How to locate the root switch of the spanning tree?

Unanswered Question
Jan 28th, 2008

I have a switch that shows the following 'show spanning-tree' output:


Spanning tree enabled protocol rstp

Root ID Priority 24588

Address 0012.d94b.fd00

When I use "show mac-address-table | inc 0012.d94b.fd00", 0012.d94b.fd00 is not found.

How do I trace which is the root switch?

In another environment, on the root switch itself, the "show spanning-tree" output displays :


Spanning tree enabled protocol rstp

Root ID Priority 24433

Address 0012.d94c.fc10

This bridge is the root

When I use the command "show mac-address-table | inc 0012.d94c.fc10", 0012.d94c.fc10 is not found too. Why is that so?

Thanks !

I have this problem too.
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shrikar.dange Mon, 01/28/2008 - 02:18


The catalyst switches typically have a pool of 1024 MAC addresses.This pool acts as the MAC address component of BIDs for vlan spanning trees (PVST +).The number of MAC addresses that are availabe depends on swich model.All switches do not support 1024 mac address.

These are the virtual MAC addresses you will not find these mac addresses in any mac table.These addresses are used for STP calculations and some redundancy protocols such as HSRP VRRP etc.



shri :)

Jon Marshall Mon, 01/28/2008 - 02:20


You probably won't see the mac-address. In your output you need to use the interface eg.

DC-BBP-F00-SW1#sh spanning-tree vlan 2


Spanning tree enabled protocol rstp

Root ID Priority 8192

Address 000f.3573.c402

Cost 3006

Port 1 (GigabitEthernet1/1) <--- YOU need to check what is at the other end of this ie. "sh cdp neigh gi1/1 det"

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 49154 (priority 49152 sys-id-ext 2)

Address 000c.cef1.1c80

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

UplinkFast enabled but inactive in rapid-pvst mode

As above, you need to check which interface the root bridge can be reached on and then trace it down this way.


matus.petrulak Mon, 01/28/2008 - 02:30


the show spanning-tree show whether the switch is root or not. If you want to trace root switch, just log on to any switch and in "sh spanning-tree" you will see port with "Root" role. (Root port is only one) If you won't see Root port you are at root bridge.

Root port is the port with "shortest path" to the root. So try "show cdp neighbors" and you will see which port is connected to "Root" port. Log to that switch and repeat until you get to the root switch.




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