vPC Domain ID Modification on an Active vPC Domain

Introduction

This document describes a problem encountered when the same virtual Port Channel (vPC) domain identifier is used for more than one vPC pair of switches. This situation causes problems when those vPC domains are connected with a double-sided vPC port-channel.

Problem

The vPC domain ID should be unique in the contiguous Layer 2 network. Problems arise when you use the same vPC domain ID for more than one vPC pair of switches.

A unique vPC domain ID is required because the domain ID is used in order to generate the system ID for many processes, such as Link Aggregation Control Protocol (LACP). Refer to the Configuring vPCs section of the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x for more information.

The vPC peer devices use the vPC domain ID that you configure in order to automatically assign a unique vPC system MAC address. Each vPC domain has a unique MAC address that is used as a unique identifier for the specific vPC-related operations. However, the devices use the vPC system MAC addresses only for link-scope operations, such as LACP. Therefore, Cisco recommends that you create each vPC domain within the contiguous Layer 2 network with a unique domain ID.

Solution

The best way to change the vPC domain ID is to configure a new vpc domain <id> command on both vPC peers. This solution is superior to the removal of the whole vPC configuration and the reapplication of it with the new vPC domain ID.

If you apply a new vPC domain command, it does not remove the current vPC configuration (peer-link, keepalive, vPC port-channels configuration). However, this change does cause the vPC interfaces to flap. The switch displays a warning message about the vPC port-channels that have flapped.

Here is an example:

bdc-n5548-3# show run vpc

!Command: show running-config vpc
!Time: Sat Dec  1 08:37:03 2012

>version 5.1(3)N2(1a)
feature vpc
logging level vpc 6

vpc domain 1
  role priority 10
  system-priority 1
  peer-keepalive destination 10.48.56.7 source 10.48.56.9
  peer-config-check-bypass
  peer-gateway
  auto-recovery

interface port-channel1
  vpc peer-link

interface port-channel24
  vpc 24

bdc-n5548-3# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
bdc-n5548-3(config)# vpc domain 10     <<<<<<Changing VPC domain here>>>>>>>>>>
Changing domain id will flap peer-link and vPCs. Continue (yes/no)? [no] yes
Note:
--------:: Re-init of peer-link and vPCs started  ::--------
2012 Dec  1 08:37:12 bdc-n5548-3 %NOHMS-2-NOHMS_ENV_FEX_OFFLINE: FEX-120 Off-line
(Serial Number FOX1409A8Y4)
2012 Dec  1 08:37:12 bdc-n5548-3 %NOHMS-2-NOHMS_ENV_FEX_OFFLINE: FEX-130 Off-line
 Serial Number SSI15430A3E)
2012 Dec  1 08:37:12 bdc-n5548-3 %PFMA-2-FEX_STATUS: Fex 120 is offline
2012 Dec  1 08:37:12 bdc-n5548-3 %PFMA-2-FEX_STATUS: Fex 130 is offline
bdc-n5548-3(config-vpc-domain)# show run vpc

!Command: show running-config vpc
!Time: Sat Dec  1 08:37:15 2012

version 5.1(3)N2(1a)
feature vpc
logging level vpc 6

vpc domain 10
  role priority 10
  system-priority 1
  peer-keepalive destination 10.48.56.7 source 10.48.56.9
  peer-config-check-bypass
  peer-gateway
  auto-recovery

interface port-channel1
  vpc peer-link

interface port-channel24
  vpc 24

bdc-n5548-3(config-vpc-domain)#

Once the domain ID is changed, the double-sided vPC can then be configured.