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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
C613-50032-01 REV D AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 1.9
Internal BGP (iBGP) Concepts
Routers that run BGP often run an IGP, such as OSPF, and have learned routes for the public
range either as one route or with subset routes. If a single route exists for the public range,
then you can add a redistribute (into BGP or BGP4+) (BGP and BGP4+) command to the
BGP configuration to redistribute that route, and only that route, into BGP.
IGPs do not handle the public routing table. Internally peered BGP routers are capable of
exchanging routing information without redistributing into an IGP. But an IGP can be used
to connect two or more distant iBGP peers, and redistribution into an IGP may be used.
iBGP between Internet connected routers
When an organization uses more than one router to connect to the Internet, and those
routers use BGP to exchange routing information with their ISPs, those same routers need
to exchange BGP routes with each other as well. The BGP neighbor relationships occur
inside that organization, making these routers iBGP peers.
iBGP with two Internet connected routers
Two Internet-connected routers in an Enterprise need to communicate BGP routes to each
other because these routers may want to forward IP packets to the other Internet-
connected router, which in turn would forward the packets into the Internet.
With an iBGP peer connection, each Internet-connected router can learn routes from the
other router and decide if that other router has a better route to reach some destinations
in the Internet. Without that iBGP connection, the routers have no way to know if the
other router has a better BGP path.
Configuring iBGP
The most basic iBGP configuration differs only slightly compared to eBGP configuration.
The configuration does not explicitly identify an eBGP versus an iBGP peer. Instead, for
iBGP, the neighbor’s ASN listed on the neighbor remote-as (BGP and BGP4+) command
lists the same ASN as the local router’s router bgp (BGP and BGP4+) command. This is
because the local router and the neighbor are in the same ASN.
For eBGP, the neighbor remote-as (BGP and BGP4+) command lists a different ASN.
The configuration to use loopback interfaces as the update source mirrors configuration
for eBGP peers, except that iBGP peers do not need to configure the neighbor ebgp-
multihop (BGP and BGP4+) command.
For iBGP connections, only the following steps are required to make two iBGP peers use a
loopback interface:
1. Configure an IP address on a loopback interface on each router.
2. Configure each router to use the loopback IP address as the source IP address, for the
neighborship with the other router, using the neighbor update-source (BGP and
BGP4+) command.
3. Configure the BGP neighbor remote-as (BGP and BGP4+) command on each router
to refer to the other router’s loopback IP address as the neighbor IP address in the
neighbor remote-as (BGP and BGP4+) command.