CCNP - Switch question - port-priority not acting as expected?

crrimson

I'm studying for CCNP switch and I have a lab consisting of 2x3550's and 2x2950 switches. I have 2 redundant connections to each switch from each switch, as shown in the attached picture.

I'm playing with STP and I'm finding that the command "spanning-tree port-priority [1]" is not working as expected.
In this example, switch DLS1 is root.



I am running commands on ALS1
ALS1#sh span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000a.b747.0980
Cost 19
Port 7 (FastEthernet0/7)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 001b.5474.6600
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

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Fa0/7 Root FWD 19 128.7 P2p
Fa0/8 Altn BLK 19 0.8 P2p <

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I've set priority for port fa 0/8 to be 0, but I can't get it to become the root port even though the cost on 0/7 & 0/8 are equal. I have tried doing shut/no shut on both ports.

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Here are details from another command:
ALS1#sh spanning-tree interface fa 0/7 detail
Port 7 (FastEthernet0/7) of VLAN0001 is forwarding
Port path cost 19, Port priority 128, Port Identifier 128.7.
Designated root has priority 24577, address 000a.b747.0980
Designated bridge has priority 24577, address 000a.b747.0980
Designated port id is 128.7, 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 7, received 1402


ALS1#sh spanning-tree interface fa 0/8 detail
Port 8 (FastEthernet0/8 ) of VLAN0001 is blocking
Port path cost 19, Port priority 0, Port Identifier 0.8.
Designated root has priority 24577, address 000a.b747.0980
Designated bridge has priority 24577, address 000a.b747.0980
Designated port id is 128.8, 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 1, received 1410

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What gives? My understanding that ports should elect root pot based on:

1) lowest root path cost (both are 19)
2) lowest bridge ID (both are on same switch)
3) lowest port-priority ( FA 0/8 is lower, but still goes into blocking )

Thanks!

crrimson

Going to answer my own question here:

I found if I set the port-priority on the opposite switch DLS1 to a lower value on FA 0/8 THEN it works, and changes the root port that ALS1 elects.

mattau

thats how port priority works. It doesnt do anything on the local switch, must be set on the upstream switch to effect the local switch you're on.

so to influence ALS1 fa0/8 to be root port you need to set DLS1 fa0/8 port priority to be less than fa0/7 on DLS1

MrBrian

What mattau said ^

Port-priority is added to the BPDU before the switch sends it out, and it's of the sending port.. To influence STP when there's two links connecting to the same upstream switch, you can either change the upstream switches port-priorities.. or.. you can change the incoming switch interfaces path costs. Also, note that influencing the local switch ports path costs has higher precedence then the upstream switch port-priority values (The port-priority value is the last possible check in the STP election process).

This video helped me out a bit as I was stuck on this in the past:
Understanding Spanning-Tree Port-Priority - YouTube

crrimson

Thanks for the replies. I just needed to step away for a minute then I figured it out. Atleast after being stuck on something like this for a while it makes the concept stick once I learned what the problem was.