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Nexus5K to Catalyst connectivity

(Sorry for the horrible drawing below) I'm under the impression that I  should be able to connect in the fashion below with VPC's made on the  N5K side and a regular etherchannel configured on the catalyst side.   When I mocked it up quickly in the lab I found that the Po on the Cat  side went active but one member went into the "suspended" state.  On the  N5K side the channel went with one member in the (H)ot Standby state.   Should I be able to get both members of this Po into a "P" state?

Cat4900  ----------  Cat4900

   |  vpc1\               / vpc2 |

   |          \           /          |

   |             \      /            |

   |               \  /              |

   |              /  \               |

   |          /         \            |

   |       /              \          |

N5K-1 ----vPC------ N5K-2


Re: Nexus5K to Catalyst connectivity


Whenever vPC's are involved in a topology, I always fall back to my trusty friend mspaint

If I understand your topology:

  - 4900-1 connected via say Po1 to N5K1 and N5K2 via vPC.

  - 4900-2 connected via say Po2 to N5K1 and N5K2 via vPC.

  - A Cross link between the 4900's

  - A vPC peer-link between the two N5K's

That should indeed work fine, just a couple of points:

-Make sure the 4900 sees his link to two N5K as a single port-channel.

-Make sure the two N5K see their links to a single 4900 as a single port-channel.

-Make sure the links to both 4900's are not in the same port-channel from an N5K perspective.

-Make sure the configuration on both N5K's match.

-The only other concern you have is spanning-tree, depending where your root is you may get a slightly unexpected topology, as we can't block on the N5K peer-link, and you have the cross link between the 4900's, so even one of the 4900s was the root you would likely block on this cross link.

Without any outputs its tough to give you an actual explanation, however the most common reasons would be:

-Links configured within a single port-channel on the N5K side connect to two different devices (different LACP AGG IDs)

-There is some config mismatch between the two N5K that force the vPC to be suspended (generally this will show as both links down rather than one).

The former can be checked with show cdp and show run interface, the latter can be checked by using "show vpc consistency-parameters global" and "show vpc consistency-parameters interface port-channel ".

Hope this helps


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