802.1D summary

BPDU format

2 flag
topology change(TC)
TC Acknowledgment(TCA)

BPDU generate
none-root bridge only generates BPDUs when it receives one on the root port.

Process:
BPDU比较的4项原则：
       address: 0x0180C2000000 and 0x0180C2000010
       1.最低的RootID   
2.最低的Root Path Cost
       3.最低的Sender BID
       4.最低的Port ID
Switch role:
?Root―A forwarding port elected for the spanning-tree topology
?Designated―A forwarding port elected for every switched LAN segment
?Alternate―A blocked port providing an alternate path to the root bridge in the spanning tree
?Backup―A blocked port in a loopback configuration
选择root,port
1.root bridge的选择：
        the LOWEST bridge id (BID).
            The BID consists of:Priority 和 MAC address
                8byte               2byte   +    6byte
2.Root Port (For example, select a Faster uplink)
        Port Cost
        Port Priority
    Root Port 的选择：
        最低的Root Path Cost
        最低的Sender (upstream) BID
        最低的Port ID
            Lowest port priority
            Lowest port number
3.designed port
    root port

Spanning-Tree Interface States
Convergence Time and procedure
Blocking-------------------listening------------------learning-------------------forwarding
               20s                               15s                               15s                         total:50s
Blocking State
    A switch initially functions as the root until it exchanges BPDUs with other switches.
?Discards frames received on the interface
?Discards frames switched from another interface for forwarding
?Does not learn addresses
?Receives BPDUs
Listening State
    (when)when the spanning tree decides that the interface should participate in frame forwarding
?Discards frames received on the interface
?Discards frames switched from another interface for forwarding
?Does not learn addresses
?Receives BPDUs
    select root bridge, root port, designated port and nondesignated port

Learning State
    prepares to participate in frame forwarding
?Discards frames received on the interface
?Discards frames switched from another interface for forwarding
?Learns MAC addresses from the frame.
?Receives BPDUs
Forwarding State
?Receives and forwards frames received on the interface
?Forwards frames switched from another interface
?Learns addresses
?Receives BPDUs

top改变
1.向root bridge发送TCN(topology change notification)
2.root bridge向所有节点发送TC for max-age+(TC flag) forward delay.