Cisco IOS Cookbook, 2nd Edition

Gateway Load-Balancing Protocol

Problem

You want to use a first hop redundancy protocol that automatically load-balances among the participating routers.

Solution

You configure GLBP on an interface using the glbp command:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface FastEthernet0/0
Router1(config-if)#ip address 172.22.1.3 255.255.255.0
Router1(config-if)#glbp 1 ip 172.22.1.1
Router1(config-if)#exit
Router1(config)#end
Router1#

As with HSRP and VRRP, you must configure other members of the same GLBP group to use the same virtual IP address:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface FastEthernet0/0
Router2(config-if)#ip address 172.22.1.2 255.255.255.0
Router2(config-if)#glbp 1 ip 172.22.1.1
Router2(config-if)#exit
Router2(config)#end
Router2#

Discussion

In Recipe 22.4, we showed a way of using HSRP to load-balance between two active redundant routers on a LAN segment. There are two reasons why this solution was somewhat less than ideal, though. First, it required configuring different default gateway addresses on half of the end devices on a LAN segment. And second, the load balancing must be done by hand and is completely static. GLBP provides a much more elegant solution to the same problem that doesn’t have these shortcomings.

In the examples, we have enabled GLBP with a single command, simply defining the group number and virtual IP address:

Router1(config-if)# ...