How to Identify Root Port in STP

Answered Question
Apr 8th, 2006
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in my Cisco 6509 Switch , the Spanning Tree for every VLAN shows me a separate mac address for the Designated Root Port

e.x Designated root has priority 32768, address 000b.fc38.f81a


But I don't see any port on this switch that shows this mac address as a result of "Show interface" command


How do i trace, Who actually owns that mac address ??


Thanks

Sanjeev

Correct Answer by Francois Tallet about 11 years 4 months ago

That's correct, there is a range of mac address reserved in the backplane that the switch is using for its consecutive bridge IDs (it's on the backplane instead of the supervisor so that you can change your supervisor without changing your bridge IDs).

Before 802.1Q was out, Cisco's PVST only supported 1K vlans. We were at that time burning 1K mac addresses for each switch we were shipping just of these bridge IDs. When 1Q defined up to 4K vlans, Cisco had to support this and there was no way we would allocate 4K mac addresses for each piece of hw sold. That's where "extended system ID" or "mac address reduction" was developed: a single mac address is used for all the vlans bridge IDs, but 12 bits are stolen from the bridge priority in order to make this ID unique per vlan. The priority can then only be set by increment of 4096 (2^12). This feature is now enabled by default on some of our latest hardware, which only have 16 or 64 built-in mac addresses reserved. It means that those switches won't even be able to go back to the behavior you observed, i.e. one different mac adress per vlan. It's of course not a problem (and was in fact added to the IEEE standard).

Regards,

Francois

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dgahm Sat, 04/08/2006 - 14:30
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Sanjeev,

That is the Bridge ID mac address of the root switch. The root port is the best path to the root switch.

You can trace to the root by going to the switch that connects to the root port (sh cdp neigh), and show spantree (CAT OS). Continue until you get to the root, which will have a root cost of 0, and Bridge ID and Designated Root with the same mac.


In most networks the root, and usually a secondary, will be chosen by setting the Bridge Priority to a lower value than the default 32768. This provides better stability, and is helpful in troubleshooting SPT issues.


Please rate helpful posts.

rais Sat, 04/08/2006 - 15:35
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Don't depend only on 'show interface' for finding mac-addresses. Do a show mac-address-table | include or show cam to find out mac addresses. There are some mac-addresses that belong to backplane etc. of the switch. STP makes use of those macs as well.


Hope this helps.

Roberto Salazar Sat, 04/08/2006 - 20:03
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It may be easier to explain with an output.


The mac address next to "Designated Root" is the mac address of the root for vlan 1. As you can see it says port 1/0 which means this switch itself is the root, if it had been some other switch, the port specified is the port to the "root" and this is direct route to the "root" hence the term root port. Another indication that this switch is the root, is the mac address next to "Bridge ID MAC ADDR" is the same as the "Designated Root". I hope this made it clear.


cat6k (enable) sh spantree

VLAN 1

Spanning tree mode PVST+

Spanning tree type ieee

Spanning tree enabled


Designated Root 00-04-9b-xx-xx-00

Designated Root Priority 32768

Designated Root Cost 0

Designated Root Port 1/0

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


Bridge ID MAC ADDR 00-04-9b-xx-xx-00

Bridge ID Priority 32768


s.pabbi Sun, 04/09/2006 - 18:44
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dgahm's response really helps in doing a Trace on Root Switch and Bosalaza's example is very easy to understand.

But I would like to now answer my own question (i discovered later) 6509 uses a range of 1024 mac addresses as Bridge IDs for 1024 Vlans (Used in sequence),which can be viewed by "Show Catalyst" command

e.x Cisco_6509#sh catalyst6000

chassis MAC addresses: 1024 addresses from 000b.fc38.f800 to 000b.fc38.fbff


where Vlan 48 would be Hex equivalent of 30 so will have an ID of 000b.fc38.f830 and Vlan 49 would be 000b.fc38.f831


Thanks to all

Sanjeev

Correct Answer
Francois Tallet Mon, 04/10/2006 - 08:31
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That's correct, there is a range of mac address reserved in the backplane that the switch is using for its consecutive bridge IDs (it's on the backplane instead of the supervisor so that you can change your supervisor without changing your bridge IDs).

Before 802.1Q was out, Cisco's PVST only supported 1K vlans. We were at that time burning 1K mac addresses for each switch we were shipping just of these bridge IDs. When 1Q defined up to 4K vlans, Cisco had to support this and there was no way we would allocate 4K mac addresses for each piece of hw sold. That's where "extended system ID" or "mac address reduction" was developed: a single mac address is used for all the vlans bridge IDs, but 12 bits are stolen from the bridge priority in order to make this ID unique per vlan. The priority can then only be set by increment of 4096 (2^12). This feature is now enabled by default on some of our latest hardware, which only have 16 or 64 built-in mac addresses reserved. It means that those switches won't even be able to go back to the behavior you observed, i.e. one different mac adress per vlan. It's of course not a problem (and was in fact added to the IEEE standard).

Regards,

Francois

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