Configuring a vss switch to 2 3750's stacked

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May 7th, 2009
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I have two distribution switches that have a VSL link between the two. I'm having an issue trying to connect these 2 switches to 2 3750 stacked switches. The way i have it currently setup is that I have one connection from vss 1 and one connection from vss 2 to each 3750. The whole thing is is configured as one port-channel, but it doesn't seem to be working properly. Is there a different way I should be doing this?

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Giuseppe Larosa Thu, 05/07/2009 - 11:02
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Hello Tito,

I think you may need to upgrade the C3750 in order for them to support PAGP extensions.


However, I see that it tells that multichassis etherchannel has no special requirements on the neighboring device.

The special requirements are for dual-active detection using PAGP on a MEC.


Are the two C3750 a single stack ? if so it should work.

Is the VSS up and running correctly ?

are the usual requisites satisfied like trunk protocol list of vlans permitted on trunk on both links and both sides ?

Hope to help


latintrpt Thu, 05/07/2009 - 11:10
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Well i was looking at Cisco's guide on 3750's and it says The PAgP protocol is not supported on cross- stack EtherChannels. Yes the 3750's are single stacked and the vss is working fine.

Reza Sharifi Sun, 06/21/2009 - 12:38
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Are you putting the right port number (on VSS site) in the port-channel?

If the port number for example was 1/1, after creating a VSS pair, it becomes 1/1/1

Can you post your VSS and port-chaneel configs?


Rahul Kachalia Sun, 06/21/2009 - 15:26
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If 3750 is in stackwise mode, then you have to use LACP as an link bundling protocol. PAgP is not supported.

For VSS Dual Active Detection, implement back-to-back cable between both virtual-switch node and enable dual active fast hello if there is no other ePAgP capable device connected to VSS except this 3750...



iyde Mon, 06/22/2009 - 04:20
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Yeah, Cisco has made a really ugly one here...

MEC requires LACP and VSS dual-active detection requires PaGP+...

Rahul Kachalia Mon, 06/22/2009 - 06:09
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MEC is an physical design. The software function over the MEC is identical to what you have it on a EtherChannel. MEC do not mandate to use LACP, it is an alternative to PAgP.

Link Bundling protocols and dual active detection/notification are two different function. It was easier to innovate and leverage existing PAgP protocol to do both function in same stack and keep it simple and transparent enough to the end user.

For dual ACTIVE detection mechanism in VSS, there is no unique requirement then you interconnect any other non-ePAgP capable device to VSS...




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