Number of switches in a MSTP domain

[Vincent1]

Hello,

I need to implement spanning-tree MSTP on a network
Number of switches on the network L2 : 400
MSTP technical specifications
1 domain
No vlan in instance 0
Vlans 1-4094 in instance 1

Is there a limitation on the number of switches supported by the MSTP protocol?

Thank you.

Vincent.

[Predrag Jovic]

As far as I know there is no limitation regarding number of switches in domain, however there can be limitation of number of "L2 hops" diameter (20 by default).

Global configuration command spanning-tree mst max-hops can be used to adjust max-hops number if necessary.

[Julio E. Moisa]

Hi

Do you keep a hierarchical L2 infrastructure?, I have never heard about the maximum switches just instances, do you have all of them on the same network segment or are they limited by routers? I think you should consider the diameter only, recommended diameter of 7. https://slaptijack.com/networking/max-spanning-tree-stp-diameter/

spanning-tree mst instance-id root secondary [ diameter net-diameter[ hello-time seconds ]]

Configures a switch as the secondary root switch. instance-id— Specifies a single instance, a range of instances separated by a hyphen, or a series of instances separated by a comma. The range is 0 to 4096. (Optional) diameter net-diameter— Specifies the maximum number of switches between any two end stations. The range is 2 to 7. This keyword is available only for MST instance 0. (Optional) hello-time seconds— Specifies the interval in seconds between the generation of configuration messages by the root switch. The range is 1 to 10 seconds; the default is 2 seconds. Use the same network diameter and hello-time values that you used when configuring the primary root switch.

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

[Predrag Jovic]

Number of 7 hops by default is applicable only to 802.1d not to 802.1w or 802.1s.

[Predrag Jovic]

Catalyst 3560 Software Configuration Guide, Release 12.2(52)SE

Configuring the Maximum-Hop Count

Beginning in privileged EXEC mode, follow these steps to configure the maximum-hop count for all MST instances. This procedure is optional.

    Command Purpose

Step 1	configure terminal	Enter global configuration mode.
Step 2	spanning-tree mst max-hops hop-count	Specify the number of hops in a region before the BPDU is discarded, and the information held for a port is aged. For hop-count, the range is 1 to 255; the default is 20.
Step 3	end	Return to privileged EXEC mode.
Step 4	show spanning-tree mst	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

[Vincent1]

Hi Julio,

Thank you for your reply.

You can find attached the target architecture.

The real estimated number of switches is around 250 (not 400).

Vlans are propagated everywhere on the network (around 200).

Do you confirm that the number of switches is not an issue (one region and one instance MSTP) ?

According to me, the diameter in the network is 6. Could you confirm?

Thanks

Vincent.

[Predrag Jovic]

There can be limitation of number of active STP logical ports and need to be calculated regarding controller that you are using (not number of switches). For more details please read article:

Spanning Tree Scalability