Cisco IOS Cookbook, 2nd Edition

Connecting to Two ISPs with Redundant Routers

Problem

You want to connect your network to two different ISPs using two routers to eliminate any single points of failure.

Solution

In this example, we have two routers in our AS, which has ASN 65500. The first router has a link to the first ISP, whose ASN is 65510:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#description connection to ISP #1, ASN 65510
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Ethernet0
Router1(config-if)#description connection to internal network, ASN 65500
Router1(config-if)#ip address 172.18.5.2 255.255.255.0
Router1(config-if)#exit
Router1(config)#ip as-path access-list 15 permit ^$
Router1(config)#router bgp 65500
Router1(config-router)#network 172.18.5.0 mask 255.255.255.0
Router1(config-router)#neighbor 172.18.5.3 remote-as 65500
Router1(config-router)#neighbor 172.18.5.3 next-hop-self
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 filter-list 15 out
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#

Then the second router connects to the second ISP, which uses ASN 65520. And because these two routers are both members of the same AS, they also must have an iBGP connection:

Router2#configure terminal Enter configuration commands, one per line. End with ...