STP root switch, specified port, root port, blocking port

STP determines each switch port (specified port: DP; root port: RP; Blocked port: AP)

Judgment steps:

1. Election of the root switch:

Judging object: Select one of all switches running the STP protocol;

Judging by: The switch selects the bridge ID value (switch priority +MAC) The smallest switch as the root bridge in the network;

After determining the root switch, all ports of the switch are designated port DP;

2. Determine the root port of the non-root switch RP, each non-root switch has and only one RP:

Judgment object: Between different ports on a non-root bridge switch;

Basis of judgment:

(1) Minimum Port-to-root bridge path overhead

(2) Minimum sender Bridge ID

(3) The sender port ID is minimal (the port ID has 16 bits, it is composed of 8-bit port priority and 8-bit port number, where the port priority value range is 0-240, the default value is 128, can be modified, but must be a multiple of 16)

3. Determine the specified port for the non-root switch:

Decision object: A designated port is elected between the different ports of each physical network segment.

Basis of judgment:

(1) Bridge-to-root bridge path overhead is minimal

(2) Minimum sender Bridge ID

(3) Minimum sender port ID

4. A blocking port AP for both non-root ports and non-specified ports;

Example Analysis:

1. (Root port Judgment) bridge to root bridge path cost, Sender Bridge ID is the same, port ID is the root port, LSW11, G0/0/1 is the root port:

2. (Specify the port) between the different ports of a physical network segment, the bridge to the root bridge path overhead is the same, all 1, the bridge ID is the specified port, LSW6 and LSW7 two ports connected to select LSW6 port for the specified port

3. (Comprehensive analysis)

Recommended blog:http://www.cnblogs.com/haitao-fan/archive/2012/09/25/2701361.html

STP root switch, specified port, root port, blocking port