Root bridge with BKL ports??!!

[krunal_shah]

Has anyone came accorss the following situation in Nexus 7K with vPC.

NEXUS1# sh spanning-tree vlan 3871

VLAN3871

  Spanning tree enabled protocol rstp

  Root ID    Priority    36639

             Address     0022.557a.7641

             This bridge is the root

             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    36639  (priority 32768 sys-id-ext 3871)

             Address     0022.557a.7641

             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

Po171            Altn BLK 1         128.4266 (vPC) P2p

Po172            Altn BLK 1         128.4267 (vPC) P2p

Po173            Altn BLK 1         128.4268 (vPC) P2p

Po174            Altn BLK 1         128.4269 (vPC) P2p

VLAN1936

  Spanning tree enabled protocol rstp

  Root ID    Priority    34704

             Address     0022.557a.7641

             This bridge is the root

             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    34704  (priority 32768 sys-id-ext 1936)

             Address     0022.557a.7641

             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

Po20             Desg FWD 1         128.4115 (vPC) P2p

Po110            Desg FWD 1         128.4205 (vPC) P2p

Po173            Desg FWD 1         128.4268 (vPC) P2p

Nexus2# sh span vlan 3871

VLAN3871

  Spanning tree enabled protocol rstp

  Root ID    Priority    36639

             Address     0022.557a.7641

             Cost        1

             Port        4105 (port-channel10)

             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    36639  (priority 32768 sys-id-ext 3871)

             Address     0026.51c9.35c1

             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
Po171            Altn BLK 1         128.4266 (vPC) P2p
Po172            Altn BLK 1         128.4267 (vPC) P2p
Po173            Altn BLK 1         128.4268 (vPC) P2p
Po174            Altn BLK 1         128.4269 (vPC) P2p

As you can see for one vlan the port channels are blocked by STP. I have exactly same configuration for both vlans on all switches.

show vpc consistancy-parameter vpc 173 does not show any of this vlan went into suspended mode in fact shw show vpc brief shows vlans are actually active.

[Zizhen Gao]

What release are you running? Might want to check out CSCtf94815  After double failure recovery, root port-channel goes int blk state. It is triggered by dual-active scenario and is fixed in 4.2(6).

zz

[krunal_shah]

Actually switch is running 4.2(6) code.

[Zizhen Gao]

Let me check on that.. the release info seems to be incorrect in the bug record. I'll get back to you shortly.

zz

[Zizhen Gao]

So developer confirmed that the bug is fixed in 4.2(6) -- appears to be a new bug to me. It should not have a BKL ports on a root, though it is possible to see ports in BKL state in transitioning state.

zz

[krunal_shah]

Down stream switches are HP switches. I am attaching some show comands when issue was occuring. The BLK state of this vlan, did not recover after bouncing the ports.

[lgijssel · Accepted solution]

This is not necessarily a bug. The ports are all in Altn BLK state.

This implies they are all connected to the same layer2 i.e. they are all receiving bpdu's from the same designated port which is also on the root bridge.

The link below explains this state:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#states

regards,

Leo

[milan.kulik]

Hi,

are you sure the Down stream switches are participating in STP?

If not, they might simply forward  STP BPDUs to other ports returning them to the Cisco switches finally.

BR,

Milan