BGP Basics in cisco

Like EGP, BGP forms a unique, unicast-based connection to each of its BGP-speaking peers. To
increase the reliability of the peer connection, BGP uses TCP (port 179) as its underlying delivery
mechanism. The update mechanisms of BGP are also somewhat simplified by allowing the TCP layer
to handle such duties as acknowledgment, retransmission, and sequencing. Because BGP rides on
TCP, a separate point-to-point connection to each peer must be established.
BGP is a distance vector protocol in that each BGP node relies on downstream neighbors to pass
along routes from their routing table; the node makes its route calculations based on those
advertised routes and passes the results to upstream neighbors. However, other distance vector
protocols quantify the distance with a single number, representing hop count or, in the case of IGRP
and EIGRP, a sum of total interface delays and lowest bandwidth. In contrast, BGP uses a list of AS
numbers through which a packet must pass to reach the destination . Because this
list fully describes the path a packet must take, BGP is called a path vector routing protocol to
contrast it with traditional distance vector protocols. The list of AS numbers associated with a BGP
route is called the AS_PATH and is one of several path attributes associated with each route. Path
attributes are described fully in a subsequent section.
Figure 2-18. BGP Determines the Shortest Loop-Free Inter-AS Path from a
List of AS Numbers Known as the AS_PATH Attribute
Recall from Chapter 1 that EGP is not a true routing protocol because it does not have a fully
developed algorithm for calculating the shortest path and it cannot detect route loops. In contrast,
the AS_PATH attribute qualifies BGP as a routing protocol on both counts. First, the shortest inter-AS
path is very simply determined by the least number of AS numbers., AS7 is receiving