28.8.4 Configuring Multiple Spanning Tree Parameters

(1) Setting BPDU Sending Interval

A short BPDU sending interval makes topology changes easier to detect. A longer interval requires more time to detect a topology change, but can reduce BPDU traffic and the load on the Spanning Tree program for the Switch.

Points to note

When no value is set, BPDUs are sent at two-second intervals. Normally, this setting is not required.

Command examples

1. (config)# spanning-tree mst hello-time 3

   Sets the BPDU sending interval for Multiple Spanning Tree to three seconds.

Notes

A short BPDU sending interval makes topology changes easier to detect, but might increase load on the Spanning Tree program due to an increase in BPDU traffic. If setting this parameter shorter than the default value (2 seconds) causes timeout messages to be output and the topology to change frequently, change it back to the default value.

(2) Setting the Max BPDU to Send

To prevent an increase in CPU load for a Spanning Tree Protocol, the maximum number of BPDUs to be sent per hello-time (BPDU sending interval) can be chosen. If topology changes frequently occur, a large quantity of BPDUs are sent to report and gather topology changes, possibly increasing BPDU traffic and CPU load. This can be controlled by limiting the maximum number of BPDUs to be sent.

Points to note

If no value is set, operation is performed with a maximum number of BPDUs per hello-time (BPDU sending interval) of 3. Normally, this setting is not required.

Command examples

1. (config)# spanning-tree mst transmission-limit 5

   Sets the maximum number of BPDUs to be sent per hello-time to 5 for Multiple Spanning Tree.

(3) Setting the Maximum Hop Count

You can set the maximum number of host for BPDUs sent from the root bridge. The BPDU counter is incremented whenever a switch is passed, and BPDUs exceeding the maximum number of hops are disabled and ignored.

For ports connected to Single Spanning Tree switches, the maximum enabled time (max-age) parameter is used instead of the maximum number of hops (max-hops). The counter for the number of hops is a valid parameter between Multiple Spanning Tree switches.

Points to note

The maximum number of hops can be increased to have BPDUs reach many switches. If no value is set, 20 is used for the maximum number of hops.

Command examples

1. (config)# spanning-tree mst max-hops 10

   Sets the maximum number of hops for BPDUs on Multiple Spanning Tree to 10.

(4) Setting BPDU Max Valid Times

For Multiple Spanning Tree, maximum enabled time (max-age) is a valid parameter only for ports connected to a Single Spanning Tree switch. It does not need to be set for configurations in which Multiple Spanning Tree runs for switches across the entire topology.

You can set the maximum enabled time for BPDUs sent from the root bridge. The BPDU counter is incremented whenever a switch is passed, and BPDUs exceeding the maximum enabled time are disabled and ignored.

Points to note

The maximum enabled time can be increased to have BPDUs reach many switches. If no value is set, 20 is used for the maximum enabled time.

Command examples

1. (config)# spanning-tree mst max-age 25

   Sets the maximum enabled time for Multiple Spanning Tree BPDUs to 25.

(5) Setting the state transition time

For timer-based operation, the port status transitions at a fixed time interval from Discarding to Learning, and then Forwarding. The time required for these status transitions can be set. A small value can be set to transition more quickly to the Forwarding status.

Points to note

If no value is set, 15 seconds is used for the status transition time. When changing this parameter to a short period of time, make sure that BPDU max. valid time (max-age) and send interval (hello-time) satisfy "2 x (forward-time-1) ≧ max-age ≧ 2 x (hello-time + 1)".

Command examples

1. (config)# spanning-tree mst forward-time 10

   Sets the state transition time of the multiple spanning tree to 10.