Brocade Converged Enhanced Ethernet Administrator's Guide v6.1.2_cee (53-1001258-01, June 2009)

Table Of Contents

Converged Enhanced Ethernet Administrator’s Guide 43

53-1001258-01

MSTP overview

The IEEE 802.1s Multiple STP (MSTP) helps create multiple loop-free active topologies on a single

physical topology. MSTP enables multiple VLANs to be mapped to the same spanning-tree instance

(forwarding path) which reduces the number of spanning-tree instances needed to support a large

number of VLANs. Each MSTP instance has a spanning-tree topology independent of other

spanning-tree instances. With MSTP you can have multiple forwarding paths for data traffic. A

failure in one instance does not affect other instances. With MSTP, you are able to more effectively

utilize the physical resources present in the network and achieve better load balancing of VLAN

traffic.

NOTE

In MSTP mode, RSTP is automatically enabled to provide rapid convergence.

Multiple switches must be configured consistently with the same MSTP configuration to participate

in multiple spanning tree (MSTP) instances. A group of interconnected switches that have the same

MSTP configuration is called an MSTP region.

NOTE

We support 16 MSTP instances and one MSTP region.

MSTP introduces a hierarchical way of managing switch domains using regions. Switches that

share common MSTP configuration attributes belong to a region. The MSTP configuration

determines the MSTP region where each switch resides. The common MSTP configuration

attributes are as follows:

• Alphanumeric configuration name (32 bytes)

• Configuration revision number (2 bytes)

• 4096-element table that maps each of the VLANs to an MSTP instance

Region boundaries are determined based on the above attributes. A multiple spanning tree

instance is an RSTP instance that operates inside an MSTP region and determines the active

topology for the set of VLANs mapping to that instance. Every region has a common internal

spanning tree (CIST) that forms a single spanning tree instance that includes all the switches in the

region. The difference between the CIST instance and the MSTP instance is that the CIST instance

operates across the MSTP region and forms a loop free topology across regions, while the MSTP

instance operates only within a region. The CIST instance can operate using RSTP if all the switches

across the regions support RSTP. However, if any of the switches operate using 802.1D STP, the

CIST instance reverts to 802.1D. Each region is viewed logically as a single STP/RSTP bridge to

other regions.

After enabling MSTP using the global protocol spanning-tree mstp command, you configure the

following from MSTP mode:

• Name of the region—Specify the region name using the region region_name command.

• Revision number—Specify the revision number using the revision number command.

• MSTP VLAN-to-instance assignment map—Map a VLAN to an MSTP instance using the instance

instance_id {vlan vlan_string | vlan_id [priority priority_id]} command.

• Hop count—Specify the maximum hops for a BPDU to prevent the messages from looping

indefinitely on the interface using the max-hops hop_count command.

To configure MSTP on your Brocade 8000 CEE switch, see “STP, RSTP, and MSTP configuration

procedures” on page 45.