Slow RSTP convergence

[hashaari]

Hi!

1. I am experiencing slow RSTP convergence in a switched network that I am currently testing. Instead of converging in 2 to 3 seconds as specified by RSTP, it took approx 30 seconds before traffic got through.

2. The setup this are depicted in the attached visio diagram.

3. To test the converge I connect a laptop to Cat3550, do continous ping to a target in the core and disconnect a GE uplink to the distibution switch so convergence time could be observed.

4. I couldn't find anything wrong with the config as it has been tested during proof of concept done earlier. I suspect issues with Gigatack.

5. What do you think? Apreciate your expert advice. Thanks.

[Francois Tallet]

Hi,

Can you post the role and state of the ports in the access switch (the stack I guess) while the problem is occurring? Looks like your alternate uplink is going slowly to forwarding.

Regards,

Francois

[ToddWarren]

Hello,

can you also verify that your designated root (DS1) is the root for all vlans, and the ports that should be forwarding are indeed forwarding for all vlans? And, how is your etherchannel configured (Pagp, ON, desirable, lacp)? This does sound like RSTP is reverting back to normal stp convergence for some reason. Thanks.

[hashaari]

Hi!

Thanks for your response. Yes, I can confirm that DS1 is root for all vlans and the ports are forwarding state.

The etherchannel are configured for ON. Is this also a contributing factor? In what way and how?

Again, apreciate your expert opinion. Thanks

[glen.grant]

One thing to check is what level code is on the 3550 , I don't think rapid spanning was supported before 12.1.13 ea1 . It would have to be newer than 12.1.13EA1.

[hashaari]

Hi!

The Cat3550 are running 12.1.13 EA1 and 12.1.14 EA1.

The Cat4506 are running 12.1.19 EW. Based on your experience, any known issues with this versions? Thanks.

[glen.grant]

12.1.13ea1 has an issue where TCN's get sent even when a port is in portfast which it shouldn't . When do a "show vlan XX" on the 3550 does it say it si running rapid pvst , should indicate it on the top of the display. Should look something like below.

VLAN0072

Spanning tree enabled protocol rstp

Root ID Priority 32768

Address 0011.bc45.c048

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

[ToddWarren]

Hello,

Glen brings up a valid point. If for whatever reason a device that is participating in RSTP encounters a device on the other end of a port that does not understand RSTP it will default back to some other flavor of STP without notification.

Also, as for the Etherchannel, if it is forced "ON" it can not take advantage of intelligent features of Pagp or LACP which can cause "unexplainable STP delays" in a failover situation. The recommended etherchannel mode between Cisco devices is "desirable".

[DWAM_2]

Hello,

If you want to use rstp with gigastack , you have to use it with point to point

so 4500A - gbic - gbic - 3550 - gigastack1 - gigastack2 - 3550 - gigastack3 - gigastack4 - 3550 - gbic - gbic - 4500B (so 4 gigastacks in this configuration) otherwise you will have a shared line and a slow convergence.

IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree independent of spanning-tree timers. Reconfiguration of the spanning tree can occur in less than one second, a feature that is critical for networks carrying delay-sensitive traffic such as voice and video.

2) I advice you to update your version (espacially for the 3550)

Best regards.