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in stp ,what's the difference between designated port and root port?

Posted on 2001-09-13
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they are both the lowest cost way to reach root bridge,are't they?
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Question by:bitsnake
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by:bsadlick
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The root port is the lowest cost path to the root bridge, but the designated port is the path to the designated bridge, which is the only device allowed to forward frames to/from a LAN. So while both ports are the lowest cost path to the root bridge, the Dport has the additional distinction of being the only path in or out.
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by:SteveJ
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The root port is the port closest to the root switch and there's only one root port per switch. Designated ports connect to switches other than the root switch, which only has designated ports. If a port is neither a root port or a designated port it is in a blocking state. However, I think that ports in a blocked state can still send and receive configuration BPDUs.


Good luck.
Steve
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scraig84 earned 100 total points
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I agree with SteveJ - mostly.  A designated port can be viewed as a downstream port, connected to other switches, or devices, away from the root.  A designated bridge is an upstream switch that another switch connects its root port to.  In other words a switch will elect one port as the root port, which connects to another switches (the designated bridge) designated port.  Each switch may act as the designated bridge for another switch, assuming that it is the lowest cost path towards the root.  Whoo, that is a mouthfull!  

However, while a blocked port still receives BPDUs it does not send them.

Cisco's documentation says the following:

A port in the blocking state performs as follows:

Discards frames received from the attached segment.

Discards frames switched from another port for forwarding.

Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)

Receives BPDUs and directs them to the system module.

Does not transmit BPDUs received from the system module.

Receives and responds to network management messages.


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