VSS Pair to VSS Pair MEC --- IP communication not working

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May 26th, 2010
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Hello Experts,

I am Stuck

VSS is up on two different pairs of 6509.One pair is Core & another pair is Access.
I have configured Layer 2 EtherChannel between these VSS's(VSS Pair to VSS Pair MEC). VTP information is passing through between VSS Pair to VSS Pair MEC but I am unable to pass layer 3 communication(simple ICMP). In the beginning, this Port-Channel was getting blocked at Core VSS side by STP, that is the strangest thing I have ever seen.  After disabling STP on this port-channel, trunk came up &  VTP started passing information successfully.

If I do a ping & debug ip packets detial I can see encapsulation failed instead of routed via FIB .

Sh ip arp : shows incomplete Mac-Address

Arp operation is failing even they are directly connected...

And I am using SUP720 Gig UTP ports for this MEC.

Performed so many known and unknown trouble shooting steps with no luck. Please clarify, if something being missed.

Your immediate response & help will be highly appreciated.

Thanks & Regards

Mudasir Abbas

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Reza Sharifi Wed, 05/26/2010 - 13:14
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Hello Mudasir,

Since you are running 2 pairs of VSS, all the links (4) between the VSS pairs must be on the same Portchannel.  Also are you going to have your servers/workstations duly connected to the VSS pair?  If you are attaching you workstations with only one link to the access layer VSS, it really does not provide any redundancy for you.



Mudasir Abbas Wed, 05/26/2010 - 13:30
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Thanks Reza for your response.

For now I am only using two links between VSS pairs. It might be problem ? by the way they are on the same port channel offcourse.

Yes, I have planned to directly connect all servers on the access VSS pair. And I'll team one server with both 6509 chassis ( Access VSS 6509 Pair) by installing two NIC card on the server.?

Do you have any idea why I am unable to pass ip communication between these VSS pairs?

Thank you very much for kind and prompt response.

Reza Sharifi Wed, 05/26/2010 - 13:37
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Hi Mudasir,

Ok, that is good that you are connecting your servers using 2 links utilizing Etherchannel, as that is whole propose of VSS. This way you have NIC redundancy and also switch redundancy.

As for 2 links versus 4, I would recommend having 4 links (fully meshed) and putting them all in the same Portchannel and try again.

I have never done it with 2 links, always with 4.  Also the link in between the switches is layer-2 only right?



Mudasir Abbas Wed, 05/26/2010 - 13:52
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Thanks again Reza

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Yeah offcourse, I will have 4 links this is only for setting up.

Ok, I'll try with 4 links. Yeah, port-channel between VSS pairs is configured as layer 2.


pzpgd1mlf Fri, 06/04/2010 - 18:07
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Did you get it working with 4 links now? I am planning to setup two VSS as well with 4 links (in two port-channels - 2xPo1, 2xPo2) so I can separate two VRF's (private/public routing table), but now I am not sure if two links will work. I did not find any requirement about having all links in the same port-channel. Please, share your experience.

Thank you,


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