09-13-2017 04:10 PM - edited 03-08-2019 12:01 PM
Attached is an image which shows the topology my question relates to. I have built the exact same topology and configs in my simulator and received the same results, so it's my understanding that this is correct but I am unsure why.
To identify the designated ports for the segments (Link X and Link Y) between SW-A and SW-C...
As a result, both Fa1/1 and Fa1/2 on SW-C enter the BLOCKING state, and Fa1/3 and Fa1/4 on SW-A become DESIGNATED ports.
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With a port-cost of 1 on SW-C Fa1/1, why has this become a BLOCKED port?
Is the port-cost only utilized in particular circumstances?
Solved! Go to Solution.
09-14-2017 01:55 PM - edited 09-14-2017 01:56 PM
So, try again.
With the designated port election it is the cost advertised on the segment that counts so your figures are not quite right.
So SW-A advertises a cost of 19 onto X and Y and this is the cost that matters ie. unlike the root port election SW-C does not add it's port cost on X and Y to the advertised cost from SW-A.
SW-C also advertises a cost to SW-A of 20 on both X and Y which means the lowest cost for both links is from SW-A which is why SW-C blocks both it's ports.
You can see that unlike the root port election SW-C's port costs on X and Y do not come into play when electing the designated ports for those links.
Jon
09-14-2017 09:27 AM
Bump, post had to be moderated so it dropped to the 2nd page =\
09-14-2017 01:13 PM - edited 09-14-2017 01:23 PM
All that typing on phone and just realised you are asking about DPs not RPs.
I'll get back to you.
Jon
09-14-2017 01:55 PM - edited 09-14-2017 01:56 PM
So, try again.
With the designated port election it is the cost advertised on the segment that counts so your figures are not quite right.
So SW-A advertises a cost of 19 onto X and Y and this is the cost that matters ie. unlike the root port election SW-C does not add it's port cost on X and Y to the advertised cost from SW-A.
SW-C also advertises a cost to SW-A of 20 on both X and Y which means the lowest cost for both links is from SW-A which is why SW-C blocks both it's ports.
You can see that unlike the root port election SW-C's port costs on X and Y do not come into play when electing the designated ports for those links.
Jon
09-14-2017 02:35 PM
09-14-2017 10:44 PM
In this specific topology the port cost on SW-C does not come into play but obviously port costs can make a big difference in the outcome in other scenarios.
Jon
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