STP clarification

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Aug 21st, 2008
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I am experimenting STP in a lab and got confused by the output of the show spanningtree command on the second switch. I have the 2 switches connected via Gig interfaces and have VLANs on both switches configured. The output from SW1 is clear but not the one from SW2. In the output, it shows on VLAN1 and VLAN51 different MAC addresses on the Priority Root ID and Bridge ID and I am confused as why these two MAC addresses are different. The second thing I am confused about is VLAN39 which says “This is the root” where the other switch is the Root Bridge!!! Attached are the two outputs from both switches.

Thanks in advance,


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cowetacoit Thu, 08/21/2008 - 09:43
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vlan 39 is not on the first switch, onyl on your second switch. so there for it will be the root for the second switch.

Giuseppe Larosa Thu, 08/21/2008 - 09:48
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Hello Sadik,

your switches are using the extension-id

Bridge ID Priority 32807 (priority 32768 sys-id-ext 39)

Address 001d.456e.dd80

in Vlan X Priority = Base_Pri +VlanX#

the base priorities are usually in steps of 4096 to accomodate all the possible Vlan-id values as per 802.1Q standard.

this feature is also called MAC address reduction: instead of using a different MAC for each Vlan they use a single MAC and modify priority.

This is thought to help troubleshooting.

2) are you sure that vlan39 is permitted on the link between switches ?

is it a trunk permitting vlans 1 and 39 ?

I think gi0/4 is permitting vlan 1, 51 on sw1 not vlan39

Hope to help


skhirbash Thu, 08/21/2008 - 10:26
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Thanks for the prompt response. I am still not clear on why there two different MAC addresses on VLAN1 and VLAN51 on SW2!

Here is the trunking status:

sh int trunk

Port Mode Encapsulation Status Native vlan

Fa0/48 on 802.1q trunking 1

Gi0/1 on 802.1q trunking 1

Gi0/4 on 802.1q trunking 1

Port Vlans allowed on trunk

Fa0/48 1-4094

Gi0/1 1-4094

Gi0/4 1-4094

Port Vlans allowed and active in management domain

Fa0/48 1,39,51,56

Gi0/1 1,39,51,56

Gi0/4 1,39,51,56

Port Vlans in spanning tree forwarding state and not pruned

Fa0/48 1,39,51,56

Gi0/1 39

Gi0/4 1,39,51,56

Giuseppe Larosa Thu, 08/21/2008 - 10:47
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Hello Sai,

let's look at this sample from your attachment:


Spanning tree enabled protocol ieee

Root ID Priority 32819

Address 0015.c663.4f00

Cost 4

Port 28 (GigabitEthernet0/4)

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

Bridge ID Priority 32819 (priority 32768 sys-id-ext 51)

Address 001d.456e.dd80

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

Aging Time 300

The reason you see two MAC addresses is:

root bridge : Sw'1 MAC = 0015.c663.4f00

bridge id = local switch MAC = 001d.456e.dd80

this is true for vlans : 1, 51

for vlan 39 you have

root bridge = this bridge

Hope to help



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