STP and uplink migrations

Unanswered Question
Oct 13th, 2008

Hello all,

I am migrating from a pair of 4510s to a pair of 6509s with as little disruption as possible. Any suggestions on how to do so when it comes to spanning tree?

The 4510s are configured as root at bridge priority 4096 and secondary at 8192. I was going to add in the 6509s as 12288 and 16384 and create a square design with all 4 switches. The access switches are a combination of 2950s, 3750s and 4948s with Etherchannel uplinks on some switches.

All access switches are configured with uplinkfast and the network is running PVST+ I'm trying to decide how to migrate the uplink ports on the access switches. I know I can move the blocking port to the new switches without disruption. I also know that I can move the root port and uplinkfast will fail to the other uplink subsecond. I also know that uplinkfast will wait 30-45 seconds to switch back from the "secondary" port to the root port, but what happens if the formerly root port is no longer in the root switch when it comes back up?

I realize there are many options so any input would be greatly appreciated.



I have this problem too.
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satish_zanjurne Mon, 10/13/2008 - 07:16

I will suggest that..

1.Make sure you are replicating the STP root config of 4510 to 6509

2.I don't think so you will require any changes in access switches.

3.Take the scheduled downtime in weekend or non-production hours & then do the migration

trust me it will be smooth & easy..

HTH..rate if helpful..

MATTHEW BECK Mon, 10/13/2008 - 07:25


Thanks for the advice, but I don't have that kind of window. Management would prefer I do it without even the 35-45 seconds of a recalc per access switch. My plan is to add the 6509s so I have 4 distribution switches, migrate all access switches to the 6509s and then shut down the 4510s. With no downtime.

Thanks again,


Giuseppe Larosa Mon, 10/13/2008 - 10:30

Hello Matt,

I would suggest to make the new C6509 the new root bridges using base priority 0 and 1: after a recalculation you will have the final setup and then you move your access switches link by link to the new devices.

I would try to get from management a time window for doing this and then later you move each uplink

I wouldn't face the risk to have an access layer switch to have a chance to be elected root bridge even during a transition

Hope to help


abdel_n Mon, 10/13/2008 - 16:25

Hi Matt,

With UplinkFast configured, the access switch will place blocked port (alternative link to a distribution switch) immediately into forwarding mode (less than one second)

*) For sake of stability:

- UplinkFast will move the returning root port immediately as soon as it is up, but the designated port in the other side of the uplink (to the root) will undertake the usual STP procedure (listening->learning) 2*ForwardDelay + 5sec = 35sec.

- To avoid flapping, uplinkFast will not allow another consecutive switchover to the backup uplink for 35 sec.

So you can take advantage of the very fast speed of UplinkFast recovery everytime an uplink failure occur(or a topology change) + 35 sec.

*) UplinkFast will make sure that the switch (access level) in which it is configured:

- have apriority of 49152 (you can can explicitly make it greater or make the priority on the root/backup very low).

- Set the port cost to 3000.

- Track alternate ports to distribution.

- instruct distribution switches to reset their CAM and then inform them about its local db (using dummy multicast): the availability of your network will be also limited by the speed at which CAM tables are updated and accurate, you can increse the frequency at which the access switch will send dummy packets.

(config) #spanning-tree uplinkfast max-update-rate

*) Though its not recommended or with a great care (and particularly in a production environment), it is possible to tune max-age down to 6 sec, so you will have 2*6 = 12sec instead of 30 sec.

Cisco Doc about UplinkFast:

I hope this will help



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