Spanning-Tree Root Port

[martino-cisco]

Hi All,

Regarding Root Port election in a Spanning-Tree topology, the first consideration for the Root Port is the Path Cost to the Root Bridge. However, I understand that by changing the bandwidth of a specific port, you can make it your choice Root Port by using the sample command:

int giX/X
bandwidth 200000

In that case if you have say interface fa0/1 and interface gi0/1, changing the bandwidth of the FastEthernet interface fa0/1 to a higher value would make it the Root Port

My question is - In what scenarios would you need to choose a Root port based on your specified bandwidth, rather than the actual speed of the links.

Hope this makes sense

[Jon Marshall]

Martin

I have to admit I have never used the bandwidth command to control which path STP will take.

If I wanted to affect the cost both as seen by the local switch or any downstream switches I would use the interface command -

"spanning-tree vlan <x> cost <cost>" 

which allows you to control the exact cost being used by the local switch on that interface and also the additive cost sent to downstream switches.

As with L3 I tend not to change the bandwidth unless I have to because it can have effects on multiple things some of which may not be intended.

In terms of what scenarios you would need to do it, well I never have had to modify path costs using ether way on a production network although I have obviously played around with it in labs.

Generally speaking STP works best with minimal configuration ie. set your root and secondary switches etc. and rely on the greater bandwidth between your core and/or distribution switches than your access layer uplinks.

I'm not sure I have answered your question fully but I guess put simply, from my experience, I have never had to manipulate costs like this and even if I did I would not use the bandwidth command to do it.

Jon

[martino-cisco]

That was helpful, Jon.

Thanks