STP incorrect root path cost

[chandra_rc16]

Hi,

I have attached my topology for practicing STP. The issue that i encountered is with switch3.

Switch3 is connected to Switch2 via Fa0/3 port (Path cost =19). Switch 2 is connected to Switch1(Root switch) via Fa0/1 (Path cost=19).

As per STP the root path cost from switch3 should be 19+19 =38. But the output of "show spanning-tree detail" is showing the root path cost to be 19.

Output of Switch3's

----------

Switch3#show spanning-tree detail

VLAN0001 is executing the ieee compatible Spanning Tree Protocol

  Bridge Identifier has priority of 32768, sysid 1, 00D0.BADC.4153

  Configured hello time 2, max age 20, forward delay 15

  Current root has priority 32769

  Root port is 3 (FastEthernet0/3), cost of root path is 19                       // I believe it should be 38 //

  Topology change flag not set, detected flag not set

  Number of topology changes 0 last change occurred 00:00:00 ago

            from FastEthernet0/1

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

Port 3 (FastEthernet0/3) of VLAN0001 is root forwarding

  Port path cost 19, Port priority 128, Port Identifier 128.3

  Designated root has priority 128, address 0001.42B7.1303

  Designated bridge has priority 32769, address 0009.7C05.C258

  Timers: message age 16, forward delay 0, hold 0

  Number of transitions to forwarding state: 1

  Link type is point-to-point by default

Port 10 (FastEthernet0/10) of VLAN0001 is designated forwarding

  Port path cost 19, Port priority 128, Port Identifier 128.10

  Designated root has priority 128, address 0001.42B7.1303

  Designated bridge has priority 32769, address 0009.7C05.C258

  Designated port id is 128.10, designated path cost 19

  Timers: message age 16, forward delay 0, hold 0

  Number of transitions to forwarding state: 1

  Link type is point-to-point by default

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

Please let me know whether i'm worng or its a bug with packet tracer.

Thanks in Advance,

Chandu

[Bharat Negi · Accepted solution]

Yes.  You are right, it should be 38. 

Also could not find address 0009.7C05.C258 in the fouth line of your output.  It must be there if S1 is chosen as STP root.

Regards

Bharat

[chandra_rc16]

Hi Bharat,

Port 3 (FastEthernet0/3) of VLAN0001 is root forwarding

  Port path cost 19, Port priority 128, Port Identifier 128.3

  Designated root has priority 128, address 0001.42B7.1303

  Designated bridge has priority 32769, address 0009.7C05.C258

  Timers: message age 16, forward delay 0, hold 0

  Number of transitions to forwarding state: 1

  Link type is point-to-point by default

For port 3. It is showing the root to be 0009.7C05.C258. I think this is fine.

Will a real equipment show the above MAC in the fourth line of orginal o/p ?

I'm using PT.

Thanks for your reply.

Chandu

[Rasika Nayanajith · Accepted solution]

You can type "show spanning-tree root" command on your Switch3 & see what rootcost value it gives as the output. As per topology if direct the link is down between SW1-SW3 path cost should be 38 to the root. (see some sample output below)

SW#show spanning-tree root

                                                           Root    Hello Max Fwd

Vlan                   Root ID                         Cost    Time  Age Dly  Root Port

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

VLAN0001             1 0006.d604.ed00         6    2   20  15  Te1/1/1        

VLAN0012          4108 1cdf.0f83.6280         2    2   20  15  Te1/1/1        

VLAN0013          4109 1cdf.0f83.6280         2    2   20  15  Te1/1/1       

HTH

Rasika

*** Pls rate all useful responses ***

[chandra_rc16]

I didn't find "root" option in PT. Are you using a real equipment ?

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

Switch3#show spanning-tree ?

  active             Report on active interfaces only

  detail             Detailed information

  inconsistentports  Show inconsistent ports

  interface          Spanning Tree interface status and configuration

  summary            Summary of port states

  vlan               VLAN Switch Spanning Trees

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

Regards,

Chandu

[Rasika Nayanajith]

Hi Chandra,

Yes, I used a real switch & not PT, may be PT does not have all these commands.

Thanks for the rating our responses

Rasika