Comments on: Spanning Tree Root Bridge http://www.xpresslearn.com/cisco/switching/spanning-tree-root-bridge Solutions to your Networking and Security questions Fri, 03 Sep 2010 09:24:29 +0000 hourly 1 http://wordpress.org/?v=3.0.1 By: Scott Pilkinton http://www.xpresslearn.com/cisco/switching/spanning-tree-root-bridge/comment-page-1#comment-510 Scott Pilkinton Wed, 17 Sep 2008 21:59:04 +0000 http://www.xpresslearn.com/?p=113#comment-510 All Cisco Catalyst switches are assigned a set of MAC addresses that can be used for spanning tree and other purposes. On a Cisco 6500 switch, you can issue the command: cat6500#show catalyst6000 all chassis MAC addresses: 1024 addresses from 00d0.634c.cc00 to 00d0.634c.cfff 00:d0:63 is of course registered to Cisco in the OUI assignments. So let's say I have one access switch and one core switch for a simple example on tracking down a root bridge: On the access switch: switch#show spanning-tree vlan 1 Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol Bridge Identifier has priority 32768, address 0005.dd45.1c00 Configured hello time 2, max age 20, forward delay 15 Current root has priority 8193, address 00d0.634c.cc00 Root port is 67, cost of root path is 4 Topology change flag not set, detected flag not set, changes 14312 Times: hold 1, topology change 35, notification 2 hello 2, max age 20, forward delay 15 Timers: hello 0, topology change 0, notification 0 Interface Gi0/1 (port 67) in Spanning tree 1 is FORWARDING Port path cost 4, Port priority 128 Designated root has priority 8193, address 00d0.634c.cc00 Designated bridge has priority 8193, address 00d0.634c.cc00 Designated port is 129, path cost 0 Timers: message age 2, forward delay 0, hold 0 BPDU: sent 819, received 35042479 I should then look to see what switch is connected to the port in FORWARDING state (in this example it is the only port connected). So connected to interface Gi0/1 is a switch named rootswitch. When I access rootswitch and issue the same command we issued on the access switch: rootswitch#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 8193 Address 00d0.634c.cc00 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 8193 (priority 8192 sys-id-ext 1) Address 00d0.634c.cc00 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 I can tell this is the root bridge because the bridge ID matches and then notice a couple of lines below the bridge IP the line: This bridge is the root Bingo! All Cisco Catalyst switches are assigned a set of MAC addresses that can be used for spanning tree and other purposes. On a Cisco 6500 switch, you can issue the command:

cat6500#show catalyst6000 all

chassis MAC addresses: 1024 addresses from 00d0.634c.cc00 to 00d0.634c.cfff

00:d0:63 is of course registered to Cisco in the OUI assignments.

So let’s say I have one access switch and one core switch for a simple example on tracking down a root bridge:

On the access switch:

switch#show spanning-tree vlan 1

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0005.dd45.1c00
Configured hello time 2, max age 20, forward delay 15
Current root has priority 8193, address 00d0.634c.cc00
Root port is 67, cost of root path is 4
Topology change flag not set, detected flag not set, changes 14312
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0

Interface Gi0/1 (port 67) in Spanning tree 1 is FORWARDING
Port path cost 4, Port priority 128
Designated root has priority 8193, address 00d0.634c.cc00
Designated bridge has priority 8193, address 00d0.634c.cc00
Designated port is 129, path cost 0
Timers: message age 2, forward delay 0, hold 0
BPDU: sent 819, received 35042479

I should then look to see what switch is connected to the port in FORWARDING state (in this example it is the only port connected). So connected to interface Gi0/1 is a switch named rootswitch.

When I access rootswitch and issue the same command we issued on the access switch:

rootswitch#show spanning-tree vlan 1

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 8193
Address 00d0.634c.cc00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address 00d0.634c.cc00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

I can tell this is the root bridge because the bridge ID matches and then notice a couple of lines below the bridge IP the line:

This bridge is the root

Bingo!

]]> By: Juan Alberto http://www.xpresslearn.com/cisco/switching/spanning-tree-root-bridge/comment-page-1#comment-509 Juan Alberto Wed, 17 Sep 2008 10:43:44 +0000 http://www.xpresslearn.com/?p=113#comment-509 Hello Scott! Very usefull article and good explanation. The best thing are the examples. I have a doubt. I have searched the MAC address displayed in Address field of the Root ID information, but I don't find it in my network. Is it the physical MAC address? It is not the management switch address. Is it a virtual MAC address? If so, how is it calculated? Thank you very much in advance!! Hello Scott!

Very usefull article and good explanation.
The best thing are the examples.
I have a doubt. I have searched the MAC address displayed in Address field of the Root ID information, but I don’t find it in my network. Is it the physical MAC address? It is not the management switch address. Is it a virtual MAC address? If so, how is it calculated?

Thank you very much in advance!!

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