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Configuring BGP BGP Overview
OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007 page 4-7
Internal vs. External BGP
Although BGP is an exterior gateway protocol, it can still be used inside an AS as a pipe to exchange BGP
updates. BGP connections inside an AS are referred to as Internal BGP (IBGP), while BGP connections
between routers in separate ASs are referred to as External BGP (EBGP).
ASs with more than one connection to the outside world are called multi-homed transit ASs, and can be
used to transit traffic by other ASs. Routers running IBGP are called transit routers when they carry the
transit traffic through an AS.
For example, the following diagram illustrates the use of IBGP in a multihomed AS:
In the above diagram, AS 100 and AS 200 can send and receive traffic via AS 300. AS 300 has become a
transit AS using IBGP between Router B and Router C.
Not all routers in an AS need to run BGP; in most cases, the internal routers use an IGP (such as RIP or
OSPF) to manage internal AS routing. This alleviates the number of routes the internal nontransit routers
must carry.
AS 100 AS 200
AS 300
Internal BGP
External BGPExternal BGP
Transit Traffic
Router C
Router B
Router A
Router D