Wrong Root port selected in PT-6.0

[chandra_rc16]

I just configured the below topology all the bridge id and port priorities are default.

Here Fa0/1 of switch2 should be ROOT port but Fa0/2 is becoming Root.

Can someone please explain me why ? Or i'm wrong somewhere.

O/P:

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

Switch1#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0001.C977.4431

             Cost        19

             Port        2(FastEthernet0/2)

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

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

             Address     00E0.F964.3896

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

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/1            Altn BLK 19        128.1    P2p

Fa0/2            Root FWD 19        128.2    P2p

Fa0/10           Desg FWD 19        128.10   P2p

Fa0/11           Desg FWD 19        128.11   P2p

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

Switch2#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0001.C977.4431

             This bridge is the root

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

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

             Address     0001.C977.4431

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

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/23           Desg FWD 19        128.23   P2p

Fa0/24           Desg FWD 19        128.24   P2p

Fa0/1            Desg FWD 19        128.1    P2p

Fa0/2            Desg FWD 19        128.2    P2p

Thanks in advance.

Regards,

Chandu

[daniel.dib · Accepted solution]

Are the two switches connected by two ports? I can't see the picture. Everything is default?

[chandra_rc16]

Yes. Both switches are connected with two ports.

Sw1 Fa0/1 ---> SW2 Fa0/24

Sw1 Fa0/2 ---> SW2 Fa0/23

I thought it was the software issue and tried using PT in another systems with similar topology but sill the same bug.

I'm attaching my topology. Please take a look. And all the values are default.

O/p from Root swtich:

Switch2#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     00D0.BA39.0927

             This bridge is the root

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

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

             Address     00D0.BA39.0927

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

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/24           Desg FWD 19        128.24   P2p                    //Priorites are default to 128//

Fa0/23           Desg FWD 19        128.23   P2p

Fa0/1            Desg FWD 19        128.1    P2p

Fa0/2            Desg FWD 19        128.2    P2p

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

O/p from non-root switch:

Switch1#show spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     00D0.BA39.0927

             Cost        19

             Port        2(FastEthernet0/2)

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

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

             Address     00E0.F7EB.E8A8

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

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/2            Root FWD 19        128.2    P2p         

Fa0/1            Altn BLK 19        128.1    P2p               // This should be Root port //

Fa0/11           Desg FWD 19        128.11   P2p

Fa0/10           Desg FWD 19        128.10   P2p

Regards,

Chandu

[devils_advocate]

Packet tracer has some odd quirks with spanning tree

[chandra_rc16]

Hi Devil,

Can you please give me a clarification on this:

Root Port Selection:

1. If cost are same then

2. Check priorties on negihbour switch if BPDUs are from one swtich

3. If priorites are same

Question:

4. then compare the Port Numbers of Sending switch or own switch.

Regards,

Chandu

[chandra_rc16]

Hi All,

Bug here is not with PT but with my understanding of Root port selection in STP.

I thought STP will compare Port numbers on switch1 in my topology. So, as per that i was thinking Fa0/1 should be Root port but STP never does that whereas it compares the Port numbers on Switch2.

So since Fa0/23 is lesser than Fa0/24. The port on Switch1 i.e, Fa0/2 connected to the Fa0/23 of Swtich2 will become Root port. That's what happended..

Output from switch1:

Interface        Role Sts Cost      Prio.Nbr Type

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

Fa0/2            Root FWD 19        128.2    P2p         

Fa0/1            Altn BLK 19        128.1    P2p              

Fa0/11           Desg FWD 19        128.11   P2p

Fa0/10           Desg FWD 19        128.10   P2p

This is absolutley correct.

Regard,

Chandu

[chandra_rc16]

Can somone please mark my answer as correct answer ? Because i cannot mark it. And this question will be unanswered forever.

Regards,

Chandu

[daniel.dib · Accepted solution]

Actually PT is behaving correctly here. SW2 is chosen as root because it has lower MAC.

SW1 has two ports connected to SW2. The algorithm is like this:

1. Lowest cost to Root

2. Lowest sender BID (tie here)

3. Lowest sender port ID

Step nr.3 is why Fa0/2 becomes root port because that is connected to Fa0/23 which is lower than Fa0/24. If you want to change the behavior, either connect Fa0/1 to Fa0/23 or on SW2 go to Fa0/24 and configure spanning-tree port-priority 64.

Daniel Dib
CCIE #37149

[chandra_rc16]

Yes Daniel thats correct. I understood it after a deep going through STP root port selection with different ports and priorities.

Regards,

Chandu