STP root path cost question

[katerina.dardoufa]

Hello community,

in our environment we have two core swithes, connected via etherchannel (2 x 1Gbps). One core-switch is the root for odd numbered vlans and the other for even. I am trying to document the existing STP topology, but I got a little mixed up (attaced you will find a diagram).

On Access-SW1 gi0/1 is forwarding for both odd and even vlans. Same goes for gi0/2 on SW2. What happens to the ports that connect the two switches together (SW1 - SW2)? Which port should become the designated port for that segment and will that port be forwarding for all vlans or forwarding for odd and blocking for even (vice versa for gi0/1 on SW2)?

The reason this puzzles me is that we have two sites with the above topology and same core switch config, as far as odd and even vlans are concerned.

For site A command output shows that SW1 gi0/2 is forwarding for odd vlans and blocking for even and SW2 gi0/1 forwarding for even and blocking for odd.

For site B command output shows that SW1 gi0/2 is forwarding for all vlans and SW2 is blocking for all vlans!

Which command output is the expected?

How will the cost path for odd and even vlans be calculated for SW1 gi0/2 and SW2 gi0/1?

As I understand SW1 will send a BPDU to SW2 through gi0/1 that says that for odd vlans the cost is 4. SW2 will add the incoming port cost so it will see that for odd vlans the total cost is (4+4) 8? Then SW2 will send a BPDU to SW1 that says that for odd vlans the cost is 4+(what is the cost to reach 65xx-A?). SW1 will add the cost for port gi0/2 so the total cost will be 8+cost to 65xx-A. That is why SW1 will be forwarding on gi0/1 for odd and blocking for even vlans. So I believe that site A has the correct output. Please correct me if I am wrong!!!

Why is site B behaving so differently???

Any insight will be highly appreciated, so that I can figure out if something is wrong.

Thanks in advance,

Katerina

[katerina.dardoufa]

I think I might have made a mistake when calculating the cost for defining the designated port! The two switches will send via BPDU their own cost to reach the root bridge for odd and even vlans and the cost of the path in between them will not be added.

So :

SW1 will send a BPDU to SW2 through gi0/1 that says that for odd vlans the cost is 4.

SW2 will send a BPDU to SW1 that says that for odd vlans the cost is 4+(what is the cost to reach 65xx-A?)

What is the cos for EtherChannels?

In the real environment the cost is 3004 instead of 4. Why is that?

Thanks again,

Katerina

[katerina.dardoufa]

As I understand it, it all comes down to one switch using uplinkfast and the other not. To be more precise, SW1 doesn't have uplinkfast configured, so for even vlans it advertises a cost of 7(I suppose due to etherchannel config) to SW2. SW2 which has uplinkfast enabled, should advertise a cost of 3004 to SW1. The shortest path method is used in spanning tree, so SW1 gi0/2 becomes forwarding and SW2 gi0/1 is blocking for even vlans.

Another post helped me come up with this explanation, which I hope is correct!!!

Thanks!

[Giuseppe Larosa]

Hello Katerina,

yes uplink-fast has these effects in an attempt to avoid local node to become transit for other switches in tha path to the root bridge.

You have answered by yourself to your question

Best Regards

Giuseppe