STP port Priority

[singh.andy]

Hi All- I'm trying to figure our the best way to transition a stp blocked port to forwarding while doing the opposite for a forwading port and set that to block.

Here's the setup. From an access switch there is a 10gig uplink which is stp forwarding for all vlans and 1 gig uplink which is blocking. I would like to perform some maintenance on the 10gig link and would like to make 1 gig forwarding. I do not want to just disconnect the 10gig link to do this and drop traffic. Is setting the port priority to the lowest (may be 1) on 1gig port cause it go forwarding and block 10gig port.

What is the best way to do this without causing too much intrupption?

Thanks All,

[Nagaraja Thanthry]

Hello,

Changing the port priority will not force a spanning-tree recalculation. Spanning-tree recalculation requires a change in the network topology. If you do not want to disconnect the cable, you can just do a "shutdown" of the 10G interface (after lowering the cost off-course) which will make the 1G take over the forwarding role. Now, when you bring the 10G back up, because of the lower cost, the 1G link will still be the preferred path. However, when you bring the 10G link up, the network has to reconverge (spanning tree calculation will happen again). Other option if you do not want it to affect your network is, put the 10G link on both sides on a seperate stale VLAN (make it an access port). Once your maintenance is complete, put it back in the original mode.

Hope this helps.

Regards,

NT

[singh.andy]

thanks for the reply. Could i initiate a topology change by plugging a host or something? i do not want to kill traffic by shuting down the link..any way to avoid doing that or am i going to lose traffic anyways during the topology change?

[milan.kulik]

Hi,

are you running RSTP? In that case, the STP convergence should be fast, but still some frames might be lost during the change time.

See http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml for details.

If you are running 802.1D, I'd definitely recommend making any LAN topology changes out of business hours.

BR,

Milan

[Nagaraja Thanthry]

Hello,

You can consider configuring uplinkfast feature that will help you reduce the packet drops.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094641.shtml

Hope this helps.

Regards,

NT

[singh.andy]

Thanks all. I'll try out few of these suggestions.