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BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers)
(Page 2 of 2)
BGP Internal and External Peers
Most BGP speakers will be connected
to more than one other speaker. This provides both greater efficiency
in the form of more direct paths to different networks, and also redundancy
to allow the internetwork to cope with either device or connection failures.
It is possible (and in many cases, likely) for a BGP speaker to have
neighbor relationships with other BGP speakers both within its own AS
and outside its AS. A neighbor within the AS is called an internal
peer, while a neighbor outside the AS is an external peer.
BGP between internal peers is sometimes called Internal BGP (IBGP)
while use of the protocol between external peers is External BGP
(EBGP). The two are similar, but differ in certain areas, especially
path
attributes and route selection. You can
see an example of BGP topology and the designation of internal and external
peers in Figure 191.
Figure 191: BGP Example Topology and Designations This diagram is a variation on Figure 171 that shows the names used by BGP to refer to different types of routers and autonomous systems. Internal routers are shown in blue while border routers are red. BGP speakers that communicate within an AS are internal peers, while those that communicate between ASes are external peers. This highly simplified internetwork shows two stub ASes, both of which only connect to the multihomed AS #2.
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 Key Concept: Each router configured to use BGP is called a BGP speaker; these devices exchange route information using the BGP messaging system. Routers that only connect to other routers in the same autonomous system are called internal routers, while those that connect to other ASes are border routers. Neighboring BGP speakers in the same AS are called internal peers, while those in different ASes are external peers.
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A peer connection between BGP speakers
can be either a direct link using some form of layer two technology,
or an indirect link using TCP. This allows the BGP speakers to establish
BGP sessions and then exchange routing information, using the
messaging system we will see later in this section.
Of course, it also is the means by which actual “end user”
traffic moves between autonomous systems. External peers are normally
connected directly, while internal peers are often linked indirectly.
In the
next topic we will see that the method
in which ASes are connected has an important impact on the overall function
of the internetwork and how traffic is carried on it.
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