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spanning-tree port selection criteria

I've got 3 links between 2 switches.

Switch 1 is the root bridge for vlan 16.

As far as I know, the port selection criteria is based on the following:

-cost

-bridge id

-port priority

-port id

(for all of these the lower the better)

As on both switches, the cost/bridge-id/port/priority is the same, by default the lower port wins.

Fair enough.

Now I'd like to modify the port selection used by Switch 2 for vlan 16 from fastethernet/13 to Fastethernet/15

To do so, I've modificated the port-priority, setting a value of 16 on Fastethernet0/15 and of 32 on Fastethernet/14, leaving the default (256) on Fastethernet/13

using the commands:

Switch1(config-if)#spanning-tree vlan 16 port-priority 16

and

Switch1(config-if)#spanning-tree vlan 16 port-priority 32

Here is the output of the command

show spanning-tree vlan 16 detail on switch 1:

Switch1(config)#show spanning-tree vlan 16 detail

Port 13 (FastEthernet0/13) of VLAN0016 is forwarding

Port path cost 19, Port priority 128, Port Identifier 128.13.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 128.13, designated path cost 0

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

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 390, received 0

Port 14 (FastEthernet0/14) of VLAN0016 is forwarding

Port path cost 19, Port priority 32, Port Identifier 32.14.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 32.14, designated path cost 0

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

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 392, received 1

Port 15 (FastEthernet0/15) of VLAN0016 is forwarding

Port path cost 19, Port priority 16, Port Identifier 16.15.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 16.15, designated path cost 0

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

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 393, received 1

And here is the output from Switch2:

Port 13 (FastEthernet0/13) of VLAN0016 is blocking

Port path cost 19, Port priority 128, Port Identifier 128.13.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 128.13, designated path cost 0

Timers: message age 1, forward delay 0, hold 0

Number of transitions to forwarding state: 0

Link type is point-to-point by default

BPDU: sent 0, received 879

Port 14 (FastEthernet0/14) of VLAN0016 is forwarding

Port path cost 19, Port priority 128, Port Identifier 128.14.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 16.15, designated path cost 0

Timers: message age 2, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 1, received 880

Port 15 (FastEthernet0/15) of VLAN0016 is blocking

Port path cost 19, Port priority 128, Port Identifier 128.15.

Designated root has priority 24592, address 000f.8f9a.9000

Designated bridge has priority 24592, address 000f.8f9a.9000

Designated port id is 32.14, designated path cost 0

Timers: message age 2, forward delay 0, hold 0

Number of transitions to forwarding state: 0

Link type is point-to-point by default

BPDU: sent 1, received 881

Any idea why Switch2 is piking up interface fastethernet 14??

Thanks!

2 REPLIES
Gold

Re: spanning-tree port selection criteria

So your intention was to get it to use port 15 on switch 2 since it has a priority of 16 on switch 1?

You have the ports cross cabled.

See the line that says designated port id. That is the port on the other end of the line. In this case its easy in that it matches the port number rather than some port offset.

Bronze

Re: spanning-tree port selection criteria

Hi.

It look like physical cable issue, you can swap fa 14 cable to fa 15 cable,

what is status of fast ethernt port all are up ?

-Minu

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