Port Selection:
-> If Non-root bridge have multiple links to Root bridge, Selects port with lower path cost to Root Bridge as Root Port.
-> If multiple paths are available to reach the Root Bridge with the same path cost, three rules are used,
1. Lowest Sending Bridge ID : Select the port connected to the neighbor bridge which has the lowest Bridge ID. If it is tie, goes to 2 point.
2. Lowest Port Priority (of sender [neighbor bridge to non root bridge]): If all the multiple paths go through the same neighboring bridge (so having same bridge ID)
to reach the Root Bridge, Non-Root bridge will select the local port which receives the lowest port priority from neighboring bridge. By default each port is
having priority value of 128 (can be 0-256 multiplier of 16).
3. Lowest Interface number (of sender [neighbor bridge to non root bridge]): Non-Root Switch will select the port which receives the lowest physical port number
from neighbor Switch.
Port states:
————-
-> Total of 5 states.
A port moves through these five states as follows:
-From initialization to blocking
-From blocking to listening or to disabled
-From listening to learning or to disabled
-From learning to forwarding or to disabled
-From forwarding to disabled
-> Blcoking:
*When a switch is first turned on, all the ports, except the root port, are set to blocking state.
*A port in the blocking state does not participate in frame forwarding.so that no traffic can be forwarded until the switch determines the root switch.
*Blocking can eliminate looping in network until all redundant links can be managed properly.
*Discards data frames received from the attached segment.
*Discards data frames switched from another port for forwarding.
*Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)
*Does not transmit BPDUs received from the system module.
*Receives BPDUs and directs them to the system module.
*Receives and responds to network management messages. (topology change BPDUs)
*port in this state is neither a designated nor root port but is recognized as an alternate port to the root.
*After receiving the first BPDU and a 20 seconds delay or,it transits to the listening state.
*Port color is changed to steady amber.
-> Listening:
*The listening state is the first transitional state a port enters after the blocking state.
*The switch enters this state when Spanning-Tree Protocol determines that the port should participate in frame forwarding.
*Learning is disabled in the listening state.
*It remains in this state for 15 seconds.
*When two or more ports are found to be creating a loop, the switch will activate the port(lowest path cost) to learning state and the other ports (higher path cost)
will be disabled.
*Discards data frames received from the attached segment.
*Discards data frames switched from another port for forwarding.
*Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)
*Receives BPDUs and directs them to the system module.
*Processes BPDUs received from the system module.
*Receives and responds to network management messages.
*It is participating in the voting, but it might not win the election.
*This port is neither designated nor root port. it is alternate port to the root.
*Port color is changed to blinking amber
-> Learning:
*A port in the learning state is preparing to participate in frame forwarding.
*This is the second transitional state through which a port moves in anticipation of frame forwarding.
*The port enters the learning state from the listening state through the operation of Spanning-Tree Protocol.
*It remains in this state for 15 seconds.
*Discards data frames received from the attached segment.
*Discards data frames switched from another port for forwarding.
*Incorporates station location into its address database i.e. it tracks MAC addresses and records new MACs (from BPDU packets and from data packets) into MAC table
to prepare for forwarding.
*Receives BPDUs and directs them to the system module.
*Receives, processes, and transmits BPDUs received from the system module.
*Receives and responds to network management messages.
*Port color is blinking amber.
-> Forwarding:
*A port in the forwarding state forwards frames.
*The port enters the forwarding state from the learning state through the operation of Spanning-Tree Protocol.
*Forwards data frames received from the attached segment.
*Forwards data frames switched from another port for forwarding.
*Incorporates station location information into its address database.
*Receives BPDUs and directs them to the system module.
*Processes BPDUs received from the system module.
*Receives and responds to network management messages.
*changes to blinking green color. At this time, STP is converged.
-> Disabled:
*Ports are disabled when they are part of a network loop.
*Discards frames received from the attached segment.
*Discards frames switched from another port for forwarding.
*Does not incorporate station location into its address database. (There is no learning, so there is no address database update.)
*Receives BPDUs, but does not direct them to the system module.
*Does not receive BPDUs for transmission from the system module.
*Receives and responds to network management messages.
vijayababuj 