Determine the Root Bridge in an MSTP-Fabric Connect Multi Homing Configuration

Identify the root bridge by determining where the Common and Internal Spanning Tree (CIST) regional root MAC address is learned for the STP-reserved I-SID. Check which MAC address has the same first five octets as the CIST regional root MAC address.

Procedure

To enter User EXEC mode, log on to the switch.
Determine the CIST regional root:

show spanning-tree mstp status
Determine which MAC address has the same first five octets as seen in the CIST regional root MAC address:

show i-sid mac-address-entry 16777003

Example

In the following example, bold text identifies the relevant information in the command output. In the output of the second command, the DEST-MAC shows 10:cd:ae:6e:d8:84, which is the system ID of the CIST regional root BEB, and the system name is BEB-1000.

Switch:1>show spanning-tree mstp status

==========================================================================================
                                  MSTP Status
==========================================================================================
------------------------------------------------------------------------------------------
Bridge Address                : b0:ad:aa:4d:b8:00
Cist Root                     : 80:00:10:cd:ae:6e:d8:00
Cist Regional Root            : 80:00:10:cd:ae:6e:d8:00
Cist Root Port                : fabric
Cist Root Cost                : 0
Cist Regional Root Cost       : 2000000
Cist Instance Vlan Mapped     : 1-1024
Cist Instance Vlan Mapped2k   : 1025-2048
Cist Instance Vlan Mapped3k   : 2049-3072
Cist Instance Vlan Mapped4k   : 3073-4050,4053-4059
Cist Max Age                  : 20 seconds
Cist Forward Delay            : 15 seconds

Switch:1>show i-sid mac-address-entry 16777003
===========================================================================================================================================
                                               I-SID Fdb Table
===========================================================================================================================================
I-SID     STATUS  MAC-ADDRESS        INTERFACE        TYPE       DEST-MAC           BVLAN DEST-SYSNAME           AREA-ROLE     AREA-NAME
-------------------------------------------------------------------------------------------------------------------------------------------
16777003  learned 10:cd:ae:6e:d8:82  Port-1/9        NON-LOCAL  10:cd:ae:6e:d8:84  4051  BEB-1000               HOME          area-20.0020                   
16777003  learned 10:cd:ae:db:a4:83  Port-1/40        NON-LOCAL  10:cd:ae:db:a4:84  4051  7208                   HOME          area-20.0020
16777003  learned b0:ad:aa:40:14:82  Port-1/40        NON-LOCAL  b0:ad:aa:40:14:84  4051  6222                   REMOTE        area-20.0020