Please confirm this basic GLBP configuration is needed

[news2010a]

Hello folks, given the GLBP configuration below, can you confirm if I need the "glbp 1 weighting track 1' command on SW2 to accomplis the following?

It seems to me it is not necessary but I would like to confirm. Thanks.

Configure GLBP between SW2 and SW4 VLAN 300.

SW4 should have the higher priority with ability for SW2 to assume control if priority of SW4 decreases.

SW2

int vlan 300

glbp 1 ip 1.1.1.1

glbp 1 preempt

glbp 1 weighting track 1

SW4:

track 1 ip route 0.0.0.0 0.0.0.0 reachability

int vlan 300

glbp 1 ip 1.1.1.1

glbp 1 preempt

glbp 1 priority 105

glbp 1 weighting track 1

glbp 1 weighting 100 lower 95

[Peter Paluch]

Hello,

For pure AVF preemptivity, the glbp weighting track command is not necessary. The AVF preemptivity is on by default, and in case of necessity, it can be controlled using the glbp forwarder preempt command. The glbp preempt allows for AVG preemption and does not influence the preemptivity of AVFs.

Using the glbp weighting track simply gave the AVF preemption a meaning here - it made the AVF weights to respond to something. Otherwise, the AVF weights would remain constant and the preemptivity would not have any reason to kick in. Note that the SW2 config as displayed here is incomplete. SW4 configuration is correct.

Best regards,

Peter

[news2010a]

Sorry I do not understand why the SW2 is incomplete?

Do you mind to indicate what you think it is missing on SW2. As far as I know, since the priority by default is 100, if SW4 interface goes down that would decrease the priority on SW4 to 95. Then at that point SW2 would take over, no?

Please let me know what you think is wrong.

[Calin C.]

Peter explanation is right, I just want to respond to your second question. I think the point of viewing things is wrong here:

 if SW4 interface goes down that would decrease the priority on SW4 to 95.

In that interface goes down, is down, meaning it cannot communicate anything to the other SW in terms of values, preferences or anything else. It's like is not there.

glbp 1 weighting 100 lower 95

Specifies the initial weighting value, and the upper and lower thresholds, for a GLBP gateway.In your case only the lower value is specified

glbp 1 weighting track 1

Specifies an object to be tracked that affects the weighting of a GLBP gateway. If you want this to work it should be

glbp 1 weighting track 1 decrement 5

as example.

This is a local value on the switch and has nothing to do with the other switch. Yes, the switches will communicate their weighting values, and the highest one will be active, but only if both interfaces (SW2 and SW4) will be up.

In more human terms what will happen here. You track on SW4 a default route. As long as the default route is reachable then the glbp will have a weight of 100. When you'll have an outage, then the tracking is coming into play and tell the glbp "hey I have a problem here, please decrement the glbp value with 5" ; the glbp reduce its weight with 5 , untip 95 value, as this is the lower threshod.

I hope this bring some more information. If not, here is the GLBP "bible" with some examples:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html#wp1046944

HTH,

Calin

[Jon Marshall]

I don't see a problem with the config -

1) you don't need the weighting command on sw2 if you are not tracking a route on sw2

2) you don't need to specify the priority on sw2 as the default is 100

3) you don't need to specify a lower value in the weighting command on sw4 because the default is to decrement by 10 so it will be lower than sw2 if the route is removed

personally i only add config that is needed so i would stick with what you have.

Jon

[Peter Paluch]

Hello,

On SW2, you are using the glbp weighting track command that references a track object 1. However, that object is not created on SW2, only on SW4, and you cannot track an object defined on another device.

Moreover, if you want to perform tracking on SW2, you should define the threshold weights when SW2 should cease and start performing as AVF.

This is what I had in mind when I indicated that your config on SW2 seems incomplete.

Best regards,

Peter

[Jon Marshall]

Hi Peter

Hope the training is going well and that you are enjoying it.

As for the question, i think that was Marlon's point ie. he is not tracking a route on SW2 so why does he need the weight tracking option.

Course i could be wrong, wouldn't be the first time

Jon

[Peter Paluch]

Hi Jon,

The training is great, I am just unraveling the mysteries of EIGRP and dissecting the hard truth from layers of Cisco marketing hype around that protocol...

Actually, yes, you may be right - I am not sure about the OP's goal myself. If he wants to perform tracking on SW2 then the object 1 is not defined. On the other hand, if no tracking is to be performed there then the tracking command itself is superfluous, just as you indicated.

Thanks!

Best regards,

Peter