STP understanding

The basic concept includes STP BID (bridge ID) switch. Port ID. Port cost. Port status. Port role.
You can change the port priority of the current port through the implementation of stp port priority. It can directly affect whether the port is elected as the designated port
to modify the default port cost calculation method by using the stp pathcost-standard command
stp pathcost-standard (do1d-1998 / dol1t / legacy)
to different configurations of path cost value, cost may be separately performed the STP
the STP selection process:
1. election root bridge
election root port 2. non-root switches
3. each line designated ports election
4. blocked non-root, non-designated port
election root bridge, a root port, a designated port , blocked ports in the following election rules are as small as possible
this information has in the BPDU.
Comparison Rule:
1.bridge ID: Priority (default 32768) + MAC address
2.Cost path cost: the ratio is calculated according to the bandwidth interfaces
3.port ID: Priority (default 128) + port number
root bridge election:
1. Select Root bridge: assuming that each switch itself as the root, send messages to each other BPUD, and then by comparing the root bridge election rules
2. Select the root port: each switch according to elect the root port BPDU transmitted by a root bridge overhead (best path), the switch receives accumulated overhead, forwarding is not accumulated
3 is selected from the specified port: for each link to the specified port is selected according to comparison rules, each link must have a designated port.
4. blocked port is selected from: In addition to the root port, designated port, the remaining ports is blocked
STP port roles:
the root Port: root port, the switch port nearest to the root bridge,
non-root has one and only one switch port RP, RP is no root switch (root port) port
located upstream network port corresponding to the port by RP must DP ( designated ports) port
to a switch port role in the determination of all, we must first determine the port RP role in determining the role of other ports
designated port: notice of a network segment BPDU port as the designated port, each segment has one and only a DP port. Copy traffic forwarded to the root bridge
ALternate port: AP current switch port to the root switch is suboptimal path, a backup port switch port RP
backup port: BP refers to an interface port receives from another interface to bridge their own more preferably when the generated BPDU, the port called backup port. BP to provide backup for the DP, it will remain in the discarding state.
Port role calculation: the calculation process to determine the root bridge, then RP.DP. and other port role.
Comparative calculation process will sequentially BPDU topological information, such as BID (bridge ID), Pathcost (port cost). And portID designated bridge ID (port ID), the smaller the priority value