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madmin.kz
Beginner

alt port role in PVST?

Please explain the output of show spanning-tree summary:

SW1-R3#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    24577

             Address     108c.cfb6.ab00

             Cost        19

             Port        15 (FastEthernet0/13)

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

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     108c.cfb6.9a00

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

             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

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

Fa0/1               Desg FWD 19        128.3    P2p

Fa0/5               Desg FWD 19        128.7    P2p

Fa0/13              Root FWD 19        128.15   P2p

Fa0/14              Altn BLK 19        128.16   P2p

Fa0/15              Altn BLK 19        128.17   P2p

Fa0/13-15 connected to the switch SW-R2

I can not understand Fa0/14-15 Port role is Alt, because the role of Alternative port was introduced in RSRP. In our case, STP in PVST Mode.

Can you explain this please?

1 ACCEPTED SOLUTION

Accepted Solutions
Arumugam Muthaiah
Cisco Employee

Hi,

Please see the below discussion which explain about your query

It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it's also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik)

Running STP nowadays should be in fact running RSTP in STP compatibility mode. And in that case, the switch would definitely display the RSTP roles.

Refer the below discussion:

https://supportforums.cisco.com/message/581353#581353

Regards,

Aru

*** Please rate if the post is useful ***

Regards, Aru *** Please rate if the post useful ***

View solution in original post

2 REPLIES 2
Arumugam Muthaiah
Cisco Employee

Hi,

Please see the below discussion which explain about your query

It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it's also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik)

Running STP nowadays should be in fact running RSTP in STP compatibility mode. And in that case, the switch would definitely display the RSTP roles.

Refer the below discussion:

https://supportforums.cisco.com/message/581353#581353

Regards,

Aru

*** Please rate if the post is useful ***

Regards, Aru *** Please rate if the post useful ***

View solution in original post

Thank you for a clear answer.

in STP has a lot of surprises