bpdu anatomy...

 

i like this diagram, from your friendly neighborhood...spiderman

 
 hulk is only neighborhood thing here...

what i like most about it is that you have to count the bytes in the field...

protocol identifier-  2 bytes, always 0
protocol version identifier- 1 byte, always 0
bpdu type- 1 byte, 0 if configuration bpdu
flags- 1 byte, for topology changes
root identifier- 8 bytes, used for bid of current root switch
root path cost- 4 bytes, path cost to root switch
sender bid- 8 bytes, bid of of current switch (sender)
port id- 2 bytes, id of of port that sent this bpdu
message age- 2 bytes,time since the root created the bpdu
max age- 2 bytes, default 20 secs(max save time for bpdu)
hello time- 2 bytes, default 2 secs(time between bpdu launches)
forward delay- 2 bytes, default 15 secs(time in listening and learning)