28.7.4 Notes on using Multiple Spanning Tree

<Structure of this section>

(1) Coexistence with other functions

[See "24.3 Coexistence of Layer 2 Switch Function and Other Functions"

(2) About MST Regions

The range of VLANs that can be handled by the Switch and other switches might differ. To handle such switches as the same MST region, make sure that the corresponding VLANs belong to MST instance 0.

(3) When convergence of the topology takes time

When the events listed in the following table occur for CIST root bridges or MST instances, the topology might take a long time to settle, during which time communication might stop and MAC address tables might be cleared.

Table 28-13: Event generation at the root bridge
Event	Description	Type of root bridge type on which the event occurs	Affected topology
Configuration change	When the region name (1), revision number (2), or correspondence between instance number and VLAN (3) is changed by configuration, and the region is split or merged: (1) name command for the MST configuration mode (2) revision command for the MST configuration mode (3) instance command for the MST configuration mode	CIST root bridge	CIST
		Root bridge on MST instance 0 (IST)	CIST
		Root bridge on MST instance 1 and those subsequent	Corresponding MST instance
	When the bridge priority is reduced by the spanning-tree mst root priority command (a larger value is currently set)	CIST root bridge	CIST
		Root bridge on MST instance 1 and those subsequent	Corresponding MST instance
Other cases	When the Switch stops	CIST root bridge	CIST
		Root bridge on MST instance 0 (IST)	CIST
		Root bridge on MST instance 1 and those subsequent	Corresponding MST instance
	When all ports are down for the Switch in a loop configuration, on the partner switch connected to the switch (and the Switch is no longer the root bridge in the corresponding loop configuration)	CIST root bridge	CIST
		Root bridge on MST instance 0 (IST)	CIST
		Root bridge on MST instance 1 and those subsequent	Corresponding MST instance