RSTP + Stack

lepavelcisco · ‎11-12-2014

Hey all,

I have a wired set up, maybe you can help me to understand 2 things.

I have 2 3750X in a stack.

then I have 5 unmanaged switched connected to the stack.

Each unmanaged switched has 2 ports to the 3750X Stack( one to switch 1 and one to switch 2)

I have rapid spanning tree, with the stack as root. 5 Vlans for each unmanaged switched + L3 int vlan for each vlan and lo0 int.

I don't use dynamic routing. Just running unicast and multicast (lo0 is RP).

everything works fine.

1. rapid spanning tree convergence, if i take one of the unmanaged switched. disconnected port 1 to 3750X 1. It takes 30 seconds until the other port starts to forward. when I reconnect it takes 1 second till the original port started to forward.

Is that logical it takes 30 second for RSTP to converge in this type of setup?

2. I have disconnected the stack cables while switches were running. what happened is that every switch run the same configuration still connected to my set up. after a few minutes I got duplicated IP messages on my 2 3750X.

And I was expecting that muticast and unicast will start to drop , but all was still working.

Had full multicast and unicast connectivity over 3 vlans with 2 hosts and multicast streamer source.

Is that logical?

hope you have any ideas or insights.

Joseph W. Doherty · ‎11-12-2014

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Posting

Taking number 2 first, breaking the stack, while the switches are still active, places two L3 switches on the network thinking they are the same switch. The assumption with a stack is a stack member fails, not that the stack partitions. One of the reasons there's two stack connections, is to avoid the situation you've created if one stack port or one stack cable fails.

Taking number 1, I would first think fail over would be faster, but then I'm thinking that should be true if your downstream switches were also running RSTP. Since you say they are not managed, I assume they are not running RSTP. This might account for the slower STP convergence. It's something I would have to think about. Also whether something like portfast on the trunk would help (which might not be possible with your topology).

lepavelcisco · ‎11-20-2014

Well for STP problem solved with MST.

spanning-tree mode mst

spanning-tree mst configuration

instance 1 vlan 1-4094

spanning-tree mst 1 root primary

spanning-tree mst forward-time 6

This gets me to 17 seconds convergence time.

can tweak more but 17 is good.

Joseph W. Doherty · ‎11-20-2014

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Interesting, because rapid-STP and MST, I believe, are very much alike.

I presume the key to your decreased convergence is the line you've highlighted.

I haven't looked, but no similar option for rapid-STP?

lepavelcisco · ‎11-20-2014

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_11_yj4/configuration/guide/lrescg/swmstp.html#wp1034497

did not find anything in rapid-STP compared to MST.

also, I was trying to avoid touching hello or max age timer.(no major reason for that)