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Spanning Tree


I have 6506 as a core and 3750E as an access layer switches, they are connected with 10G and 1 G uplink. The following is the configuration.

spanning-tree mode pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

spanning-tree uplinkfast

spanning-tree backbonefast

when i issue the command show spanning-tree, the problem is with the cost, it shows cost starting with 3002, i have read that 10G link cost is 2 and 1G link cost as 4. The output of show spanning tree. is as follows.


Spanning tree enabled protocol ieee

Root ID Priority 49153

Address 0017.0f4a.69c0

This bridge is the root

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

Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)

Address 0017.0f4a.69c0

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

Aging Time 480

Uplinkfast enabled

Interface Role Sts Cost Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Gi1/47 Desg FWD 3004 128.47 P2p

Gi1/48 Desg FWD 3004 128.48 P2p

Te4/5 Desg FWD 3002 128.389 P2p

Te4/6 Desg FWD 3002 128.390 P2p

Po1 Desg FWD 3001 128.1665 P2p

Po2 Desg FWD 3001 128.1666 P2p

can you explain me the why the cost is 3002 on Ten gig link.


Re: Spanning Tree

The spanning tree pathcost is probably set to long (it should be).

Hope that helps.

Re: Spanning Tree

You have uplink fast enabled which automatically bumps the cost up to avoid a spanning-tree loop if recalculations happen. Your bridge priority will also be set to 49152 on that switch, with your extended-id added to it. (Extended-id is the VLAN number.)


HTH, John *** Please rate all useful posts ***
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