Spanning Tree with vPC and non VPC

[BastiiGee]

Hello dear community,
I am currently trying to understand an existing topology and its spanning tree (screenshot attached).
This is probably a topology that deviates from best practice.
We have two nexus switches and a cat switch in use. nex4(secondary) and (primary) nex5 have a peer link on which is vlan 622. both nexus are connected with cat1 also on vlan 622. (not in vpc or portchannel).
The cat switch is root for all vlans. I have used VLAN 622 as an example for my question.
I wonder why nex4 disconnects the uplink to the cat1 switch, although the costs via nex5 are higher.
On the other hand, there is vlan 627, where the cost from nex4 to cat1 has been reduced to 1. But then nex5 blocks its direct link to cat1 and chooses the path via nex4, although the costs would now be the same.
Does this have anything to do with the VPC construct? How is it calculated here which link is blocked?

Here are the outputs from both nexus:(also attached)
sh spanning tree vlan 622
sh spanning tree vlan 627
Sh spanning tree vlan 622 detail

VLAN622:

Nex4#
VLAN0622
Spanning tree enabled protocol rstp
Root ID Priority 4718
Address cccc.cccc.cccc
Cost 3
Port 4105 (port-channel10)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33390 (priority 32768 sys-id-ext 622)
Address bbbb.bbbb.bbbb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po10 Root FWD 1 128.4105 (vPC peer-link) Network P2p
Eth1/29 Altn BLK 2 128.157 P2p

Nex4# sh spanning-tree vlan 622 interface po10 detail

Port 4105 (port-channel10, vPC Peer-link) of VLAN0622 is root forwarding
Port path cost 1, Port priority 128, Port Identifier 128.4105
Designated root has priority 4718, address cccc.cccc.cccc
Designated bridge has priority 33390, address aaaa.aaaa.aaaa
Designated port id is 128.4105, designated path cost 2
Timers: message age 4, forward delay 0, hold 0
Number of transitions to forwarding state: 5
The port type is network
Link type is point-to-point by default
BPDU: sent 17091466, received 17091798

Nex4# sh spanning-tree vlan 622 interface eth1/29 detail

Port 157 (Ethernet1/29) of VLAN0622 is alternate blocking
Port path cost 2, Port priority 128, Port Identifier 128.157
Designated root has priority 4718, address cccc.cccc.cccc
Designated bridge has priority 4718, address cccc.cccc.cccc
Designated port id is 128.391, designated path cost 0
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 7
Link type is point-to-point by default
BPDU: sent 4179530, received 16985131

Nex5# sh spanning-tree vlan 622

VLAN0622
Spanning tree enabled protocol rstp
Root ID Priority 4718
Address cccc.cccc.cccc
Cost 2
Port 158 (Ethernet1/30)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33390 (priority 32768 sys-id-ext 622)
Address aaaa.aaaa.aaaa
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po10 Desg FWD 1 128.4105 (vPC peer-link) Network P2p
Eth1/30 Root FWD 2 128.158 P2p

Nex5# sh spanning-tree vlan 622 interface eth1/30 detail

Port 158 (Ethernet1/30) of VLAN0622 is root forwarding
Port path cost 2, Port priority 128, Port Identifier 128.158
Designated root has priority 4718, address cccc.cccc.cccc
Designated bridge has priority 4718, address cccc.cccc.cccc
Designated port id is 128.199, designated path cost 0, Topology change is set
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 72, received 12429187

Nex5# sh spanning-tree vlan 622 interface po10 detail

Port 4105 (port-channel10, vPC Peer-link) of VLAN0622 is designated forwarding
Port path cost 1, Port priority 128, Port Identifier 128.4105
Designated root has priority 4718, address cccc.cccc.cccc
Designated bridge has priority 33390, address aaaa.aaaa.aaaa
Designated port id is 128.4105, designated path cost 2
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 4
The port type is network
Link type is point-to-point by default
BPDU: sent 17091752, received 17091420

VLAN627:

nex5# sh spanning-tree vlan 627

VLAN0627
Spanning tree enabled protocol rstp
Root ID Priority 4723
Address cccc.cccc.cccc
Cost 2
Port 4105 (port-channel10)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33395 (priority 32768 sys-id-ext 627)
Address aaaa.aaaa.aaaa
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po10 Root FWD 1 128.4105 (vPC peer-link) Network P2p
Eth1/30 Altn BLK 2 128.158 P2p

nex4# sh spanning-tree vlan 627
VLAN0627
Spanning tree enabled protocol rstp
Root ID Priority 4723
Address cccc.cccc.cccc
Cost 1
Port 157 (Ethernet1/29)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33395 (priority 32768 sys-id-ext 627)
Address bbbb.bbbb.bbbb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po10 Desg FWD 1 128.4105 (vPC peer-link) Network P2p
Eth1/29 Root FWD 1 64.157 P2p

[Deepak Kumar · Accepted solution]

Hello,

Your diagrams are not clear. Which is Nex4or Nex5? What is port-channel number?

But keep in mind that vPC have a pre-define rule as VPC will never be blocked Peer-link and that's normal because important traffic is passing through it. If now you take into consideration that VPC peer-link will never be blocked then that's might normal behavior in your topology too.

[paul.driver · Accepted solution]

Hello

Spanning costing can be appended various ways with the higher calculated value being less preferred:
Interface cost - will effect the stp costing throughout the whole stp domain NOT just that interface or any directly connected switches
Port priority - which will affect any downstream directly connected switch.

Looking at your stp readout it seems you have applied the stp costing to effect the entire stp domain and the root switch path will always take precedence