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STP cost
altjx
Member Posts: 194
in CCNA & CCENT
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?
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?
CompTIA: A+, Security+, Network+
Microsoft: MCTS: Windows 7, Configuring, MCTS: Windows Server 2008 Applications Infrastructure, Configuring
Cisco: CCENT, CCNA
Microsoft: MCTS: Windows 7, Configuring, MCTS: Windows Server 2008 Applications Infrastructure, Configuring
Cisco: CCENT, CCNA
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advanex1
Member Posts: 365 ■■■■□□□□□□
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.Currently Reading: CISM: All-in-One
New Blog: https://jpinit.com/blog -
Optionsaltjx
Member Posts: 194
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!CompTIA: A+, Security+, Network+
Microsoft: MCTS: Windows 7, Configuring, MCTS: Windows Server 2008 Applications Infrastructure, Configuring
Cisco: CCENT, CCNA -
Optionsadvanex1
Member Posts: 365 ■■■■□□□□□□
No problem.Currently Reading: CISM: All-in-One
New Blog: https://jpinit.com/blog