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Chapter 2

Protocol Attributes

The two types of BGP are iBGP and eBGP. There are several differences

between the two. Primarily, iBGP (internal BGP) is used to share BGP infor-
mation with routers within the same AS, whereas eBGP (external BGP) is
used to share route information between two different autonomous systems.
More details will be given as we discuss each type separately.

iBGP (Internal Border Gateway Protocol)

Internal BGP is used by routers that belong to the same autonomous system.
iBGP may use loopback interfaces to provide greater reachability. This is
possible because the IGP can provide multiple routes to any given destina-
tion address if the network has redundant or multiple links to each router. If
one interface on a router goes down, the TCP connection to the loopback
address can be maintained by using redundant interfaces.

It is important to understand that before any BGP route information can

be exchanged between two routers, a TCP connection has to be established.
The TCP connection is made by a three-way handshake using a SYN-ACK-
SYN sequence. Once a TCP connection has been established, route informa-
tion can be exchanged.

An important feature of iBGP is that route information from one peer is

not advertised to another iBGP peer. This avoids inconsistent route informa-
tion and routing loops. To share route information between all iBGP routers,
establish a logical mesh. (Figure 2.18 shows a picture of what is meant by
this.) Route information is exchanged only between routers. Router B can
learn BGP networks only from Router A. When Router C sends its BGP
information, only its information is sent. Routes learned from Router A are
not included.

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