2.1.f Implement and troubleshoot spanning-tree

2.1.f [i] PVST+/RPVST+/MST

MST gets short shrift in ccie… in wendell’s ccie r&s it gets barely two pages…

One of the key benefits of MST versus PVST+ is that it requires only one MST instance for a group of VLANs. If this MST region had hundreds of VLANs, and used PVST+, hundreds of sets of STP messages would be used. With MST, only one set of STP messages is needed for each MST instance.

in the ccie quick review it gets little more than a passing nod:

This provides advantages over PVST+ because typical topologies need only a few spanning-tree topologies to be optimized.

you have to dig deeper to find this:


MSTs (IEEE 802.1s) combine the best aspects from both the PVST+ and the 802.1q. The idea is that several VLANs can be mapped to a reduced number of spanning tree instances because most networks do not need more than a few logical topologies.

The CPU is spared because only (x) instances are computed.

while that last statement might appear obvious, it gets little coverage, which is funny considering cisco’s zeal in the protecting device resources department.

see also this excellent series by petr laphukov