I am confuse with the term designated and root port.
"The designated port is the port that is the single interface to forward traffic to the root bridge"
"The root port represent a switch's lowest-cost path to the root bridge"
1) Can i say all ports from root bridge port are root ports since they are the lowest-cost path to the root bridge?
2) Say Switch A is the root bridge and Switch B and C is connected through port 1 to Switch A. Can i say port 1 of Switch B and C is a designated port since it is the single interface to forward to the root bridge?
Say Switch A is the root bridge with the lowest mac address. Switch B and C is connected to Switch A through Port 1. Switch B has the second lowest mac address. And both Switch B and C is connected through port 2.
1) Port 2 of Switch C is disable because of loop. But why Port 2 of Switch B is also call a designated port. I thought designated port only apply to root bridge only?
2) You mention "Root port is a port on the switch that leads TO the root bridge" Can i say root bridge can never have root port?
3) You mention that "there is only one designated port per segment" if that is the case, can i say that even root brdge should have only one designated port? But you say all ports on root bridge are designated ports?
I think the following defintions can help sort out *any* STP topology question, regardless of the STP mode. I tried to make them as short as possible, let me know if they still make sense:
Bridges advertise the best information (i.e. the best BPDU) they know of. This information can be collected from BPDUs received or it can be the bridge own's information.
The bridge that has the best information in the network. The information of the root bridge cannot be beaten by any other bridge (root ID is the single most important parameter).
The port on which a given bridge receives the best information (the cost received in the BPDU is incremented by the port cost configured on the interface that received it for the purpose of the root port election).
The port that sends the best BPDU on a given LAN segment.
A port that receiving a better information (BPDU) from another bridge *but* that is not the root port.
A port that is not receivng better information from another port on the same bridge.
So, to come back to the "quiz". How could port X on the root R be not designated? It would mean that it receives better information than the one it would send out. Port X of R sends the following information:
-root ID: R, this value is the best (by definition)
-root path cost: 0, this value is the best
-sender bridge ID: R, this value is the best
-port ID: X
In the best case, a bridge can only match the first three parameters, and this unique bridge is the Root! So if the X is not designated, it can only be the result of a BPDU sent by a port Y on the root, with Y being a better port ID than X. According to the above definition, X will be a backup port. So basically, as you guessed, a Root bridge can have only two kinds of ports: designated or backup.
Note that backup is a blocking state. For some reason, there is a rule floating around that suggest that a root bridge can never block a port...
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