Cisco Systems: Spanning tree inconsistent port

Problem
The topology is arranged in a star, a links terminate at a pair of Cisco 4503-E switches, problem occurred that some switches could not reach the Cisco 4503-E switches after the 4503 switches became primary and secondary root.

Solution

A-4503-E#sh spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     6073.5c5c.2400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     6073.5c5c.2400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi3/1               Desg BKN*4         128.257  P2p *TYPE_Inc
Gi3/3               Desg BKN*4         128.259  P2p *TYPE_Inc
Gi3/4               Desg BKN*4         128.260  P2p *TYPE_Inc
Gi3/6               Desg FWD 4         128.262  P2p Peer(STP)
Gi3/7               Desg BKN*4         128.263  P2p *TYPE_Inc
Gi3/8               Desg FWD 4         128.264  P2p


A-4503-E#

Desg BKN*4 the port is blocked although it should be forwarding. The blocking is due to port inconsistent state as indicated by P2p *TYPE_Inc

To verify if the ports are inconsistent use this command:

A-4503-E#sh spanning-tree inconsistentports

Name                 Interface                Inconsistency
-------------------- ------------------------ ------------------
VLAN0001             GigabitEthernet3/1       Port Type Inconsistent
VLAN0001             GigabitEthernet3/3       Port Type Inconsistent
VLAN0001             GigabitEthernet3/4       Port Type Inconsistent
VLAN0001             GigabitEthernet3/7       Port Type Inconsistent

Number of inconsistent ports (segments) in the system : 4

The downstream switches have turned off DTP, the 4503-E downlink ports however are not trunk when issuing show interfaces trunk, no trunk was displayed.

The solution to this problem is to hard code the trunk link and turn off DTP on all trunk links.
The results:

A-4503-E#sh spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     6073.5c5c.2400
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     6073.5c5c.2400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi3/1               Desg FWD 4         128.257  P2p
Gi3/3               Desg FWD 4         128.259  P2p
Gi3/4               Desg FWD 4         128.260  P2p
Gi3/6               Desg FWD 4         128.262  P2p Peer(STP)
Gi3/7               Desg FWD 4         128.263  P2p
Gi3/8               Desg FWD 4         128.264  P2p


A-4503-E#

Another example
Gi3/2 is in inconsistent state, the port that should be forwarding is blocking now.

B-4503-E#sh spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     6073.5c5c.2400
             Cost        8
             Port        263 (GigabitEthernet3/7)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    28673  (priority 28672 sys-id-ext 1)
             Address     6073.5c5c.2380
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi3/2               Desg BKN*4         128.258  P2p *TYPE_Inc
Gi3/3               Altn BLK 4         128.259  P2p
Gi3/4               Altn BLK 4         128.260  P2p
Gi3/5               Desg FWD 4         128.261  P2p Peer(STP)
Gi3/6               Desg FWD 4         128.262  P2p Peer(STP)
Gi3/7               Root FWD 4         128.263  P2p
Gi3/8               Altn BLK 4         128.264  P2p

B-4503-E#sh int trunk

Port        Mode             Encapsulation  Status        Native vlan
Gi3/3       on               802.1q         trunking      1
Gi3/4       on               802.1q         trunking      1
Gi3/5       on               802.1q         trunking      1
Gi3/6       on               802.1q         trunking      1
Gi3/7       on               802.1q         trunking      1
Gi3/8       on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi3/3       1-4094
Gi3/4       1-4094
Gi3/5       1-4094
Gi3/6       1-4094
Gi3/7       1-4094
Gi3/8       1-4094

Port        Vlans allowed and active in management domain
Gi3/3       1
Gi3/4       1
Gi3/5       1
Gi3/6       1
Gi3/7       1

B-4503-E#

Gi3/2 is not a trunk, while other interfaces are trunk and turned off DTP. Because there is no DTP negotiation the gi3/2 cannot negotiate as a trunk.

B-4503-E#sh int trunk

Port        Mode             Encapsulation  Status        Native vlan
Gi3/3       on               802.1q         trunking      1
Gi3/4       on               802.1q         trunking      1
Gi3/5       on               802.1q         trunking      1
Gi3/6       on               802.1q         trunking      1
Gi3/7       on               802.1q         trunking      1
Gi3/8       on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi3/3       1-4094
Gi3/4       1-4094
Gi3/5       1-4094
Gi3/6       1-4094
Gi3/7       1-4094
Gi3/8       1-4094

Port        Vlans allowed and active in management domain
Gi3/3       1
Gi3/4       1
Gi3/5       1
Gi3/6       1
Gi3/7       1

B-4503-E#show int trunk

Port        Mode             Encapsulation  Status        Native vlan
Gi3/3       on               802.1q         trunking      1
Gi3/4       on               802.1q         trunking      1
Gi3/5       on               802.1q         trunking      1
Gi3/6       on               802.1q         trunking      1
Gi3/7       on               802.1q         trunking      1
Gi3/8       on               802.1q         trunking      1

Port        Vlans allowed on trunk
Gi3/3       1-4094
Gi3/4       1-4094
Gi3/5       1-4094
Gi3/6       1-4094
Gi3/7       1-4094
Gi3/8       1-4094

Port        Vlans allowed and active in management domain
Gi3/3       1
Gi3/4       1
Gi3/5       1
Gi3/6       1
Gi3/7       1

Port        Vlans allowed and active in management domain
Gi3/8       1

Port        Vlans in spanning tree forwarding state and not pruned
Gi3/3       none
Gi3/4       none
Gi3/5       1
Gi3/6       1
Gi3/7       1
Gi3/8       none
B-4503-E#

The gi3/2 is indeed in inconsistent state.

B-4503-E#show spanning-tree inconsistentports

Name                 Interface                Inconsistency
-------------------- ------------------------ ------------------
VLAN0001             GigabitEthernet3/2       Port Type Inconsistent

Number of inconsistent ports (segments) in the system : 1

B-4503-E#

Hard code the trunk and turn off DTP negotiation

B-4503-E(config)#int gi3/2
B-4503-E(config-if)#switchport mode trunk
B-4503-E(config-if)#switchport nonegotiate
B-4503-E(config-if)#do sh spanning vlan 1

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     6073.5c5c.2400
             Cost        8
             Port        263 (GigabitEthernet3/7)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    28673  (priority 28672 sys-id-ext 1)
             Address     6073.5c5c.2380
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi3/2               Desg FWD 4         128.258  P2p
Gi3/3               Altn BLK 4         128.259  P2p
Gi3/4               Altn BLK 4         128.260  P2p
Gi3/5               Desg FWD 4         128.261  P2p Peer(STP)
Gi3/6               Desg FWD 4         128.262  P2p Peer(STP)
Gi3/7               Root FWD 4         128.263  P2p
Gi3/8               Altn BLK 4         128.264  P2p


B-4503-E(config-if)#

When BPDU is received from a non-trunk port, the port will go into inconsistent state.

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2 Responses to Cisco Systems: Spanning tree inconsistent port

  1. mp says:

    Hi,
    happy new year all. Great post. I ran into this problem many times. The solution for this I believe is actually in the logs.
    Do a “show log” and you should see something indicating that a per-vlan BPDU has been received on an access port (something like that).

  2. cyruslab says:

    Great tip! Thanks for sharing! 🙂

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