3750 stacking and spanning-tree
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05-16-2011 08:50 AM - edited 03-06-2019 05:04 PM
Hi all,
On a stack of 5 3750, if i look at spanning-tree, i get :
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi2/0/21 Desg FWD 19 128.75 P2p Edge
St2 Back BLK 4 128.873 P2p
Gi3/0/11 Desg FWD 4 128.119 P2p Edge
Gi5/0/1 Desg FWD 4 128.217 P2p
Gi5/0/2 Desg FWD 4 128.218 P2p
Gi5/0/3 Desg FWD 4 128.219 P2p
Gi5/0/4 Desg FWD 4 128.220 P2p
Gi5/0/5 Desg FWD 4 128.221 P2p
- What is St2 interface ? Stackwise ?
- Why is it blocked ?
Thanks for your help.
Eric
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05-16-2011 03:31 PM
it's the stackwise. It's blocked to prevent loops.
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05-17-2011 01:06 AM
Thanks for the answer. That's what i guess, but what is strange to me, is that the stack is blocked rather than another link !
It seems that it appears only when uplinkfast is setup.
Eric
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05-17-2011 02:39 AM
This has nothing to do with (Uplink fast) PVST+ Feature.
The Stack port in used to Synch STP information across all Member of Stack Switches. You have another Stack port that is in Forwarding state, thats why the 2nd stack is blocked (Redundant).
The Spanning-tree (Uplink fast) feature is primarily used to spedd up the convergence of unifdirectional link failure at the Access layer , Normally STP takes 30 sec to recover from a unidirectional link failure with normal STP timer, having this command allowing the Access Switch to IMMEDIATELY Switch the path incase of a failure, So in the End, this has nothing to do with the Symptoms you are seeing.
HTH
Mohamed
