Solved: MST / Network Redundacy questions....

jasonfaraone · ‎06-06-2007

I'm planning to connect 3 3560's and a 4503 together in a ring topology, with dual fiber cables connecting each switch in a port channel group.

I want to use MST for fast restoration of service in the event of a break in the physical network.

My first question - Are there any known problems with MST and port-channel interfaces? In the MST configuration document, Cisco has redundant trunks which use the "spanning-tree mst 2 port-priority 64" command to balance between them...

I'm just getting into MST and so I want to make sure that I can skip that command, as the interface should handle balancing itself.

Also, Cisco's network topology is different than mine in the example they give; is there anything inherently wrong or undoable with my approach?

It essentially looks like this.

C's represent 3560's.

C---------C

|OOOOOO|

C------4503

Any help would be greatly appreciated.

Francois Tallet · ‎06-07-2007

Hi Jason,

By default, all the instances will have the same parameters, and will thus compute the same topology. So if you want different path for your two instances, you'll need to configure them differently. Apart that you need to have the same MST region configuration all over your network, the different instances are configured just like you would do in PVST mode (if you are used to PVST). Basically, you just need to replace the keyword "vlan" by "mst" in your commands;-)

Usually, the simplest way to enforce a topology is to force the root bridge at a particular location. This is done by tuning the priority of the bridge that you want to be root on this particular instance (for example: "spanning-tree mst 1 priority 8192" should be enough to make this bridge root for instance 1). Then, if you need to modify the path computed by default from this root, the simplest is generally to change the cost of the ports ("spanning-tree mst 1 cost X"). The higher the cost of a port, the less likely it will be elected root port.

HTH,

Francois

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Francois Tallet · ‎06-06-2007

Hi Jason,

No, there is nothing special with MST and etherchannels. If you don't plan to have MST block the ring at different for different vlans, your configuration should be quite plug and play. Just enable MST as the STP mode on all your switches.

The ring topology is not the most efficient in term of STP convergence, but considering that your ring is small, you should still have relatively fast convergence time should a channel fails completely.

Also, a note on etherchannel and IOS. IOS recomputes automatically the cost of a channel when a member is added or removed. That means that if a single fiber cable fails, you're very likely to have MST reconverge. If you don't want this (and you have enough bandwidth to operate your network with a single fiber), just hardcode a cost on your port channel interfaces.

Regards,

Francois

jasonfaraone · ‎06-06-2007

When you say "plug and play", you mean to say that as long as I have my (2) instances created on the 4503, and the vlans divided between them, that I can simply enable MST, configure the region, revision, instance / vlan mapping on each 3560 and everything should just work?

Francois Tallet · ‎06-06-2007

I did not understand you had two instances;-)

The only constraints with MST, if you want to use several instances, is that you have to put the same configuration on each and every switch (as you already know). It's something that is relatively painful if you want to change the configuration because you need to visit all your switches. That's what is not really plug and play with MST. If you only need one instance, you can go with the default configuration.

Regards,

Francois

jasonfaraone · ‎06-07-2007

Thanks a ton for your help. My test lab is somewhat limited, so its difficult to reproduce the setup.

Will MST determine different paths for my instances automatically, or is that something I will need to specify?

I'm sorry if these are all basic questions, but I'm using http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml#mst1 as a guide and I'm just not understanding how some of this works.

Thanks a ton, again.

Jason

Francois Tallet · ‎06-07-2007

Hi Jason,

By default, all the instances will have the same parameters, and will thus compute the same topology. So if you want different path for your two instances, you'll need to configure them differently. Apart that you need to have the same MST region configuration all over your network, the different instances are configured just like you would do in PVST mode (if you are used to PVST). Basically, you just need to replace the keyword "vlan" by "mst" in your commands;-)

Usually, the simplest way to enforce a topology is to force the root bridge at a particular location. This is done by tuning the priority of the bridge that you want to be root on this particular instance (for example: "spanning-tree mst 1 priority 8192" should be enough to make this bridge root for instance 1). Then, if you need to modify the path computed by default from this root, the simplest is generally to change the cost of the ports ("spanning-tree mst 1 cost X"). The higher the cost of a port, the less likely it will be elected root port.

HTH,

Francois