Question to understand the Default port

xkaijinx

Question 1. What do they mean by the "switch that forwards the lowest-cost Hello BPDU onto the segment is the DP."

I thought the Root switch sends out the lowest cost BPDU, and then when a switch receives it they add their cost onto it and then send it back out?

Question 2. Am I correct to assume that anytime a switch is connected to another switch there is a DP? So if I have 1 switch and 3 of its interfaces are connected to 3 other switches, can I am going to have at least 3 DPs on each one of these 3 other switches? (assuming that they have the lowest cost to reach the root)

NetworkVeteran

xkaijinx wrote: »

Question 1. What do they mean by the "switch that forwards the lowest-cost Hello BPDU onto the segment is the DP."

DP = Designated Port, not Default Port.

I thought the Root switch sends out the lowest cost BPDU, and then when a switch receives it they add their cost onto it and then send it back out?

The root switch is not typically connected to every LAN segment in a switched network. So while every actively forwarding port on the root switch is a Designated Port, there will typically exist some Designated Ports that do not belong to the root switch. The Designated Port will be the port sending out BPDUs with the lowest cost to reach the root switch. A key reason to choose a DP is so you don't multiple switches forwarding the packets from a particular LAN segment. Duplication is a bad thing!

Question 2. Am I correct to assume that anytime a switch is connected to another switch there is a DP?

Correct if the link is up/up and running spanning tree. Correct even if there is only one switch.

So if I have 1 switch and 3 of its interfaces are connected to 3 other switches, can I am going to have at least 3 DPs on each one of these 3 other switches?

If each of three ports on switchA are connected to each of three ports on switchB, you'd have 3 DPs and 3 RPs.

(assuming that they have the lowest cost to reach the root)

One of the switches has to have the lowest cost to reach the root switch between the two of them.