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Tuesday, August 3, 2010

Question dealing with VLAN-STP

The command on Switch 1 is Primary root bridge VLAN 10 and Secondary root bridge 1001.

But now problem is both VLAN is show "This bridge is the root" but not showing the primary or secondary.

We know we have to concentrate on BID and RID value so escape this, how can we see which is primary and secondary by another method.
!
spanning-tree vlan 1001 priority 28672
spanning-tree vlan 10 priority 4096
!
SW1#sh spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000a.4152.b880
Cost 19
Port 48 (FastEthernet0/48)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.f41a.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/48 Root FWD 19 128.48 P2p

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address 000a.f41a.5180
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
Address 000a.f41a.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/48 Desg FWD 19 128.48 P2p

VLAN1001
Spanning tree enabled protocol ieee
Root ID Priority 29673
Address 000a.f41a.5180
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 29673 (priority 28672 sys-id-ext 1001)
Address 000a.f41a.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/48 Desg FWD 19 128.48 P2p

From a Cisco switch If you want to have a summarized view of who is root for each vlan => show spanning-tree root will give a clear view of the Root bridge ID per vlan.

About secondary root, there is no way to query or listen to the network to find another potential secondary root simply because there are no advertisements or elections of secondary root bridges (only for the root).

==> Indeed in the BPDU configuration message content there is a "Root Bridge ID" field (which is exchanged and known by every switch participating in the STP instances) which identifies who is the root for each vlan, but there is no 'secondary root bridge ID' or any relative field.

As you may know for the command : " spanning-tree vlan vlan-id root {primary | secondary} [diameter diameter] " is a macro command that executes several other commands.

=> This command logic applies when the configuration command is executed; it does not dynamically change the priority if another switch later advertises a better priority.

'show spanning-tree root' you should get something like this ===>

Switch#sh spanning-tree root

Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- --------- ----- --- --- ------------
VLAN0001 4097 000e.8338.0100 0 2 20 15
VLAN0019 4115 000e.8338.0100 0 2 20 15
VLAN0020 4116 000e.8338.0100 0 2 20 15
VLAN0021 4117 000e.8338.0100 0 2 20 15
VLAN0022 4118 000e.8338.0100 0 2 20 15
VLAN0023 4119 000e.8338.0100 0 2 20 15
VLAN0254 33022 0008.21b2.b300 23 2 20 15 Gi0/12
VLAN0994 33762 0008.21b2.b300 23 2 20 15 Gi0/12

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