GLBP is a new protocol similar in function to HSRP (discussed here). It has been available since IOS version 12.2(14)S and you can read more at the Cisco site in this article on Gateway Load Balancing Protocol. GLBP and HSRP are both Cisco proprietary solutions, but VRRP is an open standard based on RFC 3768. Of course, that means you'll find HSRP and GLBP on Cisco routers only, and VRRP on almost all enterprise routers (including Cisco).

The difference with GLBP is that instead of one router being primary and the others in standby (protect) mode, with GLBP all routers that are part of theredundancy group are available to forward packets in a load sharing process. Regardless of the approach, all three protocols provide this redundancy using a virtual IP address that serves as the default gateway. The virtual IP address points to the primary or secondary routers in the case of HSRP and VRRP or to the group of routers forwarding traffic in the case of GLBP. (On a side note, while HSRP acts as an active/passive router redundancy protocol by design, you can now use Multigroup HSRP (MHSRP) to perform load-sharing, much like GLBP does.)

In the GLBP world, however, there's still an election between the multiple active routers in the redundancy group. The router that wins that election becomes the active virtual gateway (AVG). The AVG assigns virtual MAC addresses to the other routers in the group, which are active virtual forwarders (AVF's). GLBP routers communicate using the registered multicast IP address and registered UDP port 3222.

Now that you've got a basic understanding of GLBP, let's look at a basic GLBP configuration. If you've configured HSRP or VRRP before, you'll find that this configuration is very similar. You can configure GLBP in Interface Configuration Mode. Here's an example of a basic configuration on a single router:

Cell_Router(config)# interface FastEthernet 0/0
Cell_Router(config-if)# ip address
Cell_Router(config-if)# glbp 1 ip
Cell_Router(config-if)# glbp 1 priority 150
Cell_Router(config-if)# glbp 1 authentication md5 key-string 0 MySecretPassword

Note: Cisco didn't introduce GLBP MD5 authentication until Cisco IOS version 12.3(2)T.

The default priority for a GLBP router is 100 (just like HSRP). By setting the priority in this configuration to 150, we give this router a higher priority than a router at the default priority of 100. This should force the election of the router as AVG.

By default, GLBP uses round-robin load balancing for routers in the group. However, you can change this by using the glbp 1 load-balancing command. This repeats the above configuration on the other routers in the group, with varying priority. To check the status of GLBP redundancy and configuration, you can use the 'show glbp' command.

Have fun with this, and we hope we have helped you to get started with GLBP.

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