MST always uses long path costs

[huyan]

Hi everyone, 

I have a question regarding MST. 

I read here https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-mst-cost.html

It says that "The faster interface speeds indicate smaller costs" AND "MST always uses long path costs".

I wonder why it does not use small path cost, which is faster?

Thank you. 

[Deepak Kumar · Accepted solution]

Hi,

Don't be confused, both are different terms. 

The path cost is the metric STP uses to calculate the shortest path to the elected root bridge. The path cost is based on the speed of the bridge port interface. Back when Radia Perlman developed the Spanning Tree Protocol, 10 GE links were not even considered within the realm of possibility. Just like we have outgrown 32-bit IPv4 addresses, the 16-bit path cost value hasn't kept up with the pace of the networking industry.

Cisco command for change path cost is "spanning-tree pathcost method long". This command changes the path cost to increase it from a 16-bit value to a 32-bit value. More bits in the path cost value increases the range of possible link speeds. You can confirm the path cost method being used on your Cisco switch with "show spanning-tree summary" command.

By default, Cisco switches use the original spanning tree "short mode" path costs using a 16-bit value. However, as interface bandwidth has increased the 16-bit value does not provide room for future high-speed interfaces. Using the newer spanning tree "long mode" path cost using a 32-bit value provides more granularity in data centers that use extremely high-speed interfaces. 

Visit the following link to get more details:

 https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1193602

Regards,

Deepak Kumar

[huyan]

I see, thank you very much for enlightening me! Everything is clear now!

Best,

Huy.