A multihomed transit AS has more
than one connection to the outside world and can be used for transit
traffic by other ASs. Transit traffic (relative to the multihomed AS) is
any traffic with an origin and destination that does not belong to the
AS.
Even though BGP4 is an EGP, it can still be used as a
pipe to propogate routing information through an AS. BGP connections
inside an AS are called internal BGPs (IBGPs), whereas BGP connections
between ASs are called external BGPs (EBGPs). Routers that are running
IBGP are called transit routers when they carry the transit traffic
going through the AS. Routers that run EBGP with other ASs are usually
called border routers.
Not all routers inside a transit domain need to run
BGP; internal nontransit routers could have default routes to the
routers running IBGP; this prevents nontransit routers from being forced
to carry the full routing internetwork (or full Internet) routing table.
The Figure illustrates a multihomed transit AS, AS1,
connected to two different providers, ISP1 and ISP2. AS1 is learning
routes 172.18.20.0/24, 172.18.21.0/24, 172.18.18.22.0/24, and
172.18.23.0/24 from both ISP1 and ISP2 and in turn advertising all that
it learned, including its local routes, to ISP1 and ISP2. In this case,
ISP1 could use AS1 as a transit AS to reach networks 172.18.22.0/24 and
172.18.23.0/24, and ISP2 could use AS1 to reach networks 172.18.20.0/24
and 172.18.21.0/24.