Rapid-spanning tree : diameter 's limit

[marzi]

Hi all

I'have to do a ring network with 8 switches belonging 93000 (q.ty 1) and 92000 (q.ty 7) families.

I'm going to use rapid-spanning tree (802.1w).

I know that there is a limit to 7 switches about the ring's diamter if I used old spanning-tree mode (802.1d)

Is this limit still valid to rapid-spanning tree (802.1w) protocol ?

Thanks in advance

Fabrizio

[balaji.bandi]

yes as per the cisco document and limit still same :

Optional) For diameter net-diameter , specify the maximum number of devices between any two end stations. The range is 2 to 7

[MHM Cisco World]

This valid for all STP modes'

If you add more SW then the bpdu may not receive from last SW in root and assume thar there is no root' here you will get two root in one STP domain ajd that so bad.

[marzi]

Ok but I've eight switches, how I can solve this problem ?

I'm thinking to set manually the root bridge and the stp-timers in the following mode :

Switch(config)#spanning-tree vlan 1-x priority 0
Switch(config)#spanning-tree vlan 1-x max-age 22
Switch(config)#spanning-tree vlan 1-x forward-time 16

The value max-age and forwarding-time are resulting from the formulas :

max age =(4*hello)+(2*diameter)-2
forward delay=((4*hello)+(3*diameter))/2

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html

(same method for the secondary root bridge but with priority 4096)

am I on the right way or am I totally wrong ?

Thank's again for your support

Fabrizio

[MHM Cisco World]

The SW is connect as train or layer design?

[marzi]

Hi MHM thanks for your replay

I'm not sure to understand your question,

All what I can tell you is that there will be q.ty 7 9200 and 1 9300 connected in ring fashion.

It's a pure layer 2 ring.

The 9300 will be the root and one 9200 will be the secondary

Probably the 9300 will be connected to external network in L3 manner (default-route or L3 protocol I've not all detail again)

[MHM Cisco World]

I will check my note and send you private message later today.

Thanks

MHM

[Joseph W. Doherty · Accepted solution]

"Is this limit still valid to rapid-spanning tree (802.1w) protocol?"

No, but RSTP can bump into "diameter" issues too.

Have you considered something like REP (if supported)?

[marzi]

Hi Joseph

You're right, I'm considering the REP solution. I will work with 9K switches families and IOS-XE Cupertino 17.9.X.

I think that REP should be supported.

Anyway I will read the command reference

Just Right now I was reading information about REP for learn that protocol.

Thanks again Joseph