Cisco IE-5000 User Manual

847
Configuring IP Unicast Routing
Configuring BGP
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 Configuring BGP Decision Attributes, page 854 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.
This section includes the following topics:
 Default BGP Configuration, page 848
 Enabling BGP Routing, page 850
 Managing Routing Policy Changes, page 853
 Configuring BGP Decision Attributes, page 854
 Configuring BGP Filtering with Route Maps, page 857
 Configuring BGP Filtering by Neighbor, page 858
 Configuring Prefix Lists for BGP Filtering, page 859
 Configuring BGP Community Filtering, page 861
 Configuring BGP Neighbors and Peer Groups, page 863
 Configuring Aggregate Addresses, page 865
 Configuring Routing Domain Confederations, page 867
 Configuring BGP Route Reflectors, page 868
 Configuring Route Dampening, page 869
 Monitoring and Maintaining BGP, page 870

Cisco IE-5000 Specifications

General

Brand	Cisco
Model	IE-5000
Category	Switch
Language	English