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
224.0.0.102 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 10.1.1.5
Router(config-if)# glbp 1 ip 10.1.1.1
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.