Border Gateway Protocol - CCIE Routing and Switching Exam Certification Guide - page 433

Router D's configuration is shown in Example 9-3.

Router D Configuration

Example 9-3

interface loopback 0
ip address 2.1.1.1 255.255.255.255
!
router bgp 200
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop
neighbor 1.1.1.1 update-source loopback0
!
ip route 1.1.1.1 0.0.0.0 2.1.10.1

Advertising Networks

You can configure BGP in three ways to advertise networks to other peers:

1

network command

2

aggregate-address command

3

Redistribution of IGPs

network Command

The command specifies which routes in the local IP routing table are added to the BGP

table. By default, auto-summary is enabled on Cisco routers. This enables the router to
summarize subnets into the classful network boundaries. With auto-summary enabled, and the

network

network command used without a mask or with a classful mask, at least one subnet for that

network must be present in the IP routing table and the specified network will be added to the
BGP table. If auto-summary is disabled, no classful summary route is created. The network
specified in the

statement must exactly match an entry in the local routing table before
the network is added to the BGP table.

network

If the specified network does not coincide with classful network boundaries, a mask must be
specified. The command with a mask causes a route to be announced if there is an

exact match for the prefix in the routing table. If the network is not present in the routing table,
BGP does not announce it, even if the

network

command is configured. The following is a
configuration example to advertise network 4.0.0.0/8 and 5.0.0.0/16 from AS 100:

network

router bgp 100
network 4.0.0.0
network 5.0.0.0 mask 255.255.0.0

The default auto-summary is enabled. At least one subnet from network 4.0.0.0 must be present
in the routing table before network 4.0.0.0/8 is added to the BGP table. Network 5.0.0.0/16, not
a different subnet of 5.0.0.0, must be in the routing table before 5.0.0.0/16 is added to the BGP
table.