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Mario Rosi
Beginner

STP cost on VPC port channel

Hi, 

I installed a couple of Nexus 9396PX (NX-OS standalone) configured in vPC.

Unfortunately, as requested by the customer they where not the root bridge of STP (MST has been used) because the core switches are a couple of HP they are connected to in square shape through portchannel (then the Nexus ones collect other switches or UCS below as usual through vPC member ports.

That means that one of HP switches is the root bridge and one link from the Nexus has the port in STP blocking.

 

Now, I've a couple of questions:

1) We know that normally the VPC switches should be root bridge of STP and that STP makes the peerlink always UP and in forwarding; in my scenario, how could be possible that it happens however? ...I mean, I could think for instance to low the STP cost of the port from the Nexus towards the HP no root bridge and at that point STP could force the peerlink to get in blocking state, right? Or there is a hidden mechanism that override the normal STP computation? 

2) All the VPC port member are 10Gbps interfaces towards the access switches/UCS; in accord with cisco doc "MST uses the long path-cost calculation method" and that means:

Bandwidth
Short Path Cost Method Port Cost
Long Path Cost Method Port Cost
10 Mbps
100
2,000,000
100 Mbps
19
200,000
1-Gigabit Ethernet
4
20,000
10-Gigabit Ethernet
2
2,000

 

Now, what i see is:

VPC_Nexus_1# sh spanning-tree  <-- is the NExus directly connected to HP core root bridge

MST0000
  Spanning tree enabled protocol mstp
  Root ID    Priority    4096
             Address     001b.3f55.8e00   <-- is the HP switch
             Cost        0
             Port        4195 (port-channel100)
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16384  (priority 16384 sys-id-ext 0)
             Address     58f3.9ca3.6825
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Desg FWD 200       128.4096 (vPC) P2p <-- VPC towards the access switch
Po2              Desg FWD 200       128.4097 (vPC) P2p <-- VPC towards the access switch
Po10             Desg FWD 1000      128.4105 (vPC peer-link) Network P2p <-- peer link
Po100            Root FWD 200       128.4195 (vPC) P2p <-- VPC towards the HP_1

Po200            Desg FWD 200 128.4295 (vPC) P2p <-- VPC towards the HP_2

 

The only STP cost that is correct is the one concerning the peer link made of 2 X 10G; i don't understand where come from the "200" value for the vpc channels!

 

Thanks for your feedback,

Mario

4 REPLIES 4
sachinga.hcl
Enthusiast

Hi Mario,

 

Could you please share your network diagram and configuration on these switches, so that I can help you further.

 

Best Reagrds

Sachi Garg

Hi Sachi,

thanks for your support, in attachment you find the configuration of both N9K and the scheme.

The dotted links towards HP switches are because not physically connected but just pre-configured for when they will be in place there.

 

I'm looking forward to reading your email.

 

Best regards,

Mario

actyler1001
Beginner

I know this is a really old post, but I've configured a VPC link with a couple of Nexus switches that are also not setup as root and am seeing a 2 Gbps VPC uplink to the root result in a cost of 200.  Correct me if I am wrong, but the long method for a 2Gbps link should be 10,000 right?  What's going on here?

Only 3 switches in the environment.  Cisco 2960 is root, and two Nexus switches with a 20Gbps peer link between them.  Spanning tree reports that link with a cost of 1000.  The uplink from the two Nexus switches is a VPC 2Gbps link to the 2960.

vincent.jestin
Beginner

Hello,

"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."

Source: http://www.cisco.com/c/en/us/products/collateral/switches/nexus-5000-series-switches/guide_c07-673997.html

Have a good day,

Vincent

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