NX-OS STP port costs

[Isaiah]

What are the spanning-tree default port costs as derived from the interface speeds beyond 10Gb - specifically when port-channeled?  I only found the table up to speeds of 10Gb in the documentation at http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5500/sw/layer2/7x/b_5500_Layer2_Config_7x/config_multiple_spanning_tree.html.

[Reza Sharifi · Accepted solution]

See table-4 in this link:

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/6-x/layer2/configuration/guide/b_Cisco_Nexus_9000_Series_NX-OS_Layer_2_Switching_Configuration_Guide/b_Cisco_Nexus_9000_Series_NX-OS_Layer_2_Switching_Configuration_Guide_chapter_0110.html

For 40Gig is 500

So, if you have a Portchannel with 2 40Gig interface in it the priority should be 250

HTH

[Isaiah]

Thanks Reza.  Maybe what I am seeing then is due to the fact that the port-channels are also vPCs?  I am seeing every port-channel on my Nexus 5500 switches with a spanning-tree cost of 200, regardless of whether there is one or two (local) 10Gb links in the port-channel - but then, they are all vPCs as well.  Every non-vPC'ed interface cost does align with that documentation.  Does vPC change the STP cost calculation behavior?

[Reza Sharifi]

Hi,

vPC should not have any impact on STP port cost.

Here is a vPC peer link with 2 40Gig interfaces in it.

xxxxxx-x# sh spanning-tree interface port-channel 200

Mst Instance Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
MST0000 Desg FWD 250 128.4295 (vPC peer-link) Network P2p

 HTH

[Isaiah]

I found this in documentation:

vPC links have a predefined cost of 200. This cost is hard-coded and does not depend on the number of links in the PortChannel. If you need to modify the spanning-tree cost of links in the topology to obtain a certain forwarding topology, you can use the command spanning-tree cost <cost>.

It was in the "Data Center Routed Access Design with Cisco Nexus 5500 Platform and 2000 Series Fabric Extenders" guide at http://www.cisco.com/c/en/us/products/collateral/switches/nexus-5000-series-switches/guide_c07-673997.html#_Toc297326260.

[Reza Sharifi]

Isaiah,

Thanks for sharing the link.  Now, I wonder if the vPC STP cost is based on what platform we use.

Reza