GLBP Weight/threshold

[Roel Reyes]

Hi, 

I dont understand how does cisco explained GLBP weight tracking and threshold. I found this on their documents about GLBP

Why would you track an object with decrement value that will not let the weight fall below the lower threshold. As for the example below, weight is 105 and decrement is 5 only, then how this router will give up its AVF function?

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html#wp1044401

 Weighting 105 (configured 110), thresholds: lower 95, upper 105

    Track object 2 state Down decrement 5

[Francesco Molino · Accepted solution]

Hi

Let me try to answer your question, hoping I get it right.

By decrementing the weight by 5, the weight will go down the lower threshold if the track is down for more than 90 seconds. (The default preemption timer is 30 seconds).

In opposition to HSRP, GLBP is doing some load balancing. GLPB is organized in a way to have 1 AVG with glbp group with multiple AVF (up to 4 AVF per group). This means that every time a ARP arrives, AVG is replying with a vMAC corresponding to an AVF.

By decrementing slowly the GLBP weight, that means you can control the traffic and pass it through smoothly through others AVF without re-forwarding all traffic in 1 shot. In fact, the formula GLBP uses is:

In a default round-robin behaviour, every AVF has its vMAC used by hosts (sent by AVG) corresponding to a percentage of the ARP replies equal to its weight divided by the sum of all AVF weights within this group.

Then to finalize, by decrementing by 5, you're allowing few seconds (in our case, more than 90s) before this router leaves the GLBP group as AVF (lower threshold). 

This value is determined more based on your design to not switch in 1 shot all traffic to others AVF, as I said before but switch it smoothly. Usually, in a standard base, I'm more using a decrement of 10 (in average) to control the traffic flow and timers when a router is or isn't an AVF to forward that traffic.

Not sure if it's clear enough.

I'm sure there are plenty of blogs/documentation out there explaining how to configure/use GLBP weights with tracking in different designs.

Thanks

PS: Please don't forget to rate and mark as correct answer if this answered your question.

[Roel Reyes]

I don't understand it clearly sorry, but if the tracked interface is down then you let it decrement by 5 but within the threshold, meaning it is still an AVF and since the interface is down why would you still want to send or control traffic on that interface?

[Francesco Molino]

Again, why decrementing by 5 in that specific case I don't know. There are some case where you want to move the traffic to another router smoothly based on a track. The track down doesn't mean that the router can't take any traffic. You can monitor any interfaces just to trigger to load balance the traffic to another router.

If you have 2 or 3 routers let's say and as soon as a used interface (inside or outside) goes down, you wan't to remove that router as AVF then just decrement using a value of 10 or 20 and just your thresholds.

Does that make sense now?

Thanks

PS: Please don't forget to rate and mark as correct answer if this answered your question.