Conflicting information on STP's Designated Port election.

Params7

What is the first criteria for selecting a Designated Port in a segment?

Many sources including practice guides tell me that the first criteria is lower BID between the two switches. Switch with lower BID becomes the DP. However, ICND2's Press guide (Wendell Odom) says it is the Root Port cost (switch with the lowest Root Port cost becomes the DP and BID is only considered if the Root Cost's tie).

For those who have given the ICND2 exam, what does Cisco accept as its answer?

EdTheLad

STP uses BID which is a combination of the root priority and switch mac-address to elect the root-bridge. Every port on a Root-bridge becomes designated. All other switches i.e. nonRoot-bridges have one root port and potentially other ports which could be designated,blocked etc

The Root-Bridge is the switch that generates BPDUs, these BPDUs are flooded out all Designated ports. One field within the BPDU is the root-path-cost, when a nonRoot-Switch receives a BPDU it will add the local cost of the receiving interface to the received root-path-cost and the new root-path-cost is sent in BPDUs forwarded out designated ports on this switch to neighboring switches.

If a switch has multiple paths towards the Root-Bridge i.e. BPDUs are received on multiple ports, the switch needs to calculate the lowest path cost to root. The port with the lowest root path cost becomes the root port. As mentioned above, the lowest path cost is calculated using the received BPDU root-path-cost value + interface cost.
If the two ports calculated the root-path-cost, and both costs were equal, the bpdu received with the lowest bridge-id would become root. The BPDU frame has a root-id and bridge-id, whenever a switch forwards on a received bpdu it sets the bridge-id using its (bridge priority+mac-address) , so that's whats used if the root-path-cost is equal. If there happen to be 2 parrallel links to the same switch, the BID of this neighbor would be equal, in this case the switch would use the port priority field in the received bpdu. If the two bpdu's have the same received port priority, the lowest receiving port number on the local switch is the last tie-breaker.

Jon_Cisco

This thread helped me a lot when I was studying this last year.

http://www.techexams.net/forums/ccna-ccent/97465-stp-question.html

Params7

Thanks that clears DP selection for me now. Cost of Root Port, then Bridge ID as the Press book explains it. Thanks a ton folks.