Designated ports -- not only on the root bridge?

Hi All,

My understanding of designated ports were that they were all of the active ports on the root bridge.

........[SWA]
......../.......\
......./.........\
....[SWB]---[SWC]

Please ignore the dots. I had to use them to get the drawing to look correct. Basically we have a triangle of switches.

In the above example, let's say that SWA is the root bridge (because it has the lowest BID).

Both of its ports are designated ports, correct?

Since B and C are not the root bridge (but are attached to it), are any of their ports considered designated ports?

Thanks,
T800

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Comments

APA

Switch b and C ports will be root ports as they connect back to the root bridge.......

There will only ever be one designated port per segment and they are determined by the lowest cost path back to the root-bridge..... So going by your diagram I would suggest that the desginated ports will be on the root bridge as it connects to both segments....

But remember that all switchports on the root bridge are designated ports and all are in the forwarding state anyway!!!!!

The ports that connect switch C and B together will be considered non-designated ports and will be prevented from sending or receiving traffic a so a loop doesn't occur.

good question as I am studying for my CCNA at the moment as well and it helped me revise STP...... I'm pretty certain I have given the right advice... Someone correct me if I am wrong

cristi.grigore

A designated port will be elected for each switched segment (each ethernet link in you case). For the topology you provided, you will have 3 designated ports. All the ports on the root bridge are automatically considered designated ports, so the only thing that remains is to elect a designated port for the segment between SWB and SWC.

The designated port for a switched segment will be selected on the bridge with that smallest cost towards the root bridge. If the costs are equal, the tie will be broken by the smallest BridgeID.

All ports on all the bridges that participate in STP that are not designated or root ports will be put in the blocked state.

The reason behind a designated port can be visualized in the following topology:

        SwA
        / \
      /     \
    /         \
  /             \
SwB-----Hub-----SwC
         |
        Host

If the segment between SwBand SwC would not have a designated port, both ports on the segment (the one on SwB towards SwC and the one on SwC towards SwB) would be transitioned to the blocked state, cutting off the Host form the rest of the network.

By electing the port on SwB as a designated port, only the port on SwC will be blocked, leaving SwB to forward frames from an to the Host.

You can read more about it here http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Does this answer your question?

t800

Super answer! Let my buy you a beer.

T800

tech-airman

t800 wrote:

Hi All,

My understanding of designated ports were that they were all of the active ports on the root bridge.

........[SWA]
......../.......\
......./.........\
....[SWB]---[SWC]

Please ignore the dots. I had to use them to get the drawing to look correct. Basically we have a triangle of switches.

In the above example, let's say that SWA is the root bridge (because it has the lowest BID).

Both of its ports are designated ports, correct?

Since B and C are not the root bridge (but are attached to it), are any of their ports considered designated ports?

Thanks,
T800

t800,

Here's how I remember:

Root SWITCHES have Designated PORTS.
Designated SWITCHES have Root PORTS.

The Port type refers to the Switch type that is connected on the other end of the cable.

networker050184

You have to think about it in larger context than these little three switch diagrams. You will most likely have more than three switches connected in a loop and not all of them will be directly connected to the root. Then there will be segments between switches that neither are the root bridge. There would still be a designated port for the segment.

A
/...\
B.....C
/.........\
D..........E

In this simple little diagram say A is the root. So what will the be the designated port on the segment between D and B? The one advertising the lowest root path cost onto the segement which would be B. Now connect them all like spider webs (because you would in real life for redundancey) and then try to figure out the spanning tree!