Root port selection

[gregorydhenderson1]

Using the topology above: ALS2 is actually a 2960-s (Gig ports). DLS2 is a 3550. 802.1q configured on all links.

Gi1/0/9 is selected as the RP but I don't understand why. DLS2 is the Root Bridge and I was expecting Gi1/0/7 or 8 to be RP.

Here is my show spanning tree for this :

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     0011.2109.5a00
             Cost        19
             Port        9 (GigabitEthernet1/0/9)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     7c95.f32d.f880
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/7             Altn BLK 2         128.7    P2p
Gi1/0/8             Altn BLK 19        128.8    P2p
Gi1/0/9             Root FWD 19        128.9    P2p
Gi1/0/10            Altn BLK 19        128.10   P2p
Gi1/0/11            Altn BLK 19        128.11   P2p Peer(STP)
Gi1/0/12            Altn BLK 19        128.12   P2p Peer(STP)

I just changed the cost on Gi1/0/7 to see if that would change it but it did not. The speed of the links are all the same, so I thought adjusting the cost would make a difference. To me, the port priority of gi1/0/7 should make it the root port if nothing else since it is lower. I'm confused on this one and I have a good grasp on STP. All switches running RPVST and only VLAN 1 is active. Any help would be appreciated.

[Jon Marshall]

Okay thanks for the outputs.

There is definitely something wrong with the costs as you have pointed out.

I don't have access to real switches but I do have access to an online lab which emulate switches and I have used it before for STP and it worked as expected.

If you want to post all configs as attachments I can lab this up tomorrow as I would be interested to see if I can replicate it.

Your understanding matches mine ie. assuming all costs are the same for all links then the direct link should be used and not via another switch which is the case at the moment.

So I don't think you are misunderstanding anything.

Jon

[gregorydhenderson1]

That sounds good. As far as the configs go...I was ACTUALLY just setting up my switches for some lab practice and I ran into this issue. I couldn't let it go and have just been troubleshooting it all day.

So. Just standard RSTP on all switches with 802.1q configured on all links. I'm going to lab it up again as well on a different set of switches. I'm lucky, I have access to about thirty switches for practice.

I really appreciate your input and I will let you know what I find as well, Jon.

[gregorydhenderson1]

Jon, first, thank you for taking the time to look at my problem.

Now, is my initial premise correct? SHOULD Gi1/0/7 or 8 be the RP? It seems to me one of them should since the cost from ALS2 to DLS2 with the current selection of Gi1/0/9 as the RP is 38.

[Jon Marshall]

Yes, as far as I am aware your initial premise is correct.

Both ALS1 and ALS2 are seeing DLS1 as the path to the root switch even though they have direct connections to DLS2.

But the path to root via the direct link should have a better cost and so that should be a superior BPDU.

My initial thoughts -

1) running PVST on that switch may have affected it

or

2) the 3550 is an older switch and it may be an incompatibility issue

Jon

[gregorydhenderson1]

Thank you! As long as I know my STP theory is on track, I can deal with the rest. I'm going to take my CCNP Switch exam this weekend and I didn't want to go in on shaky ground. I'll keep troubleshooting.

[Jon Marshall]

No problem.

I may lab it up tomorrow and will update this thread if I find anything.

Good luck with your exam.

Jon