Because BGP relies on TCP to provide reliable and flow-controlled transmission of
routing information, the BGP protocol itself is very simple. However it also implies
that two routers can be BGP peers of each other only if they are reachable from each
other in the sense that they can exchange IP packets.
In practice this means that either of the following must be true:
â– The BGP peers must be connected to a common IP subnet.
â– The BGP peers must be in the same AS, which runs an IGP enabling the BGP
peers to reach each other.
IBGP and EBGP
When two BGP speakers are in the same autonomous system, the BGP session is
called an internal BGP session, or IBGP session. When two BGP speakers are in
different autonomous systems, the BGP session is called an external BGP session, or
EBGP session. BGP uses the same types of message on IBGP and EBGP sessions, but
the rules for when to send which message and how to interpret each message differ
slightly; for this reason some people refer to IBGP and EBGP as two separate protocols.
IBGP requires that BGP speakers within an autonomous system be fully meshed,
meaning that there must be a BGP session between each pair of peers within the AS.
IBGP does not require that all the peers be physically connected. EBGP does not
require full meshing of BGP speakers. EBGP sessions typically exist between peers
that are physically connected.
Figure 2 on page 6 shows an example of the exchange of information between
routers running IBGP and EBGP across multiple ASs.
Figure 2: Internal and External BGP
Interior Gateway Protocols
Not all the routers within an AS have to be BGP peers. For example, in some large
enterprise networks, ASs generally have many more non-BGP routers. These routers
communicate using an interior gateway protocol (IGP) such as the following:
6 â– Overview
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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