how to find the root bridge, STP/RSTP

Unanswered Question
Jan 28th, 2010

... assuming you have the mac address of the root bridge, but not its IP or hostname AND you can't console/telnet/ssh into any switch except one, which is not the root.

hmmph.

--

I have this problem too.
1 vote
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
Loading.
Ganesh Hariharan Thu, 01/28/2010 - 22:35

... assuming you have the mac address of the root bridge, but not its IP or hostname AND you can't console/telnet/ssh into any switch except one, which is not the root.

hmmph.

--

Hi,

If you want to know the details like which switch is root bridge and what stp instance running the switch and what about the states of ports then issue a command in switch as show spanning-tree

With the above command following information can be find like which switch is root bridge for which vlan in the below case the output says that this switch is roote bridge for vlan1and vlan 40 with stp timers details for each stp instance.

by default in cisco switches PVST runs and if RSPT is running the switch it will also comes in the output of the switch while you issue show sppaning-tree in switch.

Switch#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     001e.f6d6.e400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     001e.f6d6.e400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Desg FWD 3         128.56   P2p


VLAN0040
  Spanning tree enabled protocol ieee
  Root ID    Priority    32808
             Address     001e.f6d6.e400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32808  (priority 32768 sys-id-ext 40)
             Address     001e.f6d6.e400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Hope that clear out your query !!

If helpful do rate the valauble post

Ganesh.H

just do 2 steps:

1. use the command show spaning-tree vlan 1

the output look like this

root Id............priority     45468
address    ds5f.dd5fd.xx45.xxss
cost          12
port           3(fastethernet0/3)     -->     note this port\interface for the next step
hello time xxxxxxxxxxx

bridge Id........priority      56446
address     xxx.xxxx.xxxx.xxx
hello time xxxxxxxxxxx

2. use the command show cdp neighbors

check the output for the port that you're interested in from step 1's output



http://ITDualism.wordpress.com

unixdayjob Tue, 02/02/2010 - 10:40

I think I have 'forgotten' about how useful the CDP commands are.

But let's say the root bridge is not DIRECTLY connected to the port that you easily found by typing show spanning-tree.  Let's say the root is actually one or more switches away.  We can easily know which port on OUR switch goes to the root (the "root port"), but that port isn't necessarily directly connected to the root bridge -- it may be one or more switches away!  Right?

I did think of arp tables and mac-address-table

--

johnnylingo Fri, 01/29/2010 - 21:10

If you know the MAC address of the root, why not just "show mac-address-table "?

Actions

This Discussion