| A multihomed transit system has more
than one connection to the outside world and can be used for transit
traffic by other autonomous systems. From the multihomed AS' view, transit traffic is any traffic originating from
outside sources bound for outside destinations as shown in the
Figure.
A transit AS can route transit
traffic by running BGP internally so that multiple border routers in
the same AS can share BGP information. Additional routers may be
used to forward BGP information from one border router to another.
You may choose to run BGP inside an AS to facilitate this exchange.
When BGP is running inside an AS, it
is referred to as Internal BGP (IBGP). When BGP runs between
autonomous systems, it is called External BGP (EBGP). If a BGP
router's role is to route IBGP traffic, it is called a transit
router. Routers that sit on the boundary of an AS and that use EBGP
to exchange information with the ISP are called border (or edge)
routers.
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