How do I determine what mode of STP is running

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Feb 13th, 2009
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Most of my switches are running 12.2.25 or later. That version is capable of running Rapid Spanning Tree (802.1w). The show run says "spanning-tree mode pvst", and the show spanning-tree vlan 1 says "Spanning tree enabled protocol ieee". I have never changed the STP defaults. Am I running 802.1d or 802.1w? thanks. jc

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Jimmy,


>> If I attempt to move to RSTP on all my switches capable of running RSTP, what is the worst that can happen?


that the move will be not effective if most of the switches are still running PVST and so there is no real faster convergence.


So you have to evaluate how much is important for your network to improve STP convergence time.

If supporting VoIP the ability to have sub-second convergence can help.


Hope to help

Giuseppe


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Giuseppe Larosa Fri, 02/13/2009 - 11:38
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hello Jimmy,

you are running PVST+ that is based on 802.1D


to be able to run RPVST you need to change the stp mode on all devices to


spanning-tree mode rapid-pvst


see


http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/spantree.html#wp1078947


Hope to help

Giuseppe


jimmyc_2 Fri, 02/13/2009 - 12:44
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Hi Giuseppe,

I have mostly 4500s and 2960s, and they do seem to allow me to run the command "Spanning-tree mode rapid-pvst". Can I simply run the command on all the switches without any concerns? The cisco documentation is painfully unhelpful.

jimmyc

Giuseppe Larosa Fri, 02/13/2009 - 13:04
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Hello Jimmy,

the devices that don't support the command are not able to support rapid per vlan STP.


for example C4500 in release 12.2(25)SG has only support for normal PVST


while in 12.2(37)SG it is supported


http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/37sg/configuration/guides/spantree.html#wp1064592')">http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/37sg/configuration/guides/spantree.html#wp1064592


So I'm afraid that you should before upgrade the devices that can be upgraded in order to get the highest benefits.


even if the table 15-2 declares backward compatibility between Rapid PVST and PVST+.


see


http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_46_se/configuration/guide/swstp.html#wp1166519')">http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_46_se/configuration/guide/swstp.html#wp1166519


So there are IOS images that support 802.1W or also 802.1s (IEEE MST)


Be aware that there is also a pre-standard MSTP Cisco version out there.


Check your devices and plan IOS upgrades on them.


Hope to help

Giuseppe




jimmyc_2 Fri, 02/13/2009 - 13:25
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I looked at both links you provided, and they do nothing to clear things up. Cisco goes out of their way to make things complicated.


Regarding your advice to load the latest images before move to RSTP, I would rather not, since the downtime would far outweigh the gain.


If I attempt to move to RSTP on all my switches capable of running RSTP, what is the worst that can happen? thanks. jimmyc

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Giuseppe Larosa Fri, 02/13/2009 - 13:31
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Hello Jimmy,


>> If I attempt to move to RSTP on all my switches capable of running RSTP, what is the worst that can happen?


that the move will be not effective if most of the switches are still running PVST and so there is no real faster convergence.


So you have to evaluate how much is important for your network to improve STP convergence time.

If supporting VoIP the ability to have sub-second convergence can help.


Hope to help

Giuseppe


jimmyc_2 Mon, 02/16/2009 - 11:41
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Hi Guiseppe,

Many of former work-mates worked for Telespazio, as they were initial partners on Iridium. One of my friends still on the program is from Milan. Go Team Jueventus.

Jimmyc

Giuseppe Larosa Mon, 02/16/2009 - 13:38
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Hello Jimmy,

the world is small. :)


Juventus team is in a transition phase to a new major release just to use a Cisco language.


Best Regards

Giuseppe


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