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Verifying the bridge ID


I'm studying Spanning Tree Protocol

If I type the command: S1#sh spanning-tree vlan 1  

I have the following output:

    Spanning tree enabled protocol ieee
    Root ID Priority 32769
             Address 0001.42A7.A603
             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 0001.42A7.A603 him
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.25         P2p
Gi1/2 Desg FWD 4 128.26         P2p

My questiones are:

     I see that Root ID Priority and Bridge ID Priority have the same value (32769), what does it mean? What could happen if the numbers were different?

    I know that the current switch is a Root Bridge but why I have two types of ID:  Root ID and Bridge ID?

     I see (priority 32768 sys-id-ext 1) where 1 is the current VLAN 1 and 32768 + 1 = 32769 but what does it mean?

     (.......sys-id-ext 1) is the 12-bit PVST+ field that is placed into the BPDU so it can carry multiple-VLAN information, is it correct?

 2) the last rows of the output show interfaces (Gi1/1, Gi1/2), but the type shown is P2p. I thought that P2p is for serial interface used in lised lines (WAN network) where the interface are seial (not shared link).


Hello Giacomo.

Hello Giacomo.

The Bridge ID priority will always show the priority of the switch for that VLAN. The Root ID priority is the root bridge has for the VLAN. If the numbers were different, you are most likely not in the root bridge. 

As for the Bridge ID Priority field and the sys-ext-id, this is used because every switch has an identity in spanning-tree. A bridge ID is seperated into 2 parts, the 2 Byte Bridge Priority field and 6 Bytes MAC address. However, since you will be running PVST, there is a chance that some vlans can have the same MAC address and the same Bridge Priority. Therefore, the sys-ext-id is added in the Bridge Priority to allow for Per-Vlan Spanning Tree to give the switch a unique Bridge ID. 

The last P2P/SHR status shows the type of link. The P2P means that the link is using full duplex and is also running the same version of spanning-tree. If the SHR was used, it would mean that you are running at half duplex. P2P doesn't have to be set only on Serial interfaces. 

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