GLBP with backup link

Unanswered Question
Apr 19th, 2010

I have a site with three WAN connections and three routers. The first two are T-3’s and should be load balanced while they are in use. The third is a slow 3g connection that is only used to provide some basic data connectivity. I am implementing GLBP for the all three routers but how should I incorporate the third router in the GLBP group? The way I understand GLBP is that all three will become an AFV and send traffic which is not the desired result. I want the two primary connections to be load balanced; if one of the primary connections were to fail I still want all the traffic to go over the remaining T3. The only time traffic should go over the third link is when both routers have failed. If have the load sharing and redundancy working for the two primary routers but don’t know how to configure the third. Is this possible with GLBP?

I have this problem too.
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Giuseppe Larosa Mon, 04/19/2010 - 21:53

Hello Bill,

GLBP has two dimensions of "priority"

- priority: it is used to elect the AVG that is the device that answers ARP requests for the VIP address

- weighting: it is used to decide if an AVF should be used or not. This parameter can be influenced by tracking options and has a lower threshold (under which the router will resign its AVF role) and an upper threshold (over which AVF role is reenabled)

Taking in account this you can configure first two routers in the following way:

if the WAN interface  fails the weighting decrement is so big to go under the lower threshold and the router stops to be an AVF for the GLBP group.


1. enable

2. configure terminal

3. track object-number interface type number {line-protocol | ip routing}

4. exit

5. interface type number

6. glbp group weighting maximum [lower lower] [upper upper]

7. glbp group weighting track object-number [decrement value]

8. glbp group forwarder preempt [delay minimum seconds]

the third router can get an initial weighting very low like 1, using load balancing based on weights should ensure third is very little used when the other two are alive.

glbp group load-balancing weighted

this should provide a solution near to the desired one, given that default weights are 100 for the first routers and that can be incremented to 254.

Hope to help


Bill19795_2 Tue, 04/20/2010 - 05:02

I understand the workaround you have proposed and how to implement it but unfortunately it will still not work for a correct solution. Even using the third link 1% of the time is 1% too much for our desired outcome. Any other suggestions? Can I use VRRP and GLBP at the same time?

francisco_1 Tue, 04/20/2010 - 05:51

You could use weight by increasing teh weight on first and second router and low weight on the third router.  How hosts are assigned to each GLBP router can be altered by using on of 4 load balancing options such as weight. placing a weight will calculate the amount of load sharing that will occur through the MAC assignment.


Bill19795_2 Tue, 04/20/2010 - 10:27

Thanks. I understand the weighting and how it works unfortunately I don’t think it will meet my criteria. I don’t want ANY traffic to go over router 3 unless router 1 and 2 has failed or they can not use their serial link. I can accomplish load sharing and weighting to make this happen but I still can not get the third router to only load share traffic if the other AVF’s are not active.

Giuseppe Larosa Tue, 04/20/2010 - 23:26

Hello Bill,

>> The third is a slow 3g connection

And it may be expensive if it is not flat rate.

I suggest a design change:

R3 should be placed in a separate Vlan/IP subnet together with R1 and R2

R1 and R2 are the two routers members of the GLBP group in client Vlan.

R3 can use dialer watch to monitor specific routes on the primary link (the Vlan with R1 and R2) if both the primary WAN links fail R3 dialer watch trigger a DDR call using the 3G connection.

R1, R2, R3 can use a dynamic routing protocol on the common IP subnet.


In this way R3 is used only when both WAN primary links are down.

So yes GLBP alone is not enough to achieve this. The key point is to remove R3 from the client Vlan.

Vlan dot1q subinterfaces can be used to create the common subnet between R1, R2, R3.

Hope to help



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