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I have a question related to your TCPmag.com Q&A from Jan 4, 2005, about HSRP and GLBP. I'm trying to configure two 2600 routers with GLBP and a virtual IP address and I want to load balance between the two routers. Do I just configure GLBP as default for both eth0 ints on the routers?

-- Wes

Answer:

Wes,

If you don't have any particular desire for one or the other of your routers to be the one in charge [i.e., Active Virtual Gateway (AVG)] of the group, then you really don't need to do anything beyond the defaults. The devices within a particular group will elect an AVG, and it goes from there.

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GLBP, or Gateway Load Balancing Protocol, is another way to make multiple routers share a virtual IP address to be used for a default gateway by hosts. This started many years ago with Cisco's Hot Standby Router Protocol (HSRP). Well, ok, there was a variant before that called IRDP (ICMP Router Discovery Protocol), which allowed for gateway discovery by the clients -- but that's a different story.

The industry has a standard very similar to HSRP called VRRP (Virtual Router Redundancy Protocol). Both HSRP and VRRP have a single active router per group, so it's all or nothing. Both do quite well in maintaining overall network efficiency through a virtual shared IP address, so that changes in network functionality are relatively unknown to the users.

GLBP handles things a little differently. Instead of a electing a single active gateway for all traffic, an AVG is elected. The responsibility of the AVG is to actually maintain contact with all other GLBP gateways as well as be the sole device to respond to Address Resolution Protocol (ARP) requests for the gateway's virtual IP address. The AVG will respond to ARP requests in a pseudo round-robin way, going through all of the other gateways' information.

With this, we end up with multiple active gateways at the same time. Each client may receive different MAC address information for the same gateway IP address. With multiple active gateways on the network, efficiency is not only maintained like other protocols do but actually increased! This MAC address assignment is known as an Active Virtual Forwarder (AVF).

All three redundancy protocols have methods of authenticating among members of a group, as well as things like timers, tracking mechanisms, preempt capability and other fun features. But the bare minimum to make things work is simply to enable GLBP on an interface! Everything else is optional.

R1(config-if)# glbp 1 ip 172.16.1.1

Obviously you fill in whatever information is specific to your network there! But that's all that's required!

After enabling this on your network, though, I suggest you pay attention to the performance of circuits, routers and such to determine whether further tweaking is necessary. GLBP also gives you the ability to choose weighting of certain gateways over others to control the load balancing.

For more information, check out: Cisco IOS IP Application Services Configuration Guide, Release 12.4: Configuring GLBP

Hope that helps!

-- Scott

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Scott Morris, quadruple CCIE, JNCIE and all-around uber-geek, can often be seen traveling around the world consulting and delivering CCIE training. He recently accepted a new Senior CCIE Instructor position with Internetwork Expert! For more information on him check out http://www.uber-geek.net or for CCIE training check out http://www.internetworkexpert.com. You can contact Scott via editor@tcpmag.com. You can contact Scott about "Configuring Routers with GLBP and a Virtual IP Address" at editor@tcpmag.com.