cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
954
Views
5
Helpful
5
Replies

Spanning-Tree interface Cost and Designated ports

RyanB
Level 1
Level 1

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.

STP-question.png

 

To identify the designated ports for the segments (Link X and Link Y) between SW-A and SW-C...

  • The cost through SW-A to the ROOT is 19 + 19 (38) on Link X.
  • The cost through SW-A to the ROOT is 19 + 19 (38) on Link Y.
  • The cost through SW-C to the ROOT is 19 + 20 (39) on Link X.
  • The cost through SW-C to the ROOT is 19 + 20 (39) on Link Y.

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.

 

------

 

With a port-cost of 1 on SW-C Fa1/1, why has this become a BLOCKED port?

  • My initial thought process was that...
    • The cost through SW-C to the ROOT is 19 + 20 (39) on Link X
      • Making Fa1/2 on SW-C a blocked port (as per before)
    • The cost through SW-C to the ROOT is 1 + 20 (21) on Link Y.
      • Making Fa1/1 on SW-C a designated port over Fa1/4 on SW-A (cost of 38)

Is the port-cost only utilized in particular circumstances?

1 Accepted Solution

Accepted Solutions

Jon Marshall
Hall of Fame
Hall of Fame

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

View solution in original post

5 Replies 5

RyanB
Level 1
Level 1

Bump, post had to be moderated so it dropped to the 2nd page =\

Jon Marshall
Hall of Fame
Hall of Fame

All that typing on phone and just realised you are asking about DPs not RPs. 

 

I'll get back to you. 

 

Jon

 

Jon Marshall
Hall of Fame
Hall of Fame

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

Ok, this seems to make sense when you consider the cost advertisements ONTO link X/Y.

The reason for my confusion is I was following a CCNA video course by Raymond Lacoste, and he indicates that to add up the cost to the ROOT bridge, you begin with the cost of the link where the DP election is taking place. This is why I said 19+19, 19 + 20, etc...when in fact it should have just been 19 and 20 respectively.

So, would you agree that in this particular scenario - having a port-cost of 1 on SW-C Fa1/1 has zero impact on the outcome of this STP topology?
It seems that way, perhaps I was just trying to justify the configuration by implementing it into the cost calculation when it was irrelevant.

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

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: