VLANs and PVST+ limitations

Answered Question
Aug 8th, 2008

I have stepped into a network that wasn't designed by me and due to my lack of familiarity, I'm concerned about a couple items. The network consists of 3750 12Ss.

1. Some 12Ss are running PVST+ and others are running R-PVST. What sort of impact will this cause?

2. I know that we can only have 128 spanning tree instances on a switch. If we are running PVST+, does this limit us to 128 VLANS total on a switch or is there a way to combine several VLANS into a single instance?

Thanks in advance.

~Tia

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Correct Answer by Jon Marshall about 8 years 5 months ago

1) Nothing disastrous but you may find that you do not get the benefit of RSTP, for example if you have 4 switches, 3 run RSTP and 1 PVST+ then

a) if a vlan is only on the 3 RSTP switches you will get the benefit of RSTP for that vlan.

b) if the vlan is on any of the 3 RSTP switches AND the PVST+ switch then the STP for that vlan will revert to PVST+ timers.

2) Have a look at MST which allows you groups vlans into instances

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml

Jon

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Correct Answer
Jon Marshall Fri, 08/08/2008 - 07:43

1) Nothing disastrous but you may find that you do not get the benefit of RSTP, for example if you have 4 switches, 3 run RSTP and 1 PVST+ then

a) if a vlan is only on the 3 RSTP switches you will get the benefit of RSTP for that vlan.

b) if the vlan is on any of the 3 RSTP switches AND the PVST+ switch then the STP for that vlan will revert to PVST+ timers.

2) Have a look at MST which allows you groups vlans into instances

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml

Jon

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