11-09-2008 10:20 PM - edited 03-04-2019 12:15 AM
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.
Solved! Go to Solution.
11-09-2008 11:59 PM
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
11-09-2008 11:59 PM
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
11-10-2008 12:23 AM
thanks for your reply , can you explain a bit why the priority is 49152, as i know that lower is better.
11-10-2008 12:31 AM
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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