TCP connection over which routing information is exchanged according to the rules of
the BGP protocol.
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:
•
The BGP peers must be connected to a common IP subnet.
•
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
Copyright © 2010, Juniper Networks, Inc.6
JunosE 11.2.x BGP and MPLS Configuration Guide
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