Cisco 3560G 48TS Software Configuration Guide - Page 652

Configuring BGP Chapter 30 Configuring IP Unicast Routing The network has these characteristics: • Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected. • All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the requirement for a logical full mesh: confederations and route reflectors. • AS 200 is a transit AS for AS 100 and AS 300-that is, AS 200 is used to transfer packets between AS 100 and AS 300. BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions). In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (the autonomous system path), and a list of other path attributes. The primary function of a BGP system is to exchange network reachability information, including information about the list of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops, and to enforce AS-level policy decisions. A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the "Configuring BGP Decision Attributes" section on page 30-50 for information about BGP attributes. BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes. These sections briefly describe how to configure BGP and supported BGP features: • Default BGP Configuration, page 30-45 • Enabling BGP Routing, page 30-47 • Managing Routing Policy Changes, page 30-49 • Configuring BGP Decision Attributes, page 30-50 • Configuring BGP Filtering with Route Maps, page 30-52 • Configuring BGP Filtering by Neighbor, page 30-53 • Configuring Prefix Lists for BGP Filtering, page 30-54 • Configuring BGP Community Filtering, page 30-55 • Configuring BGP Neighbors and Peer Groups, page 30-57 • Configuring Aggregate Addresses, page 30-59 • Configuring Routing Domain Confederations, page 30-59 • Configuring BGP Route Reflectors, page 30-60 • Configuring Route Dampening, page 30-61 • Monitoring and Maintaining BGP, page 30-62 For detailed descriptions of BGP configuration, refer to the "Configuring BGP" chapter in the Cisco IOS IP and IP Routing Configuration Guide. For details about specific commands, refer to the Cisco IOS IP and IP Routing Command Reference for Release 12.1. 30-44 Catalyst 3560 Switch Software Configuration Guide 78-16156-01