GLBP vs. HSRP

Unanswered Question
Oct 29th, 2008

I'm trying to learn about these two HA protocols, and I have a question. From playing around with GLBP, it seems as though it doesn't change the AVG unless the Active one goes down which will then force the Standby to become active. In the case of HSRP, if you are tracking an interface that goes down, the standby will automatically come up.

In my other tests, I noticed that the AVF DID change from the active to the standby. Am I correct in the fact that the AVG physically has to die to make a standby become the active in the case of GLBP?

*** Oh, and can I equate the AVF to a physical router in HSRP? For example, in HSRP tracking an interface that goes down will make the standby active if preempt is enabled. In an AVF, weights will change the forwarders status (like HSRP), and then bring the forwarder back online when the weight is within threshold?

Thanks,

John

I have this problem too.
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Jon Marshall Wed, 10/29/2008 - 13:45

John

You can use preempt on the AVG if you want but it is disabled by default - from Cisco GLBP doc

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By default, the GLBP virtual gateway preemptive scheme is disabled. A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned to the virtual gateways. You can enable the GLBP virtual gateway preemptive scheme using the glbp preempt command. Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG.

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http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_glbp_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1054491

An AVF is a physical router that forwards packets for a certain amount of hosts.

Jon

John Blakley Wed, 10/29/2008 - 13:54

Jon,

"...if the current AVG fails."

I have the preempt, and the standby comes up only if I shut down the internal interface (the interface that shares the virtual IP). My assumption is that since it uses AVFs, the physical routers will always be in standby unless the AVG physically fails (dead power, internal interface goes down, etc.)

--John

Jon Marshall Wed, 10/29/2008 - 13:58

John

Unfortunately i don't have a lab to test with at the moment. But from the text in original post

"Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG."

ie. the first 2 sentences in that text are describing the default ie. when preemption is not enabled. But if you enable preemption then your standby AVG can preempt as long as it has a higher priority.

Perhaps i'm reading it wrong :)

Jon

John Blakley Wed, 10/29/2008 - 14:16

I'm going to do some more testing with it tonight to see what I can break. :-)

Thanks Jon!

John

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