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3750 stacking and spanning-tree

vibrisnet
Level 1
Level 1

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

3 Replies 3

Leo Laohoo
Hall of Fame
Hall of Fame

it's the stackwise.  It's blocked to prevent loops.

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

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