ProCurve 2610-PWR Technical Information download pdf (Page 154)

Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

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.

Caution 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.

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ProCurve 2610-PWR Technical Information Page 154