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[ScreenOS] How is the virtual MAC address for a pair of Active/Passive firewalls derived?



Article ID: KB7435 KB Last Updated: 27 Dec 2017Version: 6.0
How is the virtual MAC address for a pair of Active/Passive firewalls derived?


  • NSRP Active Passive
  • VMAC
  • ARP table on router or switch

Symptoms & Errors: 

  • Duplicate MAC address seen when 2 NSRP Clusters with same Cluster ID and VSD-Group are attached to the same switch.
NOTE:  If running ScreenOS 6.1 or later, also refer to KB11150 - Virtual MAC (VMAC) address for HA pair when using nsrp-max-cluster and nsrp-max-vsd variables.

The virtual MAC address for the shared interface of an Active/Passive NSRP pair has the following format:  



w - cluster ID (id:1 -> 2, id:2 -> 4, id:3 -> 6, id:4 -> 8, id:5 -> a, id:6 -> c, id:7 -> e)
xy - interface number in hex
z - vsd group

For example, you can see the virtual MAC address for eth2/1 below (shown in the 'get int' output). 

eth2/1           Trust       0010.dbff.a070    -   D   0   Root

The cluster ID is 5 (which translates to a from the above formula). 
The interface # is 07 (shown in the 'get int eth2/1' output). 
The VSD group ID is 0.

ns5200(M)-> get config | inc nsrp
set nsrp cluster id 5
set nsrp vsd-group id 0 priority 100

ns5200(M)-> get int eth2/1
Interface ethernet2/1(VSI):
  number 7, if_info 229432, if_index 0, mode nat

Modification History:
2017-12-26: Article reviewed for accuracy. Added ScreenOS tag to the title. Article is correct and complete.

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