Cisco 3750G - Catalyst Integrated Wireless LAN Controller Configuration Manual page 864

Configuring BGP
Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers
that have a TCP connection to each other for exchanging routing information are known as peers or
neighbors. In
D. The routing information is a series of AS numbers that describe the full path to the destination
network. BGP uses this information to construct a loop-free map of autonomous systems.
The network has these characteristics:
Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP
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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
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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
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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 35-51
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 contain this configuration information:
Default BGP Configuration, page 35-45
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Enabling BGP Routing, page 35-47
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Managing Routing Policy Changes, page 35-50
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Configuring BGP Decision Attributes, page 35-51
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• Configuring BGP Filtering with Route Maps, page 35-53
• Configuring BGP Filtering by Neighbor, page 35-54
Configuring Prefix Lists for BGP Filtering, page 35-55
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Configuring BGP Community Filtering, page 35-56
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Configuring BGP Neighbors and Peer Groups, page 35-57
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Configuring Aggregate Addresses, page 35-59
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Configuring Routing Domain Confederations, page 35-60
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Configuring BGP Route Reflectors, page 35-61
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Configuring Route Dampening, page 35-62
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Catalyst 3750 Switch Software Configuration Guide
35-44
Figure 35-5, Routers A and B are BGP peers, as are Routers B and C and Routers C and
Chapter 35 Configuring IP Unicast Routing
for information about BGP attributes.
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