STP cost

altjx

Hello TechExams

I'm curious about the way STP costs work. I'm reading the book again (prior to posting here), but I just want to make sure of something. In the topology below, S1 is the root bridge. I thought it sends its cost as 0 by default to all connected switches.

That being said, according to the text, S2 changed its cost on the FA0/1 port to 1. So when it forwards its Hello BPDU to S3, shouldn't the cost be 1 + 19 = 20 (which is default cost for 100 Mbps connected between S3 and S2), or do I have this wrong?

advanex1

The C portion is correct because of how it's worded. Since SW2's FA0/1 port has a cost of 1, the cost when it is sent out of SW2 will be 1. However, when it is then received at SW3, the total cost would be 20 (1+19) (if they are using 100Mbps links). Costs are added when they are RECEIVED and not when they are sent.

Also, the reason SW2 is considered to have FA0/3 as a DP for the segment of SW3/SW2 is because it advertises the lowest cost to the Root Bridge, but you seemed to have already gotten that. So, SW3 FA0/2 would be a NDP or Non-designated port.

altjx

advanex1 wrote: »

The C portion is correct because of how it's worded. Since SW2's FA0/1 port has a cost of 1, the cost when it is sent out of SW2 will be 1. However, when it is then received at SW3, the total cost would be 20 (1+19) (if they are using 100Mbps links). Costs are added when they are RECEIVED and not when they are sent.

Also, the reason SW2 is considered to have FA0/3 as a DP for the segment of SW3/SW2 is because it advertises the lowest cost to the Root Bridge, but you seemed to have already gotten that.

Ahh, gotcha. I didn't even think of it that way. Yeah, I understand the DP part of it. That makes so much more sense. Thank you!

advanex1

No problem.