Basic running STP(PVST+) on switch

Unanswered Question
Sep 14th, 2009

If three switch connect each other like to triangle. One switch(A) is root, second switch(B) is root secondary, third switch(C) is not enabed STP. Second switch(B) will take BPDU from root switch(A) and send to second Switch(B). this BPDU is spanning-tree-(for-bridges)_00(01:80:c2:00:00:00)

When BB#2(B switch) takes this BPDU, How to do BB#2 operate? Drop or Accept?

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Giuseppe Larosa Tue, 09/15/2009 - 02:20

Hello Yeo,

in real world disabling STP is always a bad idea.

the device in the middle may allow BPDUs to flow or not.

if it allows for STP the topology becomes:

SWa --- link1 --- SWB

SWa--- link2 (via SWC)--- SWB

so depending on the ports used on SWA one port on SWB is elected root bridge and the other port (for example of link2) is placed in STP blocking state.

if SWC blocks the STP BPDUs (and this is possible) you have a loop.

Hope to help


rducombl Tue, 09/15/2009 - 04:52


humm this setup is kinda looking for trouble in a real network. The problem here is that your link between BB2 and L2 switch is assymetric.

- on L2 switch side it is access port --> it will send/forward BPDU untagged likely to IEEE MAC address

- on BB2 Switch it is trunk port running PVST+ --> it is expecting tagged PVST+ cisco BPDU in vlan 10 and 100. If it received an untagged IEEE BPDU it will assign it to its native vlan which is 1 by default. and then indeed you have serious chance to get a loop.

Note that this will likely depends on how L2 switch behaves. Cisco Switches with spanning-tree disable, usually are still sending Spanning-tree BPDu to the cpu but they do not process them they simply send them unchanged to all port in the vlan. Other switches may behaves differently.

At a minimum I would avoid to have trunk to access port link.




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