Thursday, May 17, 2012

Two missing but vital Cisco Nexus vPC commands

So you come across a pair of Nexus 5548 switches and dive right into setting up a Virtual PortChannel following the configuration guides.  You'll wind up with something like this:

Things go swimmingly!  That is until you want to upgrade or have to power cycle one of the switches.  Your server can't get anywhere nor be reached for quite a few seconds.  Odd, you think.  Surely frames can happily flow along the remaining active link.  Isn't that what vPC is for?  As one switch comes back online, you then get ANOTHER period of lost connection to your server.

The following two commands come to the rescue.

  1. auto-recovery  
  2. delay restore [sec]

The first allows a switch to either takeover as primary or re-enable a vpc during loss of keepalive.  A great description of the history behind this is posted by Chris Marget.

The second helps prevent the vpc disruption when a vpc member switch comes back online.  The delay gives time to join the vpc domain and learn information before trying to forward frames.

conf t
  vpc domain 1
    delay restore 120

Have at it!  Some great Cisco operations guides are here and here.

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