RSTP blocking switches beyond 20

[Torsha]

Hi All,

I have a network where there are 31 switches connected in a loop with RSTP enabled and it works fine. Now when there is a break in the loop say at number 30 such that 1------30 break --- 31. Now I see only 20 switches are talking. Why is that happening, ideally only 31 no switch should be out of network. Is this because of max age etc configurations. Please advise.

Thanks 

[MHM Cisco World]

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

I think you are correct' cisco recommends diameter of 7 here there are 30/31 SW so last SW's dont recieve bpdu in time and assume link down.

You can check by see in each SW what the mac address of root SW 

If you see more than two root SW then your SW dont receive bpdu in time 

MHM

[Torsha]

Hi Thanks for replying.

So you mean that in loop (1-----31--1) since there are two routes

1----15(discarding)16-----1

the sw get the BPDU however when the count is 30 the last sw do not get the BPDU in time?

[MHM Cisco World]

correct 
and as I mention use show spanning tree in last SW 
in one stp domain there must be ONE root (per VLAN or for all VLAN)
if you see  two or more then there is issue 
MHM

[Torsha]

Also is there any limitation on the number of devices connected in daisy chain for RSTP enabled switches?

In my set up switches have max age as 20. I changed it 40 thinking that might be the issue but still only 20 switches communicate in daisy chain once the loop is broken 

[MHM Cisco World]

As I know NO

But let check 

Share 

Show spanning tree in SW in both side of broken link 

Show spanning tree interface (broken link) in both SW 

MHM

[M02@rt37]

Hello @Torsha 

Do you you have the STP network diameter configured ?

[Torsha]

Hi ,

no I have not configured this parameter.

Thanks

[paul driver]

Hello

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@Torsha wrote:

Also is there any limitation on the number of devices connected in daisy chain for RSTP enabled switches?

In my set up switches have max age as 20. I changed it 40 thinking that might be the issue but still only 20 switches communicate in daisy chain once the loop is broken 

---

As M02@rt37 stated the default stp diameter is 7, meaning the root path should not exceed 7 stp devices to reach the edge of the l2 network from the root switch

I would suggest NOT to manually change the stp age timers but to change the stp diameter to accommodate your network, this change will automatically adjust the stp timers.