GLBP in a campus environment

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Apr 14th, 2008
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Would GLBP be any value to me in a campus environment, if my access switches and distribution swithes ran as L3 devices and ran EIGRP? I have 2 fiber links from my access to my distribution switches and want to load balance between them. The current configuration has one vlan going over one fiber uplink to switch A, while the other fiber uplink goes to switch B and is sitting there mostly idle. Plus, there's no redundancy in this configuration, either.




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Edison Ortiz Mon, 04/14/2008 - 08:32
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Your problem is that both uplinks are Layer2 and due to Spanning-Tree, one port will be in blocking mode in order to avoid a Layer2 loop.


If you want to use both links, you need to change the uplinks from Layer2 to Layer3 and run a routing protocol such as EIGRP.


GLBP plays no part on this configuration requirement.


HTH,


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DARYLE DIANIS Mon, 04/14/2008 - 11:33
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thanks, I understand the blocking problem with STP, but from reading about GLBP, it sounds like I can have my redundancy and load balancing without using VARIANCE and EIGRP. true? I would still need EIGRP, but GLBP would present only a single router to the layer 3 access switch. Am I missing something about how GLBP operates?

Edison Ortiz Mon, 04/14/2008 - 11:36
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GLBP operates similar to HSRP as a first-hop redundancy protocol. With GLBP, the devices needing redundancy will need a static route pointing to the VIP (L3 devices) or a default gateway pointing to the VIP (end devices such as printers and workstations).


For inter-switch L3 routed design, the correct approach is implementing a routing protocol between these links.


The GLBP configuration (if needed) can be applied on Vlans facing the user community.


HTH,


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Joseph W. Doherty Mon, 04/14/2008 - 18:11
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As Edison correctly notes, if both your access and distribution devices are routing, then GLBP offers no benefit. However, GLBP can assist with L2 load balancing, so for instance if your access was L2 and distribution L3, you could use GLBP to have traffic use both uplinks, for traffic flowing to the gateway, for the same subnet.


NB: Using GLBP might cause asymmetric path issues.


PS:

BTW: You might not need to use EIGRP's variance command if the paths have equal cost.

DARYLE DIANIS Tue, 04/15/2008 - 05:48
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thanks to all that posted replies, I see I have lot to learn about proper design, this is going to help a lot.



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