Migration to Rapid PVST

[paul.kyte]

A small conundrum which I can't get my head around. can someone please help.

I have a meshed layer 2 network with multiple vlans in my server farm connecting to two core routers. All these devices are running in 802.1d Spanning Tree.

I want to migrate all these to 802.1w Rapid spanning Tree. However all the documentation I read suggets that when I reconfigure a switch to Rapid it will listen on its links to utilise the appropriate protocol (802.1w or 802.1d)in use by the connected switches.

As all my switches work in 802.1d mode as I migrate a switch it will be Rapid capable but will work in 802.1d mode because all the other switches are in this mode.

Changing an attached switch to rapid will result in that listening after the config change and it will only hear 802.1d from the other switches(whether they are rapid or not) because that is what is in use.

Eventually all but one of my switches will be rapid capable but they will all work in 802.1d mode. I'll then make the config change on this last switch, it will listen to waht is being talked (802.1d) and will then come up in that mode.

So all my switches are now rapid capable but are working in non-rapid 802.1d mode.

How do I make them use the 802.1w protocol ????

The config change doesn't take up or down the point to point links and I don't reboot after making the change. Hence from the documentation the port just listens and uses what is being sent from the other end which will be 802.1d!!!

I must be missing something or not understanding things correctly. Can someone please advise how the links become 802.1W

Thanks

[rduke]

A switch with rapid spanning tree will run common spanning tree only on the port with the old switch attached. If you do a "show span" you can see the status. On the below output I have one old switch on port 6/47. The others are using rapid spanning tree. Also, when you attach trunk between two rapid enabled switches, the link light turns green almost immediately instead of the usual 30 seconds. Don't forget to enable it at the global level on the switch.

VLAN0030

Spanning tree enabled protocol rstp

Root ID Priority 4126

Address 000e.3824.8300

This bridge is the root

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

Bridge ID Priority 4126 (priority 4096 sys-id-ext 30)

Address 000e.3824.8300

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

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- ----------

Gi2/1 Desg FWD 4 128.65 P2p

Gi2/2 Desg FWD 4 128.66 P2p

Gi2/4 Desg FWD 4 128.68 P2p

Gi2/5 Desg FWD 4 128.69 P2p

Gi2/6 Desg FWD 4 128.70 P2p

Gi4/12 Desg FWD 4 128.204 Edge P2p

Gi4/48 Desg FWD 19 128.240 P2p

Gi6/42 Desg FWD 4 128.362 P2p

Gi6/45 Desg FWD 4 128.365 P2p

Gi6/47 Desg FWD 19 128.367 P2p Peer(STP)

Gi6/48 Desg FWD 19 128.368 P2p

Po1 Desg FWD 3 128.641 P2p

R Duke

[Aaron Harrison]

Hi

Basically if a switch that only speaks old-stp gets an RSTP packet it will drop it.

If it understands the RSTP and receives a packet, this will cause it to switch to RSTP for that port.

You are correct though, all your switches will end up with RSTP and all it's ports in 802.1d mode.

You need to clear the compat mode like so (this will cause the switch to send out an RSTP BPDU which will cause the attached switches to go to RSTP for that port - most likely you'll need to do this on all your switches).

clear spanning-tree detected-protocols

See page 33 of this doc:

http://www.cisco.com/application/pdf/en/us/guest/tech/tk621/c1501/ccmigration_09186a0080174993.pdf

Regards

Aaron

[rduke]

One more thing I forgot to mention about testing it. It works so well I have moved a lot of connections in the middle of the day and no one ever knew I moved the trunks. It is great. On the other hand, if you plug a trunk in and out too many times, the switch will get mad at you and shut down the port with a "link flap" error disabled condition. I consider this good, because a bad connection on a link could get disruptive. Just don't do it too many times in a row with active users.

[paul.kyte]

Thanks for your help Gents.

I have managed to find a definitive answer to my question by much searching on the Cisco Website and comments made by yourselves.

The definitive answer is:

1. RSTP selectively sends 802.1D-configured BPDUs and topology change notification (TCN) BPDUs on a per-port basis.

2. When a port initializes, the migration-delay timer starts and RSTP BPDUs are transmitted. While the migration-delay timer is active, the bridge processes all BPDUs received on that port.

3. If the bridge receives an 802.1D BPDU after a port’s migration-delay timer expires, the bridge assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs.

4. When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU after the migration-delay expires, RSTP restarts the migration-delay timer and begins using RSTP BPDUs on that port.

The upshot of all this is that when a switch is configured for rapid spanning tree (spanning-tree mode rapid-pvst), the switch initializes its ports. It will immediately start sending RSTP BPDUs. If the Switch on the other end of the link(s) understands RSTP BPDUs it will then start to send RSTP BPDUs. When the migration delay timer expires they will be talking to each other in Rapid spanning Tree mode.

Switch links that have non Rapid switches at the other end will come up in 802.1D mode.

The Switch will then have different links operating in different modes. In essence a hybrid Switch.

Consequently converting a large meshed switch network to Rapid spanning Tree is quite easy and simple. Just change switches one at a time and the links will automatically change to 802.1w where possible and will remain 802.1d where not. Eventually the entire network will progressively become 802.1w as more switches are reconfigured, until it is completely 802.1w

Simple

[wailunip]

Hi,

Just wanted clarification on the migration process. Is it possible to make the change of spanning tree mode to Rapid PVST without it being service affecting on the switches.

Thanks