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Spanning Tree

hi,

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.

VLAN0001

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.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Spanning Tree

Hello Waseem,

the cost 3002 comes from the uplinkfast feature in order to make unlike the usage of the access switch as a transit to the root bridge.

So the port cost of all ports is incremented of 3000 and the bridge priority is increased to 49152+ Vlan-ID. The latter is done in order to make unlike the election of the access layer switch in case of current root bridge failure.

By using uplinkfast the switch is considered a leaf that should never become root bridge and never used to reach the root bridge.

The second because in traditional spanning-tree it is safe to put in forwarding state the second uplink in case of failure of the root port only if no other switches are behind, downstream, the current switch.

Hope to help

Giuseppe

3 REPLIES
Hall of Fame Super Silver

Re: Spanning Tree

Hello Waseem,

the cost 3002 comes from the uplinkfast feature in order to make unlike the usage of the access switch as a transit to the root bridge.

So the port cost of all ports is incremented of 3000 and the bridge priority is increased to 49152+ Vlan-ID. The latter is done in order to make unlike the election of the access layer switch in case of current root bridge failure.

By using uplinkfast the switch is considered a leaf that should never become root bridge and never used to reach the root bridge.

The second because in traditional spanning-tree it is safe to put in forwarding state the second uplink in case of failure of the root port only if no other switches are behind, downstream, the current switch.

Hope to help

Giuseppe

New Member

Re: Spanning Tree

thanks for your reply , can you explain a bit why the priority is 49152, as i know that lower is better.

Hall of Fame Super Blue

Re: Spanning Tree

Waseem

The default STP priority on a switch without any modifcation is 32768. So by making the STP priority 49152 you are making it very unlikely for that switch to be elected as the root bridge.

Jon

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