is GLBP (gateway load balancing) the broken promise????

Unanswered Question
Mar 12th, 2007

Can someone tell me if having the same VLAN that spans multiple switches and connects to dual distributions really works with GLBP???

I have two interconnected distribution switches (Distribution-A and Distribution-B) that have multiple access layer switches (access-A & access-B) connected to each distribution switch. Each access layer switch has the same VLAN (Vlan2) across both switches. .

Please take a look at this PDF:

On page 40, figure 41 - with VLAN 2 present on two access layer switches, and both switches are uplinked to the two distribution switches.

In this configuration, wouldn't one of the uplinks on Distribution switch B be blocked not only on the link that connects to distribution-A and also to one of the access layer switches.

I don't see how GLBP can work if you have dual distribution switches connected to multiple access layer switches that span Vlans. This document seems wrong

Can someone correct me if I?m wrong.

I have this problem too.
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attrgautam Mon, 03/12/2007 - 21:06

No I think the diagram is right if the distribution switches connect to 2 different Layer 3 routers which dont run STP between them. Then there is a loop free path between the access/distribution and core. I am sure it has to be diff Core routers as single core routers wudnt help with GLBP anyways.

Let me know if i clarified.

I'm not so sure - The text seems very clear that the STP is running on the core/distribution switches hende the migration of the STP block from the Access switch links (to the right-hand distribution switch) to the link between the two distribution switches. This still leaves the STP loop from A1 to D1 to A2 to D2 back.

I would agree that, if it is following best practice then the VLAN2 on A1 is not the same IP subnet as VLAN2 on A2 and that the D1 and D2 switches would be running as IP routed interfaces. The point from the original question is that this is not clear in the text and not clear in the diagram?

Of course, I could have misunderstood the original question, the first answer and/or the diagram.

olavarrm Tue, 03/13/2007 - 06:39

this issue simple. If you have a multilayer switch in the distribution layers and all your access layer switches are layer 2 only and you have a vlan (vlan 2 for example) that is configures on multiple access layers switches. You can't run GLBP. The diagram is wrong.

You can also look at this doc from networks 2005 cdccont_0900aecd804ab67d.pdf. it shows the same thing but it's impossible.

Any insight?

Darren Ramsey Tue, 03/13/2007 - 20:49

GLBP will work in either situation. Vlan per switch/closet is optimal as there is no STP blocking and both uplinks will be utilized. If you are spanning vlans across switches/closets, then GLBP will take a suboptimal path. See attachment for examples of both. To summarize I'd recommend GLBP when there are no loops, and HSRP when spanning vlans across switches.



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