explanation required 5

Answered Question
Jun 27th, 2007

What exactly is a designated bridge? What does one mean by "it is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge."

I am confused as to what exactly "to and from the LAN" means.

I have this problem too.
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Correct Answer by mohammedmahmoud about 9 years 5 months ago

Hi,

Despite that your example is wired, but the answer is correct, switch B will be the designated bridge for this segment of your network.

HTH,

Mohammed Mahmoud.

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mohammedmahmoud Wed, 06/27/2007 - 06:01

Hi,

A converged STP topology results in only one switch forwarding onto each LAN segment (One Designated Switch per each LAN segment). The switch that forwards onto a LAN segment is called the designated switch for that segment, and the port that it uses to forward frames onto that segment is called the Designated Port (DP). By definition, only the DP on that segment should forward frames onto the segment. (The DP forwards the traffic to the network segment ? In the direction from the Root switch)

HTH,

Mohammed Mahmoud.

neerav_kumar Wed, 06/27/2007 - 06:16

Just to confirm that I got that please verify the designated bridge in the following configuration.

Switch A (root bridge) is connected directly to switch B. Switch B, C and D form a cyclic loop. So in this case switch B is the designated bridge. Right?

Correct Answer
mohammedmahmoud Wed, 06/27/2007 - 06:23

Hi,

Despite that your example is wired, but the answer is correct, switch B will be the designated bridge for this segment of your network.

HTH,

Mohammed Mahmoud.

Wilson Samuel Wed, 06/27/2007 - 20:44

Hi Neerav,

STP at times could be bit confusing in terms, however its pretty simple to understand. Incontinuation to Mohd. please remember the following:-

1. STP or Spanning Tree Protocol is a L2 Protocol to have redundancy in a Looped network, at the same time the Loop is 'dynamically blocked' to avoid any 'looped packets'

2. In terms of STP you will find the term Bridges e.g. Root Bridge, Designated Bridge etc., simply put Switches are an enhanced prototype of Brdige however the term continued despite the fact that Bridges bid their farewell a long time agon.

3. Every STP Domain will have Root Bridge (on which all the ports are forwarding + Would be 'Central reference point for the Spanning Tree'

4. Designated Bridge, is always for a given segment (Root Bridge is for entire Spanning Tree instance), on that particular Segment, the Designated Bridge would have the port in forwarding mode or in other terms if 2 Bridges/Switches from the same STP instance are connected to a LAN Segment, the Designated Bridge (or Switch) would have its port in FWD mode on all segments (for that STP instance)

I hope that it did some value add to your KB.

Kind Regards,

Wilson Samuel

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