Cisco geeks - Why would I use PVST/PVRST+ over MST?

[Cliotoby]

OK Im really confused...

So I get the whole MST thing where it maps multiple VLANs to a single spanning tree instance. What I dont get is why I would want to use a Per VLAN version which requires a spanning tree instance for each VLAN. What is the benefit of this. Surely in larger networks it would be a ballache in terms of processing, which is where MST comes in surely? Also in a smaller network which required only a single Spanning tree I would use 802.1w RSTP, so again what is the benefit of Per Vlan Spanning tree?

Does the Per VLAN version simply allow for load balancing or something? Or does having different switches be the root bridge for different VLANs mean that cpu cycles are reduced - Switch A the root for VOIP VLAN while Switch B would be the root for VLAN 3 (A different department). ?????

MISTP or MST - Multiple Instance of Spanning Tree Protocol (802.1S) is an IEEE standard which allows several VLANs to be mapped to a reduced number of spanning-tree instances. This is possible since most networks do not need more than a few logical topologies. Each instance handles multiple VLANs that have the same Layer 2 topology. Combines the best of aspects from both PVST+ and 802.1Q. Relies heavily on RSTP and is more complex thas usual STP.

PVST - Per-VLAN Spanning Tree maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop. Cisco proprietary version of STP that offers more flexibility than the CST version. Per-VLAN Spanning Tree (PVST) operates a separate instance of STP for each individual VLAN. This allows the STP on each VLAN to be configured independently, offering better performance and tuning for specific conditions. The default when using ISL.

Just found this explenation... Is this right? Its from a manufacturer website (HP i think).

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. This causes redundant links to be blocked; they are then available on a standby basis. 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 network while still providing fault tolerance through rapid, automatic reconfiguration if there is a failure in a network's physical topology.

 

[bmh.01]

That explanation is how I understand it, allows different spanning tree topo for different vlans. One link may be forwarding for Vlans 100,101,102 but blocked for 103,104,105 and vise versa. Spread the load.

 

[Cliotoby]

Cheers. Think i get it now, i think the title should have been RSTP vs MST/PVST+

CST or similar, RSTP etc are basically just a waste of resources. May as well share the love with MST.

 

[Cr33do]

Yep. That's about it.

In the simplest form you may have a few edge switches (or a switch stack comprising of many switches) dual linked back to the core (which is the root bridge) forming a physical loop topology. Imagine you have a data vlan and a voice vlan on the edge. In single spanning tree instance you don't utilise the second link back to the core. It's there for resilience only. Mst allows you to use both links - data up one and voice up the other.

You may want to position the root bridge in different places depending on the environment.