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New Member

Help with STP (redundant connections, fail-over)

Hi guys,

Please take a look at the design at:

G5484: 1000BaseSX GBIC

G5483: 1000BaseT GBIC

1000T: fixed uplink 10/100/1000T ports

I know it's not the best design (trying to facilitate existing one) but I need to figure out the configuration of the STP so the whole cluster fails over to the blocked link (unblocks it and starts forwarding) if either the top 2950G or 4006 Pri Root switch dies.

Right now if I turn off connection to Pri Root (either by powering off top 2950G, or powering off 4006 Pri Root) the whole cluster just sits there and never enables forwarding on the bottom blocked link.

I know that if I used the GBic's to stack up the 2950 switches there is a possibility to enable the GigaStack Uplink Fast, which uses some protocol to inform all GBIC's about the path to root failure. It's not the case here since the 2950T-24's use 10/100/1000 for uplink ports and do not have the same capability.

I'm pretty much at loss here.

any ideas are appreciated...



Re: Help with STP (redundant connections, fail-over)

If you have multiple VLANs, then it sounds like maybe one or more of the VLANs is not being carried across one (or more) of the switch-to-switch links. Check your config to see that all your inter-switch links are VLAN trunks. Then check to see whether you're manually removing any of the VLANs from the VLAN trunk. Remember, Cisco does a Spanning Tree Per VLAN; so each VLAN has to have its own loop, otherwise it can get isolated from the Root Bridge when there's a link failure in the right place. (Or wrong place, depending on how you look at it.)

Also, if you're using 802.1Q encapsulation, make sure that the native VLAN on each end of a switch-to-switch link is the same.

How are you configuring your network so that it specifically blocks the link between your 4006 Backup Root Bridge and the bottom 2950G-24 in your picture? Without human intervention (for example, manually increasing STP costs on certain links, or using UplinkFast on a switch), STP would naturally want to block the link between the 4th and 5th 2950T-24, counting down from the top in your pic.

For what it's worth, the way your switches are chained together is creating a Network Diameter of 11. STP's Max Age and Forwarding Delay timer default values are based on a Network Diameter of 7. So you may also be having some reconvergence timing issues, too. If you are restricted to this topology, and all the other stuff checks out, try extending the Max Age and Forwarding Delay parameters on both your Root Bridge and Backup Root Bridge. (Configuring it on the Root Bridges, it will propagate to all the other switches.) Remember to do this for each and every VLAN.

Values to try: if your Hello Time interval is the default of 2 seconds, then MA=28sec and FD=21sec. Reconvergence will take place in 42 to 70 seconds, vs. STP default 30 to 50 seconds. If your HT=1sec, then try MA=24sec and FD=19sec, for a reconvergence of 38 to 62 seconds. Yes, it's a painfully long time -- but until you shrink that ND, that's what you're stuck with.

Hope this helps.

New Member

Re: Help with STP (redundant connections, fail-over)


thanks for the info. I didn't know about the diameter parameters - I saw the configuration option but didn't pay any attention to it. The bottom link is down because there is a human intervention - I've turned on the bottom switch last.

All the links between the switches are 802.1q trunks. The 2950 configures them automatically, not sure how but it does.

Regarding the MST or PVST: the 2950T allow me to configure spanning-tree mode PVST, but 2950G do not. Do 2950 switches automatically implement PVST?

Should I also enable backbone fast on any of these switches? Will it provide any benefit?

anyway, thanks for the info - i'll try it out, see if we can do anything about it.


P.S. I understand how you came up with MA and FD values ( The FD I think would be 18, not 19, as "2 x forward delay=14+10+10.5+1=35.5" But how did you calculate the STP convergence period?

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