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spanning-tree gateway load balancing (STP & GLBP)

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:

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/cdccont_0900aecd801a8a2d.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.

2 REPLIES
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Re: spanning-tree gateway load balancing (STP & GLBP)

STP can be used for L2 load balancing which acn be done using spanning-tree and multiple-vlans, and with etherchannels. If you have 2 uplinks, 1 vlan would be fwd in 1 link, and 2nd vlan would be fwd in the 2nd link.

Following link may help you

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml#intro_vlan

GLBP, used at layer 3, Load balancing is achieved with up to 4 gateways on the same subnet , sharing the same IP but with different virtual macs.

Following link may help you

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hiap_c/ch20/haipglbp.htm

Re: spanning-tree gateway load balancing (STP & GLBP)

You are correct. I believe the diagram is in error. Access switch A should have a different vlan than Access switch B, and there should be a L3 connection between the two distribution boxes. GLBP works best when there is only one subnet per switch/closet (no vlan spanning). HSRP makes more sense when spanning vlans across multiple switches and remember to match the STP root with the HSRP active gateway.

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