[c-nsp] configuring spanning-tree to block on the backplane

Jason Lixfeld jason at lixfeld.ca
Sat Dec 27 12:58:22 EST 2008

Here's the scenario:

server 1 has a trunk to switch 1.  One active vlan on the trunk, 15.
server 2 has a trunk to switch 1.  One active vlan on the trunk, 15.
switch 2 has 2 trunks to switch 1.  One active vlan on the trunks, 15.

Spanning-tree is setup as PVST on switch 1 and switch 2.  Spanning- 
tree for vlan 15 blocks one of the ports on switch 2, which is  
expected.  What I need to do is change the path from server 1 to  
server 2 so it goes via switch 2, not directly through switch 1, but  
I'm not sure if it's possible to block a path between two ports on the  
same switch in order to do what I want.  I've tried variations of  
disabling spanning-tree on vlan 15 on switch 1 and/or fudging the vlan  
port costs on switch 1 and/or switch 2, but any way I cut the cake,  
there is always a physical port being blocked between switch 1 and  
switch 2.  I can always just wire the servers directly to switch 2,  
but if I can find a way to do it remotely by fudging the tree, it'd  
save me a trip.		

Incase it matters, switch 1 is a 3550 running 12.2(25)SEE1.  Switch 2  
is a 6509/SUP720 running 12.2(33)SXH3a.  The 6509 connects to the 3550  
via two ports on a WS-X6148A-GE-TX.

Thanks in advance.

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