CCNP Switch - Diameter ?

Hi Everyone,
I hope that you are having a nice day. I have a question. Can anyone explain 'Diameter'? Its mentioned in Chapter 8 - Spanning Tree configuration, but only barely. Its not even listed in the index or glossary. Here is an example:

Switch(config)# spanning-tree vlan 'vlan-id' root primary [diameter diameter]

Any advice is appreciated.
Thanks!

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Comments

Heero

It is basically hop count for BPDUs. I've never messed with it so I can't tell you any specifics about it, but there is definitely documentation out there.

fredrikjj

It's a macro that adjusts the timers based on the maximum number of hops in the spanning tree. Though, if I remember correctly, it's not actually the number of hops, but rather the number of devices which, I guess, would make it number of hops plus one (correct me if I'm wrong). It doesn't do anything that couldn't do by simply adjusting them manually. Basically, 15-20 years ago someone at Cisco came up some kind of relationship between the number of devices and the timers and created this command. Considering how far removed we are from those times and that we're not even using the same STP version, I would consider it a legacy command.

Coolhandluke

Just to confirm what others have said, It adjusts the timers for STP to allow BPDU's to span more/less of the network tree (hops?). Apparently once you get to 7 bridge devices SPT starts to fall apart with the default timers. There was a great paper about a hospital network (All systems down | Computerworld) having issues with STP.

theodoxa

It adjusts the STP Timers based on the size of the Layer 2 network (maximum number of switches that a frame would have to traverse to cross the network). The default timers are based on a Diameter of 7 (a frame would have to cross no more than 7 switches to reach the other end of the Layer 2 network), but your network might be smaller than that. By adjusting the diameter, STP will calculate new [shorter] timers, allowing for faster transitions and recovery.

Node Man

Thank you Everyone!