User's Manual

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Chapter 61 Configure BGP
61.1 Overview
This chapter describes how to configure border gateway protocol (BGP). For complete description about BGP
commands in this chapter, please refer to other sections related to “BGP command”. BGP is an Exterior
Gateway Protocol (EGP) defined in RFC1163, 1267 and 1771. It permits to establish a route selection
mechanism among different autonomous systems, this mechanism can automatically guarantee the loop-free
routing information exchange between the autonomous systems.
61.1.1 The BGP implementation of the router
In BGP, each route includes a network number, the autonomous system list this route has tranverse (called
As-path) and other attribute lists. Our router software supports BGP v4 defined in RFC1771. The basic function
of BGP is to exchange network reachability information with other BGP systems, including information about
AS-path information. This information can be used to construct the AS connection graph which can eliminate
route loop, and it can implement AS level routing policy with AS connection graph. BGP v4 supports classless
inter-domain router (CIDR), CIDR can reduce the size of the routing table through creating summary routes
and thus creates a super network. CIDR removes the concept of network level in BGP, and supports IP prefix
broadcasting. CIDR route can be transferred through OSPF, Enhanced IGRP, ISIS-IP and RIP2.
An important difference between exterior gateway routing and interior gateway router is the former has better
controllability. In order to control the route, the implementation of BGP provides several optional methods:
In order to filter routes, it can be based on access-list based on neighbour, aspath-list,
prefix-list and also use the access-list based on interface, prefix-list to filter routes or the
Nexthop attribute of the routes.
In order to change the attribute of the routes, you can use the route-map to mend the
attributes of BGP routes including MED, Local preference, route value and etc…
In order to interact with the interior gateway dynamic routing protocol (OSPF, RIP, etc…),
you can redistribute route, so as to automatically generate BGP routing information. You
can also generate BGP routes through manual configuration of network, aggregation.
While generating BGP routes, you can use route-map to configure the attributes of the
routes.
In order to control the precedence of BGP routes in the system, you can use command
“distance” to configure the management distance of BGP routes.
61.1.2 How does BGP select the path
The decision process of BGP is established on the basis of comparing route attribute value. When the same
network has several routes, BGP selects the best route to the destination. The following process summarizes
how BGP selects the best route:
If it cannot arrive at the next hop, it will not be considered.
If the path is internal and the synchronization is activated, and if the route is not in IGP, the
route will not be considered.
Select preferable path with the maximum precedence.