Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

C a u t i o n

802.1s Multiple Spanning Tree Protocol (MSTP)

The 802.1D and 802.1w spanning tree protocols operate without regard to a network’s VLAN configuration, and maintain one common spanning tree throughout a bridged network. Thus, these protocols map one loop-free, logical topology on a given physical topology. The 802.1s Multiple Spanning Tree protocol (MSTP) uses VLANs to create multiple spanning trees in a network, which significantly improves network resource utilization while maintaining a loop-free environment.

While the per-VLAN spanning tree approach adopted by some vendors over­ comes the network utilization problems inherent in using STP or RSTP, using a per-VLAN technology with multiple VLANs can overload the switch’s CPU. MSTP on the switches covered by this guide complies with the IEEE 802.1s standard, and extends STP and RSTP functionality to map multiple indepen­ dent spanning tree instances onto a physical topology. With MSTP, each spanning tree instance can include one or more VLANs and applies a separate, per-instance forwarding topology. Thus, where a port belongs to multiple VLANs, it may be dynamically blocked in one spanning tree instance, but forwarding in another instance. This achieves load-balancing across the net­ work while keeping the switch’s CPU load at a moderate level (by aggregating multiple VLANs in a single spanning tree instance). Like RSTP, MSTP provides fault tolerance through rapid, automatic reconfiguration if there is a failure in a network’s physical topology.

Spanning tree interprets a switch mesh as a single link. Because the switch automatically gives faster links a higher priority, the default MSTP parameter settings are usually adequate for spanning tree operation. Also, because incorrect MSTP settings can adversely affect network performance, you should not change the MSTP settings from their default values unless you have a strong understanding of how spanning tree operates.

In a mesh environment, the default MSTP timer settings (Hello Time and Forward Delay) are usually adequate for MSTP operation. Because a packet crossing a mesh may traverse several links within the mesh, using smaller- than-default settings for the MSTP Hello Time and Forward Delay timers can cause unnecessary topology changes and end-node connectivity problems.

For MSTP information beyond what is provided in this manual, refer to the IEEE 802.1s standard.

5-46