Root Bridge vs. Designated Switch?
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Author:  Router47 [ Tue Nov 19, 2013 11:27 pm ]
Post subject:  Root Bridge vs. Designated Switch?

I know what the Root Bridge is but my study material has also been talking about something called a "Designated Switch" for a given LAN...that's DIFFERENT from the Root Bridge?

What is this?

Author:  rednectar [ Wed Nov 20, 2013 12:16 am ]
Post subject:  Re: Root Bridge vs. Designated Switch?

You need to learn about Spanning Tree - might be a good start, but it is a bit dry

Here is Spanning Tree in a nutshell
  1. The bridge with the lowest BRIDGE_ID becomes the ROOT_BRIDGE
  2. Bridges send BPDUs out all their DESIGNATED_PORTS - in the case of the root, this will be all ports normally
  3. If a bridge receives BPDUs on more than one port,
    • Decide which port gives the best path back to the ROOT_BRIDGE and mark that port as its ROOT_PORT
    • The bridge that sent the BPDU to that ROOT_PORT is that bridge's DESIGNATED_BRIDGE
    • BLOCK the non-root ports, and (since IEEE802.1D 2004) mark them as ALTERNATE ports
    • Bridges determine the best path back to the root bridge based on cost. If the cost for two paths is the same, then it will choose to go via the neighbouring bridge with the lowest BRIDGE_ID. If more than one path is advertised by the same BRIDGE_ID (ie you have parallel links between two switches), then the bridge will choose the path with the lower PORT_ID

Build yourself a topology in GNS3 using 3 RouterSwitch devices. Connect them in a triangle making sure you use ports 1/x (the switch ports) and not ports 0/x (the router ports) and explore the spanning tree in action. At the same time, draw a diagram of what you have and follow the BPDUs, marking DESIGNATED_PORTS, ROOT_PORTS and ALTERNATE_PORTS (Blocking) as you go.

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