Only desg port state in some Vlan no Root is this normal

[alanhong]

Hi guys

am not sure if this is correct need to get some more idea on this

i have 4 Vlans on my switch and when i run a spanning-tree on my switch  ce500 or 2960 i get

VLAN0050

  Spanning tree enabled protocol rstp

  Root ID    Priority    32818

             Address     0016.c8e3.3d00

             Cost        31

             Port        1 (GigabitEthernet1)

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

  Bridge ID  Priority    32818  (priority 32768 sys-id-ext 50)

             Address     0018.1963.6980

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

             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Gi1              Root FWD 19        128.1    P2p

Gi2              Desg FWD 4         128.2    P2p

Fa1              Desg FWD 19        128.3    P2p

i have aleast one port on the switch in root port role in a particular Vlan except for  one VLAN which has all ports in DESG role and no root port is this normal ?

VLAN0002

  Spanning tree enabled protocol rstp

  Root ID    Priority    32770

             Address     0018.1963.6980

             This bridge is the root

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

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)

             Address     0018.1963.6980

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

             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Gi1              Desg FWD 19        128.1    P2p

Gi2              Desg FWD 4         128.2    P2p

Fa1              Desg FWD 19        128.3    P2p

Fa3              Desg FWD 19        128.5    P2p VLAN0002
  Spanning tree enabled protocol rstp
  Root ID    Priority    32770
             Address     0018.1963.6980
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0018.1963.6980
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1              Desg FWD 19        128.1    P2p
Gi2              Desg FWD 4         128.2    P2p
Fa1              Desg FWD 19        128.3    P2p
Fa3              Desg FWD 19        128.5    P2p

[Peter Paluch · Accepted solution]

Hello Alan,

i have aleast one port on the  switch in root port role in a particular Vlan except for  one VLAN which  has all ports in DESG role and no root port is this normal ?

If, for a particular VLAN, this output occurs on a single switch only then it is normal - it would mean that the particular switch is the root switch for the VLAN.

If multiple switches in a single VLAN are displaying the similar output, i.e. all ports being Designated, then there is something wrong with your network, and it would need closer attention.

Best regards,

Peter

[Somasundaram Jayaraman · Accepted solution]

Hi,

Root Switches will not have root ports. All the port states will be in designated forwarding.

++ Least cost path from the bridge.

After the root bridge has been  chosen, each bridge determines the cost of each possible path from  itself to the root. From these, it picks one with the smallest cost (a  least-cost path). The port connecting to that path becomes the

root port of the bridge

++ Non root switches will learn the root switch Mac by the root port.

++ This is a expected behaviour.

In order to make the interested switch as a root for all the vlans you can use the command "spanning-tree root primary" globally

Link for troubleshooting spanning-tree issues:

================================

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml

- You can use the command show spanning-tree detail | i ieee|from|occur|is exec, in order to find out from which interface you received the TCN from and at what time.

Hope this helps.

Cheers

Somu

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