Provide redundancy on your Cisco routers with GLBP

GLBP is a router redundancy protocol introduced in Cisco IOS version 12.2(14)S. Not familiar with GLBP? Let David Davis introduce you to this protocol and show you how to configure it.

Are you familiar with GLBP? I'm willing to bet that a good number of you are not. In fact, I hadn't heard of it until recently.

GLBP stands for Gateway Load Balancing Protocol, and it's been on the Cisco scene for about two years. GLBP is a router redundancy protocol introduced in Cisco IOS version 12.2(14)S.

To give you an idea where GLBP fits into your network, let's look at how it compares to its competition. Two such protocols are Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP). Both are router redundancy protocols that are similar to GLBP.

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).

Since all three are router redundancy protocols, what differentiates GLBP from the rest? With GLBP, all routers that are part of the group are available to forward packets. (In the server world, we might call this an active/active cluster.) On the other hand, with HSRP or VRRP, only one router forwards packets at a time, and the others are waiting in case the primary goes down (an active/passive cluster).

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 (AVFs). GLBP routers communicate using the multicast IP address and 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:

Router(config)# interface FastEthernet 0/0
Router(config-if)# ip address
Router(config-if)# glbp 1 ip
Router(config-if)# glbp 1 priority 150
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.

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David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.

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