STP and Trunk

Unanswered Question
Nov 17th, 2007

If Sw1 and Sw2 Connected to Core1 and Core2 and there is a trunk link between the 2 cores.Sw1 has vlan 10 and sw2 has vlan 20.

If i allowed vlan 10 only on the trunk link between the 2 cores.

Does it mean that the STP for vlan 20 is disabled beacuse there is no closed loop now for vlan 20 ? Can anyone confirm that pls?

I have this problem too.
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Jon Marshall Sat, 11/17/2007 - 06:28


Assuming Sw1 and Sw2 are not connected to each other then STP will not run across the trunk link so you have in effect 2 separate vlan 20's, one on each switch.

STP will still run on the switches for vlan 20, it just won't be one instance of STP across all the switches.



welcomeccie Sat, 11/17/2007 - 06:32

the topolgy is the following


| |


and the same for SW2.

and the trunk allow for vlan 10 only?

so the STP will be for Vlan 10 only and Vlan 20 will not right?

glen.grant Sat, 11/17/2007 - 06:35

vlan 20 is not allowed unless you allow it . By default when the trunk is created all vlans are allowed so you have to manually prune off any unwanted vlans that you don't want to traverse that trunk link.

Jon Marshall Sat, 11/17/2007 - 06:41

Sorry, it's not clear from your diagram. I'm assuming that Sw1 has a link to Core1 and Core2 and the same for Sw2.

Assuming the connections are trunk links and you only allow vlan 10 across the trunk link between core1 and core2

One instance of vlan 10 STP will be run across all 4 switches

STP will still run for vlan 20 across the switches. There just won't be any loops so no ports will need to block.


welcomeccie Sat, 11/17/2007 - 06:46

YEs, Sw1 has a link to Core1 and Core2 and the same for Sw2.

So i think there will no STP instance for Vlan 20 through the trunk what do you think?

Jon Marshall Sat, 11/17/2007 - 06:49


You are correct STP for vlan 20 will not run across the trunk link between Core1 and Core2.



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