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BGP Peers
are peers, then they assume that they are equal in relationship. Usually an
administrator has decided that it is beneficial for his customers to reach one
another. These peers advertise their customers' routes to one another. This
does not mean that they exchange their full Internet routing tables.
Let's take a more in-depth look at iBGP and eBGP.
The internal Border Gateway Protocol (iBGP) is used by routers that all
belong to the same autonomous system. These routers may use loopback
interfaces to provide greater reachability in the AS. This is possible because
an IGP can provide multiple routes to any given destination 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.
Before any BGP route information can be exchanged between two rout-
ers, a TCP connection has to be established. And another routing protocol
other than BGP can be used to establish the TCP connection. The TCP con-
nection is made by a three-way handshake using a SYN, ACK, SYN
sequence. Once a TCP connection has been established, route information
can be exchanged.
Routing information from one peer is not advertised from one iBGP to
another iBGP peer. This prevents inconsistent route information and routing
loops in the network. To share route information among all iBGP routers,
you must establish a logical mesh, as shown in Figure 7.5. Routing informa-
tion is then exchanged only between routers who are members of this mesh.
RouterB can learn BGP networks only from RouterA. When RouterC sends
its BGP information, only its own information is sent. Routing information
learned from RouterA is not included.
Copyright ©2001 SYBEX , Inc., Alameda, CA