802.1s MST - why bother?

joseph-chapman · ‎04-09-2004

I'm looking for compelling reasons why the typical campus environment would need to use MST.

I know it reduces the number of trees used however, if you follow current recommendations you don't need more than 2-3 spanning trees on an edge switch. Additionally, it is now capable to route between the edge switch (C3550s) and the distribution/core layer.

Since Cisco is encouraging and making it possible to do more routing in the campus LAN environment without the latency penalty previously experience in older switch/router combinations, why would I be concerned with large numbers of spanning tree instances on a particular switch?

As far as I can tell, MST helps fix poor campus switched-network designs the same way that EIGRP helped fix poor WAN topologies with discontiguous/non-VLSM'able IP addressing schemes.

And the biggest drawback to MST is that as soon as you add a new VLAN and map it to something other than instance 0 (the IST/CST) you just broke the MST region since the config of that switch no longer matches other neighbor switches. This means you just lost all the benefits of MST as you now have multiple regions and you only get the one IST/CST between the regions.

Does this sound about right, or am I just completely missing something here?

ebreniz · ‎04-16-2004

MST extends the IEEE 802.1w rapid spanning tree (RST) algorithm to multiple spanning trees. This extension provides both rapid convergence and load balancing in a VLAN environment. MST converges faster than Per VLAN Spanning Tree Plus (PVST+) and is backward compatible with 802.1D STP, 802.1w (Rapid Spanning Tree Protocol [RSTP]), and the Cisco PVST+ architecture.

MST allows you to build multiple spanning trees over trunks. You can group and associate VLANs to spanning tree instances. Each instance can have a topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic and enables load balancing. Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect other instances.

More details read

http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_guide_chapter09186a00800dde9e.html#1021469

joseph-chapman · ‎05-13-2004

I know about the benefits that MST provides, but I'm concerned with the limitations that by changing a VLAN-instance mapping on one switch causes that switch to no longer be in the same 'region' as its neighbors. I then lose all MST 'load balancing' capabilities as they will only communicate CST.

I can run Rapid-PVST and if I'm going to follow current recommended best practices I'm not going to be distributing many VLANs across multiple switches (the same VLANs, at any rate) as I will try to instead use more point-to-point routed links.

So, I'm still left with trying to come up with a compelling arguement for why I need to use MST instead of Rapid-PVST and fewer multi-switch VLAN deployments.