election of avfs in GLBP

Answered Question
Jan 25th, 2009
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Hi every body!

In GLBP, routers with highest priority is elected as AVG and the router with 2nd highest priority is elected as standby router.If priorities are same, then router with higher ip address , will become AVG.


Now 2nd step comes, elections of AVFs .

Since AVG has 4 mac addresses to assigns , there could be 4 AVF at most including AVG which also acts as AVF.

My question is what is the criteria to elect AVF?

Say we have we have 8 routers, r1 through r8

r1 is AVG, r2 is standby(both also act as AVF)

Now two more AVF could be selected from the rest of 6 routers,

How will these two AVF be elected? using priority and ip address or weight?


thanks a lot!

Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Sarah,

the document you have linked is about HSRP and not GLBP.


My understanding is that GLBP has a two dimensions criteria:

priority then ip address to elect AVG and backup AVG

weighting then ip address to elect AVF (but AVF is assigned by AVG)


The only way to demonstrate this would be to have 5 routers in the same LAN segment configuring all them for the same GLBP group and then playing with the parameters to see what happens.


Hope to help

Giuseppe

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Giuseppe Larosa Mon, 01/26/2009 - 04:58
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Hello Sarah,


GLBP has a two dimension criteria:


priority is used to elect the AVG and backup AVG.


weigthing is used to elect a primary AVF for each of the 4 Virtual MAC addresses:


see


http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_glbp_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1055542


glbp group forwarder preempt [delay minimum seconds]


Configures the router to take over as AVF for a GLBP group if the current AVF for a GLBP group falls below >>its low weighting threshold.<<<


•This command is enabled by default with a delay of 30 seconds.


•Use the optional delay and minimum keywords and the seconds argument to specify a minimum delay interval in seconds before preemption of the AVF takes place.



Additional routers can listen to GLBP messages and act as secondary AVF ready to take the active role if primary AVF fails.


Hope to help

Giuseppe


sarahr202 Mon, 01/26/2009 - 19:01
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Thanks Giuseppe!


Please consider the following scenario:


Scenario #1

suppose we have 6 routers, r1,r2,r3,r4,r5,r6

r1 priority=200

r2 priority =150

r3, r4, r5 have default weight 100, and priority 100.

r1 becomes AVG

r2 would standby router.

These two, r1 and r2 are also avfs and use two of four mac.

Two more avfs could be selected because two macs are left.

Which two would be selected as priority and weight are same?

scenario # 2

r1,r2,r3,r4,r5,r6

r1 priority =200, weight=50

r2 priority =150, weight=40

r3,r4,r5, r6 have default weight 100, and priority 100.

Would r1 become Avg ? will it become avf or not as weight is 50?

Is it possible for a router to be avg but not avf at the same time?


how about r2? will it be standby router and avf? but its weight is 40.



Thanks a lot!


viyuan700 Mon, 01/26/2009 - 22:06
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Scenario #1

Which two would be selected as priority and weight are same?


In my opinion between r3,r4,r5,r6, 2 with highest IP address willbe selected as other 2 AVF.


scenario # 2

Would r1 become Avg ? will it become avf or not as weight is 50?


I think r1 will be AVG but not AVF. As weight goes down when a interface is tracked and if interface is down it cannot be a forwarder.


Is it possible for a router to be avg but not avf at the same time?


In HSRP when you track interface priority goes down by value defined. Using logic (not tested)In GLBP becuase of weighting as one more order of redundancy, if AVG fails (AVG router is down) then it cannot be AVG and AVF but if only an interface is down then it is still acting as AVG but not AVF. It is assigning virtual MAC but not forwarding.


how about r2? will it be standby router and avf? but its weight is 40.


As mentioned above, it can be standby router but not AVF as interface is down through which it sends traffic out.

Giuseppe Larosa Tue, 01/27/2009 - 01:07
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Hello Sarah,


>> Is it possible for a router to be avg but not avf at the same time?


yes I think it is possible in GLBP and is what I was meaning with two dimensions criteria.


if the weighting is lowest the router cannot qualify as primary AVF when more then 4 routers are involved.


Actually, the AVG performs a separate function that is that of answering clients ARP requests for the VIP address using a load balancing algorithm.

With GLBP different client PCs will associate a different MAC address (that of a different AVF) to the VIP ip address.

As a result of this the AVG could be a device that is not involved in actual forwarding of user traffic out the lan segment.


Hope to help

Giuseppe


sarahr202 Tue, 01/27/2009 - 18:12
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Thanks Giuseppe!

In Avf election, if weights are same, what is next criteria is used to choose avf? priority?

thanks a lot!

Giuseppe Larosa Wed, 01/28/2009 - 05:33
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Hello Sarah,

documentation is not clear but I think the criteria for forwarding capabilities are:


weighting

then

highest ip address on LAN


I would expect priority to be used to elect the AVG and backup AVG only.


see


http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_glbp_ps6017_TSD_Products_Configuration_Guide_Chapter.html#wp1054557


Hope to help

Giuseppe


sarahr202 Wed, 01/28/2009 - 08:55
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Hi Giuseppe!

according to the follwing side, priority is determined first by configured priority value then by the highest ip address.


javascript:newWin('http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a91.shtml')


" Priority is determined first by the configured priority value, and then by the IP address"



Any thoughts?



thanks a lot!

Correct Answer
Giuseppe Larosa Wed, 01/28/2009 - 12:27
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Hello Sarah,

the document you have linked is about HSRP and not GLBP.


My understanding is that GLBP has a two dimensions criteria:

priority then ip address to elect AVG and backup AVG

weighting then ip address to elect AVF (but AVF is assigned by AVG)


The only way to demonstrate this would be to have 5 routers in the same LAN segment configuring all them for the same GLBP group and then playing with the parameters to see what happens.


Hope to help

Giuseppe

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