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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.
Routers within each AS usually communicate with each other using either a interior routing
protocol (such as OSPF or RIP) or an interior variant of BGP (called iBGP).
Boundary routers at the border of the AS communicate with their peers
in the neighboring AS'es via exterior BGP (eBGP).
<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>),
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>),
and 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>).
<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 over incomplete.</LI>
<LI>Prefer the lowest value of the Multiple Exit Discriminator.</LI>
<LI>Prefer internal routes over external ones.</LI>
<LI>Prefer the route with the lowest value of router ID of the
advertising router.</LI>
</UL>
<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 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.)
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> via <I>ip</I></CODE><DD><P>Configure multihop BGP to a
neighbor which is connected at most <I>number</I> hops far and to which
we should route via our direct neighbor with address <I>ip</I>.
Default: switched off.
<P>
<DT><CODE>next hop self</CODE><DD><P>Avoid calculation of the Next Hop
attribute and always advertise our own source address (see
below) as a next hop. This needs to be used only occasionally
to circumvent misconfigurations of other routers.
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>drop</CODE>, because route servers usually
does not forward packets ifselves.
<P>
<DT><CODE>source address <I>ip</I></CODE><DD><P>Define local address we should use
for next hop calculation. Default: the address of the local end
of the interface our neighbor is connected to.
<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 keep MED attribute). Default: disabled.
<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 error. 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>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 on external (inter-AS) links to
convey to an adjacent AS the optimal entry point into the local AS.
The received attribute may be also propagated over internal BGP links
(and this is default behavior). The attribute value is zeroed when a route
is exported from a routing table to a 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 a 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>quad <CODE>bgp_originator_id</CODE> [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> [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 62.168.0.130 as 5588; # Our neighbor ...
multihop 20 via 62.168.0.13; # ... 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((65000,5678));
# 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 62.168.0.1; # 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, because some
routing protocols (for example OSPFv2) suppose there is only
one network address per interface. 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>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 2.2), 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>
<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>
<P>The Kernel protocol doesn't define any route attributes.
<P>A simple configuration can look this way:
<P>
<P>
<HR>
<PRE>
protocol kernel {
import all;
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 area witch different id included. 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 <name> {
rfc1583compat <switch>;
tick <num>;
area <id> {
stub cost <num>;
networks {
<prefix>;
<prefix> hidden;
}
stubnet <prefix>;
stubnet <prefix> {
hidden <switch>;
summary <switch>;
cost <num>;
}
interface <interface pattern> {
cost <num>;
stub <switch>;
hello <num>;
poll <num>;
retransmit <num>;
priority <num>;
wait <num>;
dead count <num>;
dead <num>;
rx buffer [normal|large|<num>];
type [broadcast|nonbroadcast|pointopoint];
strict nonbroadcast <switch>;
authentication [none|simple|cryptographic];
password "<text>";
password "<text>" {
id <num>;
generate from "<date>";
generate to "<date>";
accept from "<date>";
accept to "<date>";
};
neighbors {
<ip>;
<ip> eligible;
};
};
virtual link <id> {
hello <num>;
retransmit <num>;
wait <num>;
dead count <num>;
dead <num>;
authentication [none|simple|cryptographic];
password "<text>";
};
};
}
</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>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 cost <I>num</I></CODE><DD><P>No external (except default) routes are flooded into stub areas.
Setting this value marks area stub with defined cost of default route.
Default value is no. (Area is not stub.)
<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>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>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></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.
<P>
<DT><CODE>virtual link <I>id</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.
<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 - 65536.
<P>
<DT><CODE>type broadcast</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, flooding and Hello messages are sent using multicasts
(a single packet for all the neighbors).
<P>
<DT><CODE>type pointopoint</CODE><DD><P>Point-to-point networks connect just 2 routers together. No election
is performed there which reduces the number of messages sent.
<P>
<DT><CODE>type nonbroadcast</CODE><DD><P>On nonbroadcast networks, the packets are sent to each neighbor
separately because of lack of multicast capabilities.
<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 any non-NBMA network. Default is No.
<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 nonbroadcast networks
are to be sent. Some of them could be marked as eligible.
<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 opaque mode or in the transparent
mode. 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. The opaque mode is a default mode.
<P>
<P>In 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 mode can be set 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 opaque.
</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">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 two options that can be specified per-interface. First is <CODE>metric</CODE>, with
default one. Second is <CODE>mode multicast|broadcast|quiet|nolisten|version1</CODE>, it selects 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 but not listen to it.
<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 >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 10.
<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 10;
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="ss6.8">6.8</A> <A HREF="bird.html#toc6.8">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 three types of static routes: `classical' routes telling to
forward packets to a neighboring router, device routes specifying forwarding
to hosts on a directly connected network 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 has no configuration options. Instead, the
definition of the protocol contains 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> 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> drop|reject|prohibit</CODE><DD><P>Special routes
specifying to drop the packet, return it as unreachable or return
it as administratively prohibited.
</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 62.168.0.13; # Default route
route 62.168.0.0/25 reject; # Sink route
route 10.2.0.0/24 via "arc0"; # Secondary network
}
</PRE>
<HR>
<P>
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