designated bridge and Root bridge

[v.j]

Hi could you explain wats the diff btwn Designated brige and root bridge. Thanx

[r.cheung]

A designated bridge is the switch just downstream of the root bridge. The root bridge controls the spanning tree topology and is the hub for connecting the other switches. Root bridges are elected based on its bridge id (bridge priority (default of 32768 for all switches) + lowest mac address of the switches)Its recommended to place the root bridge at the center of the vlan.

[david.sisco]

I disagree with the statement that a designated bridge is the switch just downstream of the root bridge. If a LAN segment has more that one bridge which *could* forward packets toward the root bridge, than 1 of them will be elected the designated bridge and all of the other bridges on that segment will be put in a blocked state. If there are no bridging loops anywhere in the LAN, then every bridge is a designated bridge.

PS I have no credentials to answer authoritativley on this subject. However, we have had some spanning tree configuration problems and I have read and reread everything I could find on the Cisco site.

[j.chenevey]

Designated bridge (switch) is the bridge closest to the root switch through which frames will be forwarded to the root.

Root bridge (switch) is the logical center of the spanning tree topology (per VLAN STP in Cisco switches) in a switched network.

[graemerappoport]

Hi all,

Is another way to say it, that the Designated Bridge is the secondary or the backup root bridge?

As in PVSTP, you would configure:

spanning-tree vlan 10 root primary (on the root bridge) and:

spanning-tree vlan 10 root secondary (on the designated bridge, or backup root bridge).

Is this correct or incorrect explaination?

[ADEDEJDA]

I have a different reading of these concepts, designated port and designated bridge should be viewed in terms of LAN segment, including role and port state, and considered together. 

Designated bridge is a switch representing a LAN segment, it is not necessarily the bridge closest to the root bridge, although it is most likely the case. What does it mean to be closest, is it root path cost or what?  In terms of "proximity", the root port is the port on each bridge closest to the root bridge.

This now leads us to the concept of Designated port. This is the designated bridge forwarding port on a LAN segment. i.e the port used by a designated bridge to receive traffic from the direction of the root to the LAN or send traffic from the LAN towards the root.