Cisco NCS 5500 Series

CHAPTER 2
Implementing BGP
Border Gateway Protocol (BGP) is an Exterior Gateway Protocol (EGP) that allows you to create loop-free
interdomain routing between autonomous systems. An autonomous system is a set of routers under a single
technical administration. Routers in an autonomous system can use multiple Interior Gateway Protocols
(IGPs) to exchange routing information inside the autonomous system and an EGP to route packets outside
the autonomous system.
This module provides conceptual and configuration information on BGP.
ModificationRelease
This feature was introduced.Release 6.0
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BGP Functional Overview, page 3
BGP Functional Overview
BGP uses TCP as its transport protocol. Two BGP routers form a TCP connection between one another (peer
routers) and exchange messages to open and confirm the connection parameters.
BGP routers exchange network reachability information. This information is mainly an indication of the full
paths (BGP autonomous system numbers) that a route should take to reach the destination network. This
information helps construct a graph that shows which autonomous systems are loop free and where routing
policies can be applied to enforce restrictions on routing behavior.
Any two routers forming a TCP connection to exchange BGP routing information are called peers or neighbors.
BGP peers initially exchange their full BGP routing tables. After this exchange, incremental updates are sent
as the routing table changes. BGP keeps a version number of the BGP table, which is the same for all of its
BGP peers. The version number changes whenever BGP updates the table due to routing information changes.
Keepalive packets are sent to ensure that the connection is alive between the BGP peers and notification
packets are sent in response to error or special conditions.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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