selected route, no other route is propagated for
that network. This option allows to try the next route in
order until one that is accepted is found or all routes for
that network are rejected. This can be used for route servers
that need to propagate different tables to each client but do
not want to have these tables explicitly (to conserve memory).
This option requires that the connected routing table is
<A HREF="bird-2.html#dsc-sorted">sorted</A>. Default: off.
<P>
<DT><CODE>enable route refresh <I>switch</I></CODE><DD><P>When BGP speaker
changes its import filter, it has to re-examine all routes
received from its neighbor against the new filter. As these
routes might not be available, there is a BGP protocol
extension Route Refresh (specified in RFC 2918) that allows
BGP speaker to request re-advertisement of all routes from its
neighbor. This option specifies whether BIRD advertises this
capability and accepts such requests. Even when disabled, BIRD
can send route refresh requests. Default: on.
<P>
<DT><CODE>interpret communities <I>switch</I></CODE><DD><P>RFC 1997 demands
that BGP speaker should process well-known communities like
no-export (65535, 65281) or no-advertise (65535, 65282). For
example, received route carrying a no-adverise community
should not be advertised to any of its neighbors. If this
option is enabled (which is by default), BIRD has such
behavior automatically (it is evaluated when a route is
exported to the BGP protocol just before the export filter).
Otherwise, this integrated processing of well-known
communities is disabled. In that case, similar behavior can be
implemented in the export filter. Default: on.
<P>
<DT><CODE>enable as4 <I>switch</I></CODE><DD><P>BGP protocol was designed to use 2B AS numbers
and was extended later to allow 4B AS number. BIRD supports 4B AS extension,
but by disabling this option it can be persuaded not to advertise it and
to maintain old-style sessions with its neighbors. This might be useful for
circumventing bugs in neighbor's implementation of 4B AS extension.
Even when disabled (off), BIRD behaves internally as AS4-aware BGP router.
Default: on.
<P>
<DT><CODE>capabilities <I>switch</I></CODE><DD><P>Use capability advertisement
to advertise optional capabilities. This is standard behavior
for newer BGP implementations, but there might be some older
BGP implementations that reject such connection attempts.
When disabled (off), features that request it (4B AS support)
are also disabled. Default: on, with automatic fallback to
off when received capability-related error.
<P>
<DT><CODE>advertise ipv4 <I>switch</I></CODE><DD><P>Advertise IPv4 multiprotocol capability.
This is not a correct behavior according to the strict interpretation
of RFC 4760, but it is widespread and required by some BGP
implementations (Cisco and Quagga). This option is relevant
to IPv4 mode with enabled capability advertisement only. Default: on.
<P>
<DT><CODE>route limit <I>number</I></CODE><DD><P>The maximal number of routes
that may be imported from the protocol. If the route limit is
exceeded, the connection is closed with an error. Limit is currently implemented as
<CODE>import limit <I>number</I> action restart</CODE>. This option is obsolete and it is
replaced by
<A HREF="bird-3.html#import-limit">import limit option</A>. Default: no limit.
<P>
<DT><CODE>disable after error <I>switch</I></CODE><DD><P>When an error is encountered (either
locally or by the other side), disable the instance automatically
and wait for an administrator to fix the problem manually. Default: off.
<P>
<DT><CODE>hold time <I>number</I></CODE><DD><P>Time in seconds to wait for a Keepalive
message from the other side before considering the connection stale.
Default: depends on agreement with the neighboring router, we prefer
240 seconds if the other side is willing to accept it.
<P>
<DT><CODE>startup hold time <I>number</I></CODE><DD><P>Value of the hold timer used
before the routers have a chance to exchange open messages and agree
on the real value. Default: 240 seconds.
<P>
<DT><CODE>keepalive time <I>number</I></CODE><DD><P>Delay in seconds between sending
of two consecutive Keepalive messages. Default: One third of the hold time.
<P>
<DT><CODE>connect retry time <I>number</I></CODE><DD><P>Time in seconds to wait before
retrying a failed attempt to connect. Default: 120 seconds.
<P>
<DT><CODE>start delay time <I>number</I></CODE><DD><P>Delay in seconds between protocol
startup and the first attempt to connect. Default: 5 seconds.
<P>
<DT><CODE>error wait time <I>number</I>,<I>number</I></CODE><DD><P>Minimum and maximum delay in seconds between a protocol
failure (either local or reported by the peer) and automatic restart.
Doesn't apply when <CODE>disable after error</CODE> is configured. If consecutive
errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.
<P>
<DT><CODE>error forget time <I>number</I></CODE><DD><P>Maximum time in seconds between two protocol
failures to treat them as a error sequence which makes the <CODE>error wait time</CODE>
increase exponentially. Default: 300 seconds.
<P>
<DT><CODE>path metric <I>switch</I></CODE><DD><P>Enable comparison of path lengths
when deciding which BGP route is the best one. Default: on.
<P>
<DT><CODE>med metric <I>switch</I></CODE><DD><P>Enable comparison of MED
attributes (during best route selection) even between routes
received from different ASes. This may be useful if all MED
attributes contain some consistent metric, perhaps enforced in
import filters of AS boundary routers. If this option is
disabled, MED attributes are compared only if routes are
received from the same AS (which is the standard behavior).
Default: off.
<P>
<DT><CODE>deterministic med <I>switch</I></CODE><DD><P>BGP route selection
algorithm is often viewed as a comparison between individual
routes (e.g. if a new route appears and is better than the
current best one, it is chosen as the new best one). But the
proper route selection, as specified by RFC 4271, cannot be
fully implemented in that way. The problem is mainly in
handling the MED attribute. BIRD, by default, uses an
simplification based on individual route comparison, which in
some cases may lead to temporally dependent behavior (i.e. the
selection is dependent on the order in which routes appeared).
This option enables a different (and slower) algorithm
implementing proper RFC 4271 route selection, which is
deterministic. Alternative way how to get deterministic
behavior is to use <CODE>med metric</CODE> option. This option is
incompatible with
<A HREF="bird-2.html#dsc-sorted">sorted tables</A>.
Default: off.
<P>
<DT><CODE>igp metric <I>switch</I></CODE><DD><P>Enable comparison of internal
distances to boundary routers during best route selection. Default: on.
<P>
<DT><CODE>prefer older <I>switch</I></CODE><DD><P>Standard route selection algorithm
breaks ties by comparing router IDs. This changes the behavior
to prefer older routes (when both are external and from different
peer). For details, see RFC 5004. Default: off.
<P>
<DT><CODE>default bgp_med <I>number</I></CODE><DD><P>Value of the Multiple Exit
Discriminator to be used during route selection when the MED attribute
is missing. Default: 0.
<P>
<DT><CODE>default bgp_local_pref <I>number</I></CODE><DD><P>A default value
for the Local Preference attribute. It is used when a new
Local Preference attribute is attached to a route by the BGP
protocol itself (for example, if a route is received through
eBGP and therefore does not have such attribute). Default: 100
(0 in pre-1.2.0 versions of BIRD).
</DL>
<P>
<H3>Attributes</H3>
<P>BGP defines several route attributes. Some of them (those marked with `<CODE>I</CODE>' in the
table below) are available on internal BGP connections only, some of them (marked
with `<CODE>O</CODE>') are optional.
<P>
<DL>
<DT><CODE>bgppath <CODE>bgp_path</CODE></CODE><DD><P>Sequence of AS numbers describing the AS path
the packet will travel through when forwarded according to the particular route.
In case of internal BGP it doesn't contain the number of the local AS.
<P>
<DT><CODE>int <CODE>bgp_local_pref</CODE> [I]</CODE><DD><P>Local preference value used for
selection among multiple BGP routes (see the selection rules above). It's
used as an additional metric which is propagated through the whole local AS.
<P>
<DT><CODE>int <CODE>bgp_med</CODE> [O]</CODE><DD><P>The Multiple Exit Discriminator of the route
is an optional attribute which is used on external (inter-AS) links to
convey to an adjacent AS the optimal entry point into the local AS.
The received attribute is also propagated over internal BGP links.
The attribute value is zeroed when a route is exported to an external BGP
instance to ensure that the attribute received from a neighboring AS is
not propagated to other neighboring ASes. A new value might be set in
the export filter of an external BGP instance.
See RFC 4451
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt</A>
for further discussion of BGP MED attribute.
<P>
<DT><CODE>enum <CODE>bgp_origin</CODE></CODE><DD><P>Origin of the route: either <CODE>ORIGIN_IGP</CODE>
if the route has originated in an interior routing protocol or
<CODE>ORIGIN_EGP</CODE> if it's been imported from the <CODE>EGP</CODE> protocol
(nowadays it seems to be obsolete) or <CODE>ORIGIN_INCOMPLETE</CODE> if the origin
is unknown.
<P>
<DT><CODE>ip <CODE>bgp_next_hop</CODE></CODE><DD><P>Next hop to be used for forwarding of packets
to this destination. On internal BGP connections, it's an address of the
originating router if it's inside the local AS or a boundary router the
packet will leave the AS through if it's an exterior route, so each BGP
speaker within the AS has a chance to use the shortest interior path
possible to this point.
<P>
<DT><CODE>void <CODE>bgp_atomic_aggr</CODE> [O]</CODE><DD><P>This is an optional attribute
which carries no value, but the sole presence of which indicates that the route
has been aggregated from multiple routes by some router on the path from
the originator.
<P>
<DT><CODE>clist <CODE>bgp_community</CODE> [O]</CODE><DD><P>List of community values associated
with the route. Each such value is a pair (represented as a <CODE>pair</CODE> data
type inside the filters) of 16-bit integers, the first of them containing the number of the AS which defines
the community and the second one being a per-AS identifier. There are lots
of uses of the community mechanism, but generally they are used to carry
policy information like "don't export to USA peers". As each AS can define
its own routing policy, it also has a complete freedom about which community
attributes it defines and what will their semantics be.
<P>
<DT><CODE>eclist <CODE>bgp_ext_community</CODE> [O]</CODE><DD><P>List of extended community
values associated with the route. Extended communities have similar usage
as plain communities, but they have an extended range (to allow 4B ASNs)
and a nontrivial structure with a type field. Individual community values are
represented using an <CODE>ec</CODE> data type inside the filters.
<P>
<DT><CODE>quad <CODE>bgp_originator_id</CODE> [I, O]</CODE><DD><P>This attribute is created by the
route reflector when reflecting the route and contains the router ID of the
originator of the route in the local AS.
<P>
<DT><CODE>clist <CODE>bgp_cluster_list</CODE> [I, O]</CODE><DD><P>This attribute contains a list
of cluster IDs of route reflectors. Each route reflector prepends its
cluster ID when reflecting the route.
</DL>
<P>
<H3>Example</H3>
<P>
<HR>
<PRE>
protocol bgp {
local as 65000; # Use a private AS number
neighbor 198.51.100.130 as 64496; # Our neighbor ...
multihop; # ... which is connected indirectly
export filter { # We use non-trivial export rules
if source = RTS_STATIC then { # Export only static routes
# Assign our community
bgp_community.add((65000,64501));
# Artificially increase path length
# by advertising local AS number twice
if bgp_path ~ [= 65000 =] then
bgp_path.prepend(65000);
accept;
}
reject;
};
import all;
source address 198.51.100.14; # Use a non-standard source address
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.2">6.2</A> <A HREF="bird.html#toc6.2">Device</A>
</H2>
<P>The Device protocol is not a real routing protocol. It doesn't generate
any routes and it only serves as a module for getting information about network
interfaces from the kernel.
<P>
<P>Except for very unusual circumstances, you probably should include
this protocol in the configuration since almost all other protocols
require network interfaces to be defined for them to work with.
<P>
<H3>Configuration</H3>
<P>
<DL>
<DT><CODE>scan time <I>number</I></CODE><DD><P>Time in seconds between two scans
of the network interface list. On systems where we are notified about
interface status changes asynchronously (such as newer versions of
Linux), we need to scan the list only in order to avoid confusion by lost
notification messages, so the default time is set to a large value.
<P>
<DT><CODE>primary [ "<I>mask</I>" ] <I>prefix</I></CODE><DD><P>If a network interface has more than one network address, BIRD
has to choose one of them as a primary one. By default, BIRD
chooses the lexicographically smallest address as the primary
one.
<P>This option allows to specify which network address should be
chosen as a primary one. Network addresses that match
<I>prefix</I> are preferred to non-matching addresses. If more
<CODE>primary</CODE> options are used, the first one has the highest
preference. If "<I>mask</I>" is specified, then such
<CODE>primary</CODE> option is relevant only to matching network
interfaces.
<P>In all cases, an address marked by operating system as
secondary cannot be chosen as the primary one.
</DL>
<P>
<P>As the Device protocol doesn't generate any routes, it cannot have
any attributes. Example configuration looks like this:
<P>
<P>
<HR>
<PRE>
protocol device {
scan time 10; # Scan the interfaces often
primary "eth0" 192.168.1.1;
primary 192.168.0.0/16;
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.3">6.3</A> <A HREF="bird.html#toc6.3">Direct</A>
</H2>
<P>The Direct protocol is a simple generator of device routes for all the
directly connected networks according to the list of interfaces provided
by the kernel via the Device protocol.
<P>
<P>The question is whether it is a good idea to have such device
routes in BIRD routing table. OS kernel usually handles device routes
for directly connected networks by itself so we don't need (and don't
want) to export these routes to the kernel protocol. OSPF protocol
creates device routes for its interfaces itself and BGP protocol is
usually used for exporting aggregate routes. Although there are some
use cases that use the direct protocol (like abusing eBGP as an IGP
routing protocol), in most cases it is not needed to have these device
routes in BIRD routing table and to use the direct protocol.
<P>
<P>There is one notable case when you definitely want to use the
direct protocol -- running BIRD on BSD systems. Having high priority
device routes for directly connected networks from the direct protocol
protects kernel device routes from being overwritten or removed by IGP
routes during some transient network conditions, because a lower
priority IGP route for the same network is not exported to the kernel
routing table. This is an issue on BSD systems only, as on Linux
systems BIRD cannot change non-BIRD route in the kernel routing table.
<P>
<P>The only configurable thing about direct is what interfaces it watches:
<P>
<P>
<DL>
<DT><CODE>interface <I>pattern [, ...]</I></CODE><DD><P>By default, the Direct
protocol will generate device routes for all the interfaces
available. If you want to restrict it to some subset of interfaces
(for example if you're using multiple routing tables for policy
routing and some of the policy domains don't contain all interfaces),
just use this clause.
</DL>
<P>
<P>Direct device routes don't contain any specific attributes.
<P>
<P>Example config might look like this:
<P>
<P>
<HR>
<PRE>
protocol direct {
interface "-arc*", "*"; # Exclude the ARCnets
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.4">6.4</A> <A HREF="bird.html#toc6.4">Kernel</A>
</H2>
<P>The Kernel protocol is not a real routing protocol. Instead of communicating
with other routers in the network, it performs synchronization of BIRD's routing
tables with the OS kernel. Basically, it sends all routing table updates to the kernel
and from time to time it scans the kernel tables to see whether some routes have
disappeared (for example due to unnoticed up/down transition of an interface)
or whether an `alien' route has been added by someone else (depending on the
<CODE>learn</CODE> switch, such routes are either ignored or accepted to our
table).
<P>
<P>Unfortunately, there is one thing that makes the routing table
synchronization a bit more complicated. In the kernel routing table
there are also device routes for directly connected networks. These
routes are usually managed by OS itself (as a part of IP address
configuration) and we don't want to touch that. They are completely
ignored during the scan of the kernel tables and also the export of
device routes from BIRD tables to kernel routing tables is restricted
to prevent accidental interference. This restriction can be disabled using
<CODE>device routes</CODE> switch.
<P>
<P>If your OS supports only a single routing table, you can configure
only one instance of the Kernel protocol. If it supports multiple
tables (in order to allow policy routing; such an OS is for example
Linux), you can run as many instances as you want, but each of them
must be connected to a different BIRD routing table and to a different
kernel table.
<P>
<P>Because the kernel protocol is partially integrated with the
connected routing table, there are two limitations - it is not
possible to connect more kernel protocols to the same routing table
and changing route destination/gateway in an export
filter of a kernel protocol does not work. Both limitations can be
overcome using another routing table and the pipe protocol.
<P>
<H3>Configuration</H3>
<P>
<DL>
<DT><CODE>persist <I>switch</I></CODE><DD><P>Tell BIRD to leave all its routes in the
routing tables when it exits (instead of cleaning them up).
<DT><CODE>scan time <I>number</I></CODE><DD><P>Time in seconds between two consecutive scans of the
kernel routing table.
<DT><CODE>learn <I>switch</I></CODE><DD><P>Enable learning of routes added to the kernel
routing tables by other routing daemons or by the system administrator.
This is possible only on systems which support identification of route
authorship.
<P>
<DT><CODE>device routes <I>switch</I></CODE><DD><P>Enable export of device
routes to the kernel routing table. By default, such routes
are rejected (with the exception of explicitly configured
device routes from the static protocol) regardless of the
export filter to protect device routes in kernel routing table
(managed by OS itself) from accidental overwriting or erasing.
<P>
<DT><CODE>kernel table <I>number</I></CODE><DD><P>Select which kernel table should
this particular instance of the Kernel protocol work with. Available
only on systems supporting multiple routing tables.
</DL>
<P>
<H3>Attributes</H3>
<P>The Kernel protocol defines several attributes. These attributes
are translated to appropriate system (and OS-specific) route attributes.
We support these attributes:
<P>
<DL>
<DT><CODE>int <CODE>krt_source</CODE></CODE><DD><P>The original source of the imported
kernel route. The value is system-dependent. On Linux, it is
a value of the protocol field of the route. See
/etc/iproute2/rt_protos for common values. On BSD, it is
based on STATIC and PROTOx flags. The attribute is read-only.
<P>
<DT><CODE>int <CODE>krt_metric</CODE></CODE><DD><P>The kernel metric of
the route. When multiple same routes are in a kernel routing
table, the Linux kernel chooses one with lower metric.
<P>
<DT><CODE>ip <CODE>krt_prefsrc</CODE></CODE><DD><P>(Linux) The preferred source address.
Used in source address selection for outgoing packets. Have to
be one of IP addresses of the router.
<P>
<DT><CODE>int <CODE>krt_realm</CODE></CODE><DD><P>(Linux) The realm of the route. Can be
used for traffic classification.
</DL>
<P>
<H3>Example</H3>
<P>A simple configuration can look this way:
<P>
<P>
<HR>
<PRE>
protocol kernel {
export all;
}
</PRE>
<HR>
<P>
<P>Or for a system with two routing tables:
<P>
<P>
<HR>
<PRE>
protocol kernel { # Primary routing table
learn; # Learn alien routes from the kernel
persist; # Don't remove routes on bird shutdown
scan time 10; # Scan kernel routing table every 10 seconds
import all;
export all;
}
protocol kernel { # Secondary routing table
table auxtable;
kernel table 100;
export all;
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.5">6.5</A> <A HREF="bird.html#toc6.5">OSPF</A>
</H2>
<H3>Introduction</H3>
<P>Open Shortest Path First (OSPF) is a quite complex interior gateway
protocol. The current IPv4 version (OSPFv2) is defined in RFC
2328
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt</A> and
the current IPv6 version (OSPFv3) is defined in RFC 5340
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt">ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt</A> It's a link state
(a.k.a. shortest path first) protocol -- each router maintains a
database describing the autonomous system's topology. Each participating
router has an identical copy of the database and all routers run the
same algorithm calculating a shortest path tree with themselves as a
root. OSPF chooses the least cost path as the best path.
<P>
<P>In OSPF, the autonomous system can be split to several areas in order
to reduce the amount of resources consumed for exchanging the routing
information and to protect the other areas from incorrect routing data.
Topology of the area is hidden to the rest of the autonomous system.
<P>
<P>Another very important feature of OSPF is that
it can keep routing information from other protocols (like Static or BGP)
in its link state database as external routes. Each external route can
be tagged by the advertising router, making it possible to pass additional
information between routers on the boundary of the autonomous system.
<P>
<P>OSPF quickly detects topological changes in the autonomous system (such
as router interface failures) and calculates new loop-free routes after a short
period of convergence. Only a minimal amount of
routing traffic is involved.
<P>
<P>Each router participating in OSPF routing periodically sends Hello messages
to all its interfaces. This allows neighbors to be discovered dynamically.
Then the neighbors exchange theirs parts of the link state database and keep it
identical by flooding updates. The flooding process is reliable and ensures
that each router detects all changes.
<P>
<H3>Configuration</H3>
<P>In the main part of configuration, there can be multiple definitions of
OSPF areas, each with a different id. These definitions includes many other
switches and multiple definitions of interfaces. Definition of interface
may contain many switches and constant definitions and list of neighbors
on nonbroadcast networks.
<P>
<HR>
<PRE>
protocol ospf &lt;name&gt; {
rfc1583compat &lt;switch&gt;;
stub router &lt;switch&gt;;
tick &lt;num&gt;;
ecmp &lt;switch&gt; [limit &lt;num&gt;];
area &lt;id&gt; {
stub;
nssa;
summary &lt;switch&gt;;
default nssa &lt;switch&gt;;
default cost &lt;num&gt;;
default cost2 &lt;num&gt;;
translator &lt;switch&gt;;
translator stability &lt;num&gt;;
networks {
&lt;prefix&gt;;
&lt;prefix&gt; hidden;
}
external {
&lt;prefix&gt;;
&lt;prefix&gt; hidden;
&lt;prefix&gt; tag &lt;num&gt;;
}
stubnet &lt;prefix&gt;;
stubnet &lt;prefix&gt; {
hidden &lt;switch&gt;;
summary &lt;switch&gt;;
cost &lt;num&gt;;
}
interface &lt;interface pattern&gt; [instance &lt;num&gt;] {
cost &lt;num&gt;;
stub &lt;switch&gt;;
hello &lt;num&gt;;
poll &lt;num&gt;;
retransmit &lt;num&gt;;
priority &lt;num&gt;;
wait &lt;num&gt;;
dead count &lt;num&gt;;
dead &lt;num&gt;;
rx buffer [normal|large|&lt;num&gt;];
type [broadcast|bcast|pointopoint|ptp|
nonbroadcast|nbma|pointomultipoint|ptmp];
strict nonbroadcast &lt;switch&gt;;
real broadcast &lt;switch&gt;;
ptp netmask &lt;switch&gt;;
check link &lt;switch&gt;;
ecmp weight &lt;num&gt;;
ttl security [&lt;switch&gt;; | tx only]
tx class|dscp &lt;num&gt;;
tx priority &lt;num&gt;;
authentication [none|simple|cryptographic];
password "&lt;text&gt;";
password "&lt;text&gt;" {
id &lt;num&gt;;
generate from "&lt;date&gt;";
generate to "&lt;date&gt;";
accept from "&lt;date&gt;";
accept to "&lt;date&gt;";
};
neighbors {
&lt;ip&gt;;
&lt;ip&gt; eligible;
};
};
virtual link &lt;id&gt; [instance &lt;num&gt;] {
hello &lt;num&gt;;
retransmit &lt;num&gt;;
wait &lt;num&gt;;
dead count &lt;num&gt;;
dead &lt;num&gt;;
authentication [none|simple|cryptographic];
password "&lt;text&gt;";
};
};
}
</PRE>
<HR>
<P>
<DL>
<DT><CODE>rfc1583compat <I>switch</I></CODE><DD><P>This option controls compatibility of routing table
calculation with RFC 1583
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt">ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt</A>. Default
value is no.
<P>
<DT><CODE>stub router <I>switch</I></CODE><DD><P>This option configures the router to be a stub router, i.e.,
a router that participates in the OSPF topology but does not
allow transit traffic. In OSPFv2, this is implemented by
advertising maximum metric for outgoing links, as suggested
by RFC 3137
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc3137.txt">ftp://ftp.rfc-editor.org/in-notes/rfc3137.txt</A>.
In OSPFv3, the stub router behavior is announced by clearing
the R-bit in the router LSA. Default value is no.
<P>
<DT><CODE>tick <I>num</I></CODE><DD><P>The routing table calculation and clean-up of areas' databases
is not performed when a single link state
change arrives. To lower the CPU utilization, it's processed later
at periodical intervals of <I>num</I> seconds. The default value is 1.
<P>
<DT><CODE>ecmp <I>switch</I> [limit <I>number</I>]</CODE><DD><P>This option specifies whether OSPF is allowed to generate
ECMP (equal-cost multipath) routes. Such routes are used when
there are several directions to the destination, each with
the same (computed) cost. This option also allows to specify
a limit on maximal number of nexthops in one route. By
default, ECMP is disabled. If enabled, default value of the
limit is 16.
<P>
<DT><CODE>area <I>id</I></CODE><DD><P>This defines an OSPF area with given area ID (an integer or an IPv4
address, similarly to a router ID). The most important area is
the backbone (ID 0) to which every other area must be connected.
<P>
<DT><CODE>stub</CODE><DD><P>This option configures the area to be a stub area. External
routes are not flooded into stub areas. Also summary LSAs can be
limited in stub areas (see option <CODE>summary</CODE>).
By default, the area is not a stub area.
<P>
<DT><CODE>nssa</CODE><DD><P>This option configures the area to be a NSSA (Not-So-Stubby
Area). NSSA is a variant of a stub area which allows a
limited way of external route propagation. Global external
routes are not propagated into a NSSA, but an external route
can be imported into NSSA as a (area-wide) NSSA-LSA (and
possibly translated and/or aggregated on area boundary).
By default, the area is not NSSA.
<P>
<DT><CODE>summary <I>switch</I></CODE><DD><P>This option controls propagation of summary LSAs into stub or
NSSA areas. If enabled, summary LSAs are propagated as usual,
otherwise just the default summary route (0.0.0.0/0) is
propagated (this is sometimes called totally stubby area). If
a stub area has more area boundary routers, propagating
summary LSAs could lead to more efficient routing at the cost
of larger link state database. Default value is no.
<P>
<DT><CODE>default nssa <I>switch</I></CODE><DD><P>When <CODE>summary</CODE> option is enabled, default summary route is
no longer propagated to the NSSA. In that case, this option
allows to originate default route as NSSA-LSA to the NSSA.
Default value is no.
<P>
<DT><CODE>default cost <I>num</I></CODE><DD><P>This option controls the cost of a default route propagated to
stub and NSSA areas. Default value is 1000.
<P>
<DT><CODE>default cost2 <I>num</I></CODE><DD><P>When a default route is originated as NSSA-LSA, its cost
can use either type 1 or type 2 metric. This option allows
to specify the cost of a default route in type 2 metric.
By default, type 1 metric (option <CODE>default cost</CODE>) is used.
<P>
<DT><CODE>translator <I>switch</I></CODE><DD><P>This option controls translation of NSSA-LSAs into external
LSAs. By default, one translator per NSSA is automatically
elected from area boundary routers. If enabled, this area
boundary router would unconditionally translate all NSSA-LSAs
regardless of translator election. Default value is no.
<P>
<DT><CODE>translator stability <I>num</I></CODE><DD><P>This option controls the translator stability interval (in
seconds). When the new translator is elected, the old one
keeps translating until the interval is over. Default value
is 40.
<P>
<DT><CODE>networks { <I>set</I> }</CODE><DD><P>Definition of area IP ranges. This is used in summary LSA origination.
Hidden networks are not propagated into other areas.
<P>
<DT><CODE>external { <I>set</I> }</CODE><DD><P>Definition of external area IP ranges for NSSAs. This is used
for NSSA-LSA translation. Hidden networks are not translated
into external LSAs. Networks can have configured route tag.
<P>
<DT><CODE>stubnet <I>prefix</I> { <I>options</I> }</CODE><DD><P>Stub networks are networks that are not transit networks
between OSPF routers. They are also propagated through an
OSPF area as a part of a link state database. By default,
BIRD generates a stub network record for each primary network
address on each OSPF interface that does not have any OSPF
neighbors, and also for each non-primary network address on
each OSPF interface. This option allows to alter a set of
stub networks propagated by this router.
<P>Each instance of this option adds a stub network with given
network prefix to the set of propagated stub network, unless
option <CODE>hidden</CODE> is used. It also suppresses default stub
networks for given network prefix. When option
<CODE>summary</CODE> is used, also default stub networks that are
subnetworks of given stub network are suppressed. This might
be used, for example, to aggregate generated stub networks.
<P>
<DT><CODE>interface <I>pattern</I> [instance <I>num</I>]</CODE><DD><P>Defines that the specified interfaces belong to the area being defined.
See
<A HREF="bird-3.html#dsc-iface">interface</A> common option for detailed description.
In OSPFv3, you can specify instance ID for that interface
description, so it is possible to have several instances of
that interface with different options or even in different areas.
<P>
<DT><CODE>virtual link <I>id</I> [instance <I>num</I>]</CODE><DD><P>Virtual link to router with the router id. Virtual link acts
as a point-to-point interface belonging to backbone. The
actual area is used as transport area. This item cannot be in
the backbone. In OSPFv3, you could also use several virtual
links to one destination with different instance IDs.
<P>
<DT><CODE>cost <I>num</I></CODE><DD><P>Specifies output cost (metric) of an interface. Default value is 10.
<P>
<DT><CODE>stub <I>switch</I></CODE><DD><P>If set to interface it does not listen to any packet and does not send
any hello. Default value is no.
<P>
<DT><CODE>hello <I>num</I></CODE><DD><P>Specifies interval in seconds between sending of Hello messages. Beware, all
routers on the same network need to have the same hello interval.
Default value is 10.
<P>
<DT><CODE>poll <I>num</I></CODE><DD><P>Specifies interval in seconds between sending of Hello messages for
some neighbors on NBMA network. Default value is 20.
<P>
<DT><CODE>retransmit <I>num</I></CODE><DD><P>Specifies interval in seconds between retransmissions of unacknowledged updates.
Default value is 5.
<P>
<DT><CODE>priority <I>num</I></CODE><DD><P>On every multiple access network (e.g., the Ethernet) Designed Router
and Backup Designed router are elected. These routers have some
special functions in the flooding process. Higher priority increases
preferences in this election. Routers with priority 0 are not
eligible. Default value is 1.
<P>
<DT><CODE>wait <I>num</I></CODE><DD><P>After start, router waits for the specified number of seconds between starting
election and building adjacency. Default value is 40.
<P>
<DT><CODE>dead count <I>num</I></CODE><DD><P>When the router does not receive any messages from a neighbor in
<I>dead count</I>*<I>hello</I> seconds, it will consider the neighbor down.
<P>
<DT><CODE>dead <I>num</I></CODE><DD><P>When the router does not receive any messages from a neighbor in
<I>dead</I> seconds, it will consider the neighbor down. If both directives
<I>dead count</I> and <I>dead</I> are used, <I>dead</I> has precendence.
<P>
<DT><CODE>rx buffer <I>num</I></CODE><DD><P>This sets the size of buffer used for receiving packets. The buffer should
be bigger than maximal size of any packets. Value NORMAL (default)
means 2*MTU, value LARGE means maximal allowed packet - 65535.
<P>
<DT><CODE>type broadcast|bcast</CODE><DD><P>BIRD detects a type of a connected network automatically, but
sometimes it's convenient to force use of a different type
manually. On broadcast networks (like ethernet), flooding
and Hello messages are sent using multicasts (a single packet
for all the neighbors). A designated router is elected and it
is responsible for synchronizing the link-state databases and
originating network LSAs. This network type cannot be used on
physically NBMA networks and on unnumbered networks (networks
without proper IP prefix).
<P>
<DT><CODE>type pointopoint|ptp</CODE><DD><P>Point-to-point networks connect just 2 routers together. No
election is performed and no network LSA is originated, which
makes it simpler and faster to establish. This network type
is useful not only for physically PtP ifaces (like PPP or
tunnels), but also for broadcast networks used as PtP links.
This network type cannot be used on physically NBMA networks.
<P>
<DT><CODE>type nonbroadcast|nbma</CODE><DD><P>On NBMA networks, the packets are sent to each neighbor
separately because of lack of multicast capabilities.
Like on broadcast networks, a designated router is elected,
which plays a central role in propagation of LSAs.
This network type cannot be used on unnumbered networks.
<P>
<DT><CODE>type pointomultipoint|ptmp</CODE><DD><P>This is another network type designed to handle NBMA
networks. In this case the NBMA network is treated as a
collection of PtP links. This is useful if not every pair of
routers on the NBMA network has direct communication, or if
the NBMA network is used as an (possibly unnumbered) PtP
link.
<P>
<DT><CODE>strict nonbroadcast <I>switch</I></CODE><DD><P>If set, don't send hello to any undefined neighbor. This switch
is ignored on other than NBMA or PtMP networks. Default value is no.
<P>
<DT><CODE>real broadcast <I>switch</I></CODE><DD><P>In <CODE>type broadcast</CODE> or <CODE>type ptp</CODE> network
configuration, OSPF packets are sent as IP multicast
packets. This option changes the behavior to using
old-fashioned IP broadcast packets. This may be useful as a
workaround if IP multicast for some reason does not work or
does not work reliably. This is a non-standard option and
probably is not interoperable with other OSPF
implementations. Default value is no.
<P>
<DT><CODE>ptp netmask <I>switch</I></CODE><DD><P>In <CODE>type ptp</CODE> network configurations, OSPFv2
implementations should ignore received netmask field in hello
packets and should send hello packets with zero netmask field
on unnumbered PtP links. But some OSPFv2 implementations
perform netmask checking even for PtP links. This option
specifies whether real netmask will be used in hello packets
on <CODE>type ptp</CODE> interfaces. You should ignore this option
unless you meet some compatibility problems related to this
issue. Default value is no for unnumbered PtP links, yes
otherwise.
<P>
<DT><CODE>check link <I>switch</I></CODE><DD><P>If set, a hardware link state (reported by OS) is taken into
consideration. When a link disappears (e.g. an ethernet cable is
unplugged), neighbors are immediately considered unreachable
and only the address of the iface (instead of whole network
prefix) is propagated. It is possible that some hardware
drivers or platforms do not implement this feature. Default value is no.
<P>
<DT><CODE>ttl security [<I>switch</I> | tx only]</CODE><DD><P>TTL security is a feature that protects routing protocols
from remote spoofed packets by using TTL 255 instead of TTL 1
for protocol packets destined to neighbors. Because TTL is
decremented when packets are forwarded, it is non-trivial to
spoof packets with TTL 255 from remote locations. Note that
this option would interfere with OSPF virtual links.
<P>If this option is enabled, the router will send OSPF packets
with TTL 255 and drop received packets with TTL less than
255. If this option si set to <CODE>tx only</CODE>, TTL 255 is used
for sent packets, but is not checked for received
packets. Default value is no.
<P>
<DT><CODE>tx class|dscp|priority <I>num</I></CODE><DD><P>These options specify the ToS/DiffServ/Traffic class/Priority
of the outgoing OSPF packets. See
<A HREF="bird-3.html#dsc-prio">tx class</A> common option for detailed description.
<P>
<DT><CODE>ecmp weight <I>num</I></CODE><DD><P>When ECMP (multipath) routes are allowed, this value specifies
a relative weight used for nexthops going through the iface.
Allowed values are 1-256. Default value is 1.
<P>
<DT><CODE>authentication none</CODE><DD><P>No passwords are sent in OSPF packets. This is the default value.
<P>
<DT><CODE>authentication simple</CODE><DD><P>Every packet carries 8 bytes of password. Received packets
lacking this password are ignored. This authentication mechanism is
very weak.
<P>
<DT><CODE>authentication cryptographic</CODE><DD><P>16-byte long MD5 digest is appended to every packet. For the digest
generation 16-byte long passwords are used. Those passwords are
not sent via network, so this mechanism is quite secure.
Packets can still be read by an attacker.
<P>
<DT><CODE>password "<I>text</I>"</CODE><DD><P>An 8-byte or 16-byte password used for authentication.
See
<A HREF="bird-3.html#dsc-pass">password</A> common option for detailed description.
<P>
<DT><CODE>neighbors { <I>set</I> } </CODE><DD><P>A set of neighbors to which Hello messages on NBMA or PtMP
networks are to be sent. For NBMA networks, some of them
could be marked as eligible. In OSPFv3, link-local addresses
should be used, using global ones is possible, but it is
nonstandard and might be problematic. And definitely,
link-local and global addresses should not be mixed.
<P>
</DL>
<P>
<H3>Attributes</H3>
<P>OSPF defines four route attributes. Each internal route has a <CODE>metric</CODE>.
Metric is ranging from 1 to infinity (65535).
External routes use <CODE>metric type 1</CODE> or <CODE>metric type 2</CODE>.
A <CODE>metric of type 1</CODE> is comparable with internal <CODE>metric</CODE>, a
<CODE>metric of type 2</CODE> is always longer
than any <CODE>metric of type 1</CODE> or any <CODE>internal metric</CODE>.
<CODE>Internal metric</CODE> or <CODE>metric of type 1</CODE> is stored in attribute
<CODE>ospf_metric1</CODE>, <CODE>metric type 2</CODE> is stored in attribute <CODE>ospf_metric2</CODE>.
If you specify both metrics only metric1 is used.
<P>Each external route can also carry attribute <CODE>ospf_tag</CODE> which is a
32-bit integer which is used when exporting routes to other protocols;
otherwise, it doesn't affect routing inside the OSPF domain at all.
The fourth attribute <CODE>ospf_router_id</CODE> is a router ID of the router
advertising that route/network. This attribute is read-only. Default
is <CODE>ospf_metric2 = 10000</CODE> and <CODE>ospf_tag = 0</CODE>.
<P>
<H3>Example</H3>
<P>
<P>
<HR>
<PRE>
protocol ospf MyOSPF {
rfc1583compat yes;
tick 2;
export filter {
if source = RTS_BGP then {
ospf_metric1 = 100;
accept;
}
reject;
};
area 0.0.0.0 {
interface "eth*" {
cost 11;
hello 15;
priority 100;
retransmit 7;
authentication simple;
password "aaa";
};
interface "ppp*" {
cost 100;
authentication cryptographic;
password "abc" {
id 1;
generate to "22-04-2003 11:00:06";
accept from "17-01-2001 12:01:05";
};
password "def" {
id 2;
generate to "22-07-2005 17:03:21";
accept from "22-02-2001 11:34:06";
};
};
interface "arc0" {
cost 10;
stub yes;
};
interface "arc1";
};
area 120 {
stub yes;
networks {
172.16.1.0/24;
172.16.2.0/24 hidden;
}
interface "-arc0" , "arc*" {
type nonbroadcast;
authentication none;
strict nonbroadcast yes;
wait 120;
poll 40;
dead count 8;
neighbors {
192.168.120.1 eligible;
192.168.120.2;
192.168.120.10;
};
};
};
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.6">6.6</A> <A HREF="bird.html#toc6.6">Pipe</A>
</H2>
<H3>Introduction</H3>
<P>The Pipe protocol serves as a link between two routing tables, allowing routes to be
passed from a table declared as primary (i.e., the one the pipe is connected to using the
<CODE>table</CODE> configuration keyword) to the secondary one (declared using <CODE>peer table</CODE>)
and vice versa, depending on what's allowed by the filters. Export filters control export
of routes from the primary table to the secondary one, import filters control the opposite
direction.
<P>
<P>The Pipe protocol may work in the transparent mode mode or in the opaque mode.
In the transparent mode, the Pipe protocol retransmits all routes from
one table to the other table, retaining their original source and
attributes. If import and export filters are set to accept, then both
tables would have the same content. The transparent mode is the default mode.
<P>
<P>In the opaque mode, the Pipe protocol retransmits optimal route
from one table to the other table in a similar way like other
protocols send and receive routes. Retransmitted route will have the
source set to the Pipe protocol, which may limit access to protocol
specific route attributes. This mode is mainly for compatibility, it
is not suggested for new configs. The mode can be changed by
<CODE>mode</CODE> option.
<P>
<P>The primary use of multiple routing tables and the Pipe protocol is for policy routing,
where handling of a single packet doesn't depend only on its destination address, but also
on its source address, source interface, protocol type and other similar parameters.
In many systems (Linux being a good example), the kernel allows to enforce routing policies
by defining routing rules which choose one of several routing tables to be used for a packet
according to its parameters. Setting of these rules is outside the scope of BIRD's work
(on Linux, you can use the <CODE>ip</CODE> command), but you can create several routing tables in BIRD,
connect them to the kernel ones, use filters to control which routes appear in which tables
and also you can employ the Pipe protocol for exporting a selected subset of one table to
another one.
<P>
<H3>Configuration</H3>
<P>
<DL>
<DT><CODE>peer table <I>table</I></CODE><DD><P>Defines secondary routing table to connect to. The
primary one is selected by the <CODE>table</CODE> keyword.
<P>
<DT><CODE>mode opaque|transparent</CODE><DD><P>Specifies the mode for the pipe to work in. Default is transparent.
</DL>
<P>
<H3>Attributes</H3>
<P>The Pipe protocol doesn't define any route attributes.
<P>
<H3>Example</H3>
<P>Let's consider a router which serves as a boundary router of two different autonomous
systems, each of them connected to a subset of interfaces of the router, having its own
exterior connectivity and wishing to use the other AS as a backup connectivity in case
of outage of its own exterior line.
<P>
<P>Probably the simplest solution to this situation is to use two routing tables (we'll
call them <CODE>as1</CODE> and <CODE>as2</CODE>) and set up kernel routing rules, so that packets having
arrived from interfaces belonging to the first AS will be routed according to <CODE>as1</CODE>
and similarly for the second AS. Thus we have split our router to two logical routers,
each one acting on its own routing table, having its own routing protocols on its own
interfaces. In order to use the other AS's routes for backup purposes, we can pass
the routes between the tables through a Pipe protocol while decreasing their preferences
and correcting their BGP paths to reflect the AS boundary crossing.
<P>
<HR>
<PRE>
table as1; # Define the tables
table as2;
protocol kernel kern1 { # Synchronize them with the kernel
table as1;
kernel table 1;
}
protocol kernel kern2 {
table as2;
kernel table 2;
}
protocol bgp bgp1 { # The outside connections
table as1;
local as 1;
neighbor 192.168.0.1 as 1001;
export all;
import all;
}
protocol bgp bgp2 {
table as2;
local as 2;
neighbor 10.0.0.1 as 1002;
export all;
import all;
}
protocol pipe { # The Pipe
table as1;
peer table as2;
export filter {
if net ~ [ 1.0.0.0/8+] then { # Only AS1 networks
if preference>10 then preference = preference-10;
if source=RTS_BGP then bgp_path.prepend(1);
accept;
}
reject;
};
import filter {
if net ~ [ 2.0.0.0/8+] then { # Only AS2 networks
if preference>10 then preference = preference-10;
if source=RTS_BGP then bgp_path.prepend(2);
accept;
}
reject;
};
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.7">6.7</A> <A HREF="bird.html#toc6.7">RAdv</A>
</H2>
<H3>Introduction</H3>
<P>The RAdv protocol is an implementation of Router Advertisements,
which are used in the IPv6 stateless autoconfiguration. IPv6 routers
send (in irregular time intervals or as an answer to a request)
advertisement packets to connected networks. These packets contain
basic information about a local network (e.g. a list of network
prefixes), which allows network hosts to autoconfigure network
addresses and choose a default route. BIRD implements router behavior
as defined in
RFC 4861
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc4861.txt">ftp://ftp.rfc-editor.org/in-notes/rfc4861.txt</A>
and also the DNS extensions from
RFC 6106
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc6106.txt">ftp://ftp.rfc-editor.org/in-notes/rfc6106.txt</A>.
<P>
<H3>Configuration</H3>
<P>There are several classes of definitions in RAdv configuration --
interface definitions, prefix definitions and DNS definitions:
<P>
<DL>
<DT><CODE>interface <I>pattern [, ...]</I> { <I>options</I> }</CODE><DD><P>Interface definitions specify a set of interfaces on which the
protocol is activated and contain interface specific options.
See
<A HREF="bird-3.html#dsc-iface">interface</A> common options for
detailed description.
<P>
<DT><CODE>prefix <I>prefix</I> { <I>options</I> }</CODE><DD><P>Prefix definitions allow to modify a list of advertised
prefixes. By default, the advertised prefixes are the same as
the network prefixes assigned to the interface. For each
network prefix, the matching prefix definition is found and
its options are used. If no matching prefix definition is
found, the prefix is used with default options.
<P>Prefix definitions can be either global or interface-specific.
The second ones are part of interface options. The prefix
definition matching is done in the first-match style, when
interface-specific definitions are processed before global
definitions. As expected, the prefix definition is matching if
the network prefix is a subnet of the prefix in prefix
definition.
<P>
<DT><CODE>rdnss { <I>options</I> }</CODE><DD><P>RDNSS definitions allow to specify a list of advertised
recursive DNS servers together with their options. As options
are seldom necessary, there is also a short variant <CODE>rdnss
<I>address</I></CODE> that just specifies one DNS server. Multiple
definitions are cumulative. RDNSS definitions may also be
interface-specific when used inside interface options. By
default, interface uses both global and interface-specific
options, but that can be changed by <CODE>rdnss local</CODE> option.
<P>
<DT><CODE>dnssl { <I>options</I> }</CODE><DD><P>DNSSL definitions allow to specify a list of advertised DNS
search domains together with their options. Like <CODE>rdnss</CODE>
above, multiple definitions are cumulative, they can be used
also as interface-specific options and there is a short
variant <CODE>dnssl <I>domain</I></CODE> that just specifies one DNS
search domain.
<P>
<A NAME="dsc-trigger"></A>
<DT><CODE>trigger <I>prefix</I></CODE><DD><P>RAdv protocol could be configured to change its behavior based
on availability of routes. When this option is used, the
protocol waits in suppressed state until a <I>trigger route</I>
(for the specified network) is exported to the protocol, the
protocol also returnsd to suppressed state if the
<I>trigger route</I> disappears. Note that route export depends
on specified export filter, as usual. This option could be
used, e.g., for handling failover in multihoming scenarios.
<P>During suppressed state, router advertisements are generated,
but with some fields zeroed. Exact behavior depends on which
fields are zeroed, this can be configured by
<CODE>sensitive</CODE> option for appropriate fields. By default, just
<CODE>default lifetime</CODE> (also called <CODE>router lifetime</CODE>) is
zeroed, which means hosts cannot use the router as a default
router. <CODE>preferred lifetime</CODE> and <CODE>valid lifetime</CODE> could
also be configured as <CODE>sensitive</CODE> for a prefix, which would
cause autoconfigured IPs to be deprecated or even removed.
</DL>
<P>
<P>Interface specific options:
<P>
<DL>
<DT><CODE>max ra interval <I>expr</I></CODE><DD><P>Unsolicited router advertisements are sent in irregular time
intervals. This option specifies the maximum length of these
intervals, in seconds. Valid values are 4-1800. Default: 600
<P>
<DT><CODE>min ra interval <I>expr</I></CODE><DD><P>This option specifies the minimum length of that intervals, in
seconds. Must be at least 3 and at most 3/4 * <CODE>max ra interval</CODE>.
Default: about 1/3 * <CODE>max ra interval</CODE>.
<P>
<DT><CODE>min delay <I>expr</I></CODE><DD><P>The minimum delay between two consecutive router advertisements,
in seconds. Default: 3
<P>
<DT><CODE>managed <I>switch</I></CODE><DD><P>This option specifies whether hosts should use DHCPv6 for
IP address configuration. Default: no
<P>
<DT><CODE>other config <I>switch</I></CODE><DD><P>This option specifies whether hosts should use DHCPv6 to
receive other configuration information. Default: no
<P>
<DT><CODE>link mtu <I>expr</I></CODE><DD><P>This option specifies which value of MTU should be used by
hosts. 0 means unspecified. Default: 0
<P>
<DT><CODE>reachable time <I>expr</I></CODE><DD><P>This option specifies the time (in milliseconds) how long
hosts should assume a neighbor is reachable (from the last
confirmation). Maximum is 3600000, 0 means unspecified.
Default 0.
<P>
<DT><CODE>retrans timer <I>expr</I></CODE><DD><P>This option specifies the time (in milliseconds) how long
hosts should wait before retransmitting Neighbor Solicitation
messages. 0 means unspecified. Default 0.
<P>
<DT><CODE>current hop limit <I>expr</I></CODE><DD><P>This option specifies which value of Hop Limit should be used
by hosts. Valid values are 0-255, 0 means unspecified. Default: 64
<P>
<DT><CODE>default lifetime <I>expr</I> [sensitive <I>switch</I>]</CODE><DD><P>This option specifies the time (in seconds) how long (after
the receipt of RA) hosts may use the router as a default
router. 0 means do not use as a default router. For
<CODE>sensitive</CODE> option, see
<A HREF="#dsc-trigger">trigger</A>.
Default: 3 * <CODE>max ra interval</CODE>, <CODE>sensitive</CODE> yes.
<P>
<DT><CODE>rdnss local <I>switch</I></CODE><DD><P>Use only local (interface-specific) RDNSS definitions for this
interface. Otherwise, both global and local definitions are
used. Could also be used to disable RDNSS for given interface
if no local definitons are specified. Default: no.
<P>
<DT><CODE>dnssl local <I>switch</I></CODE><DD><P>Use only local DNSSL definitions for this interface. See
<CODE>rdnss local</CODE> option above. Default: no.
</DL>
<P>
<P>
<P>Prefix specific options:
<P>
<DL>
<DT><CODE>skip <I>switch</I></CODE><DD><P>This option allows to specify that given prefix should not be
advertised. This is useful for making exceptions from a
default policy of advertising all prefixes. Note that for
withdrawing an already advertised prefix it is more useful to
advertise it with zero valid lifetime. Default: no
<P>
<DT><CODE>onlink <I>switch</I></CODE><DD><P>This option specifies whether hosts may use the advertised
prefix for onlink determination. Default: yes
<P>
<DT><CODE>autonomous <I>switch</I></CODE><DD><P>This option specifies whether hosts may use the advertised
prefix for stateless autoconfiguration. Default: yes
<P>
<DT><CODE>valid lifetime <I>expr</I> [sensitive <I>switch</I>]</CODE><DD><P>This option specifies the time (in seconds) how long (after
the receipt of RA) the prefix information is valid, i.e.,
autoconfigured IP addresses can be assigned and hosts with
that IP addresses are considered directly reachable. 0 means
the prefix is no longer valid. For <CODE>sensitive</CODE> option, see
<A HREF="#dsc-trigger">trigger</A>. Default: 86400 (1 day), <CODE>sensitive</CODE> no.
<P>
<DT><CODE>preferred lifetime <I>expr</I> [sensitive <I>switch</I>]</CODE><DD><P>This option specifies the time (in seconds) how long (after
the receipt of RA) IP addresses generated from the prefix
using stateless autoconfiguration remain preferred. For
<CODE>sensitive</CODE> option, see
<A HREF="#dsc-trigger">trigger</A>.
Default: 14400 (4 hours), <CODE>sensitive</CODE> no.
</DL>
<P>
<P>
<P>RDNSS specific options:
<P>
<DL>
<DT><CODE>ns <I>address</I></CODE><DD><P>This option specifies one recursive DNS server. Can be used
multiple times for multiple servers. It is mandatory to have
at least one <CODE>ns</CODE> option in <CODE>rdnss</CODE> definition.
<P>
<DT><CODE>lifetime [mult] <I>expr</I></CODE><DD><P>This option specifies the time how long the RDNSS information
may be used by clients after the receipt of RA. It is
expressed either in seconds or (when <CODE>mult</CODE> is used) in
multiples of <CODE>max ra interval</CODE>. Note that RDNSS information
is also invalidated when <CODE>default lifetime</CODE> expires. 0
means these addresses are no longer valid DNS servers.
Default: 3 * <CODE>max ra interval</CODE>.
</DL>
<P>
<P>
<P>DNSSL specific options:
<P>
<DL>
<DT><CODE>domain <I>address</I></CODE><DD><P>This option specifies one DNS search domain. Can be used
multiple times for multiple domains. It is mandatory to have
at least one <CODE>domain</CODE> option in <CODE>dnssl</CODE> definition.
<P>
<DT><CODE>lifetime [mult] <I>expr</I></CODE><DD><P>This option specifies the time how long the DNSSL information
may be used by clients after the receipt of RA. Details are
the same as for RDNSS <CODE>lifetime</CODE> option above.
Default: 3 * <CODE>max ra interval</CODE>.
</DL>
<P>
<P>
<H3>Example</H3>
<P>
<HR>
<PRE>
protocol radv {
interface "eth2" {
max ra interval 5; # Fast failover with more routers
managed yes; # Using DHCPv6 on eth2
prefix ::/0 {
autonomous off; # So do not autoconfigure any IP
};
};
interface "eth*"; # No need for any other options
prefix 2001:0DB8:1234::/48 {
preferred lifetime 0; # Deprecated address range
};
prefix 2001:0DB8:2000::/48 {
autonomous off; # Do not autoconfigure
};
rdnss 2001:0DB8:1234::10; # Short form of RDNSS
rdnss {
lifetime mult 10;
ns 2001:0DB8:1234::11;
ns 2001:0DB8:1234::12;
};
dnssl {
lifetime 3600;
domain "abc.com";
domain "xyz.com";
};
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.8">6.8</A> <A HREF="bird.html#toc6.8">RIP</A>
</H2>
<H3>Introduction</H3>
<P>The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol, where each router broadcasts (to all its neighbors)
distances to all networks it can reach. When a router hears distance to another network, it increments
it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes
unreachable, routers keep telling each other that its distance is the original distance plus 1 (actually, plus
interface metric, which is usually one). After some time, the distance reaches infinity (that's 15 in
RIP) and all routers know that network is unreachable. RIP tries to minimize situations where
counting to infinity is necessary, because it is slow. Due to infinity being 16, you can't use
RIP on networks where maximal distance is higher than 15 hosts. You can read more about RIP at
<A HREF="http://www.ietf.org/html.charters/rip-charter.html">http://www.ietf.org/html.charters/rip-charter.html</A>. Both IPv4
(RFC 1723
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt">ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt</A>)
and IPv6 (RFC 2080
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt</A>) versions of RIP are supported by BIRD, historical RIPv1 (RFC 1058
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt">ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt</A>)is
not currently supported. RIPv4 MD5 authentication (RFC 2082
<A HREF="ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt">ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt</A>) is supported.
<P>
<P>RIP is a very simple protocol, and it has a lot of shortcomings. Slow
convergence, big network load and inability to handle larger networks
makes it pretty much obsolete. (It is still usable on very small networks.)
<P>
<H3>Configuration</H3>
<P>In addition to options common for all to other protocols, RIP supports the following ones:
<P>
<DL>
<DT><CODE>authentication none|plaintext|md5</CODE><DD><P>selects authentication method to be used. <CODE>none</CODE> means that
packets are not authenticated at all, <CODE>plaintext</CODE> means that a plaintext password is embedded
into each packet, and <CODE>md5</CODE> means that packets are authenticated using a MD5 cryptographic
hash. If you set authentication to not-none, it is a good idea to add <CODE>password</CODE>
section. Default: none.
<P>
<DT><CODE>honor always|neighbor|never </CODE><DD><P>specifies when should requests for dumping routing table
be honored. (Always, when sent from a host on a directly connected
network or never.) Routing table updates are honored only from
neighbors, that is not configurable. Default: never.
</DL>
<P>
<P>There are some options that can be specified per-interface:
<P>
<DL>
<DT><CODE>metric <I>num</I></CODE><DD><P>This option specifies the metric of the interface. Valid
<P>
<DT><CODE>mode multicast|broadcast|quiet|nolisten|version1</CODE><DD><P>This option selects the mode for RIP to work in. If nothing is
specified, RIP runs in multicast mode. <CODE>version1</CODE> is
currently equivalent to <CODE>broadcast</CODE>, and it makes RIP talk
to a broadcast address even through multicast mode is
possible. <CODE>quiet</CODE> option means that RIP will not transmit
any periodic messages to this interface and <CODE>nolisten</CODE>
means that RIP will send to this interface butnot listen to it.
<P>
<DT><CODE>ttl security [<I>switch</I> | tx only]</CODE><DD><P>TTL security is a feature that protects routing protocols
from remote spoofed packets by using TTL 255 instead of TTL 1
for protocol packets destined to neighbors. Because TTL is
decremented when packets are forwarded, it is non-trivial to
spoof packets with TTL 255 from remote locations.
<P>If this option is enabled, the router will send RIP packets
with TTL 255 and drop received packets with TTL less than
255. If this option si set to <CODE>tx only</CODE>, TTL 255 is used
for sent packets, but is not checked for received
packets. Such setting does not offer protection, but offers
compatibility with neighbors regardless of whether they use
ttl security.
<P>Note that for RIPng, TTL security is a standard behavior
(required by RFC 2080), but BIRD uses <CODE>tx only</CODE> by
default, for compatibility with older versions. For IPv4 RIP,
default value is no.
<P>
<DT><CODE>tx class|dscp|priority <I>num</I></CODE><DD><P>These options specify the ToS/DiffServ/Traffic class/Priority
of the outgoing RIP packets. See
<A HREF="bird-3.html#dsc-prio">tx class</A> common option for detailed description.
</DL>
<P>
<P>The following options generally override behavior specified in RFC. If you use any of these
options, BIRD will no longer be RFC-compliant, which means it will not be able to talk to anything
other than equally configured BIRD. I have warned you.
<P>
<DL>
<DT><CODE>port <I>number</I></CODE><DD><P>selects IP port to operate on, default 520. (This is useful when testing BIRD, if you
set this to an address &gt;1024, you will not need to run bird with UID==0).
<P>
<DT><CODE>infinity <I>number</I></CODE><DD><P>selects the value of infinity, default is 16. Bigger values will make protocol convergence
even slower.
<P>
<DT><CODE>period <I>number</I></CODE><DD><P>specifies the number of seconds between periodic updates. Default is 30 seconds. A lower
number will mean faster convergence but bigger network
load. Do not use values lower than 12.
<P>
<DT><CODE>timeout time <I>number</I></CODE><DD><P>specifies how old route has to be to be considered unreachable. Default is 4*<CODE>period</CODE>.
<P>
<DT><CODE>garbage time <I>number</I></CODE><DD><P>specifies how old route has to be to be discarded. Default is 10*<CODE>period</CODE>.
</DL>
<P>
<H3>Attributes</H3>
<P>RIP defines two route attributes:
<P>
<DL>
<DT><CODE>int <CODE>rip_metric</CODE></CODE><DD><P>RIP metric of the route (ranging from 0 to <CODE>infinity</CODE>).
When routes from different RIP instances are available and all of them have the same
preference, BIRD prefers the route with lowest <CODE>rip_metric</CODE>.
When importing a non-RIP route, the metric defaults to 5.
<P>
<DT><CODE>int <CODE>rip_tag</CODE></CODE><DD><P>RIP route tag: a 16-bit number which can be used
to carry additional information with the route (for example, an originating AS number
in case of external routes). When importing a non-RIP route, the tag defaults to 0.
</DL>
<P>
<H3>Example</H3>
<P>
<HR>
<PRE>
protocol rip MyRIP_test {
debug all;
port 1520;
period 12;
garbage time 60;
interface "eth0" { metric 3; mode multicast; };
interface "eth*" { metric 2; mode broadcast; };
honor neighbor;
authentication none;
import filter { print "importing"; accept; };
export filter { print "exporting"; accept; };
}
</PRE>
<HR>
<P>
<H2><A NAME="ss6.9">6.9</A> <A HREF="bird.html#toc6.9">Static</A>
</H2>
<P>The Static protocol doesn't communicate with other routers in the network,
but instead it allows you to define routes manually. This is often used for
specifying how to forward packets to parts of the network which don't use
dynamic routing at all and also for defining sink routes (i.e., those
telling to return packets as undeliverable if they are in your IP block,
you don't have any specific destination for them and you don't want to send
them out through the default route to prevent routing loops).
<P>
<P>There are five types of static routes: `classical' routes telling
to forward packets to a neighboring router, multipath routes
specifying several (possibly weighted) neighboring routers, device
routes specifying forwarding to hosts on a directly connected network,
recursive routes computing their nexthops by doing route table lookups
for a given IP and special routes (sink, blackhole etc.) which specify
a special action to be done instead of forwarding the packet.
<P>
<P>When the particular destination is not available (the interface is down or
the next hop of the route is not a neighbor at the moment), Static just
uninstalls the route from the table it is connected to and adds it again as soon
as the destination becomes adjacent again.
<P>
<P>The Static protocol does not have many configuration options. The
definition of the protocol contains mainly a list of static routes:
<P>
<DL>
<DT><CODE>route <I>prefix</I> via <I>ip</I></CODE><DD><P>Static route through
a neighboring router.
<DT><CODE>route <I>prefix</I> multipath via <I>ip</I> [weight <I>num</I>] [via ...]</CODE><DD><P>Static multipath route. Contains several nexthops (gateways), possibly
with their weights.
<DT><CODE>route <I>prefix</I> via <I>"interface"</I></CODE><DD><P>Static device
route through an interface to hosts on a directly connected network.
<DT><CODE>route <I>prefix</I> recursive <I>ip</I></CODE><DD><P>Static recursive route,
its nexthop depends on a route table lookup for given IP address.
<DT><CODE>route <I>prefix</I> blackhole|unreachable|prohibit</CODE><DD><P>Special routes
specifying to silently drop the packet, return it as unreachable or return
it as administratively prohibited. First two targets are also known
as <CODE>drop</CODE> and <CODE>reject</CODE>.
<P>
<DT><CODE>check link <I>switch</I></CODE><DD><P>If set, hardware link states of network interfaces are taken
into consideration. When link disappears (e.g. ethernet cable
is unplugged), static routes directing to that interface are
removed. It is possible that some hardware drivers or
platforms do not implement this feature. Default: off.
<P>
<DT><CODE>igp table <I>name</I></CODE><DD><P>Specifies a table that is used
for route table lookups of recursive routes. Default: the
same table as the protocol is connected to.
</DL>
<P>
<P>Static routes have no specific attributes.
<P>
<P>Example static config might look like this:
<P>
<P>
<HR>
<PRE>
protocol static {
table testable; # Connect to a non-default routing table
route 0.0.0.0/0 via 198.51.100.130; # Default route
route 10.0.0.0/8 multipath # Multipath route
via 198.51.100.10 weight 2
via 198.51.100.20
via 192.0.2.1;
route 203.0.113.0/24 unreachable; # Sink route
route 10.2.0.0/24 via "arc0"; # Secondary network
}
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