Specifications

Spanning Tree Operation
9-8 Ethernet Bridge Operation
With the understanding of our shortened BPDU, we now begin our
explanation of the Spanning Tree Operation. In Figure 9-3, SAM is sending
out BPDUs across all LANs to which it is connected. SAM sends BPDUs
out through its port 2 onto LAN C and through port 1 onto LAN A.
Figure 9-3. SAM’s Initial BPDUs
SAM’s BPDUs are represented by the following examples:
Each of these BPDUs represent SAM as their root bridge. This is shown by
the Path Cost to the root from LANs A & C = 0 (if SAM was root, then
there would be no bridge hops going from LAN A to the root, hence a cost
of 0). They also indicate that SAM was the transmitting bridge (Bridge ID).
The only difference between the BPDUs is the Port Identifier; 8001 for port
1 and 8002 for port 2.
BPDU (Port 1) BPDU (Port 2)
Root ID 80-00-00-00-1d-23-56-a2 80-00-00-00-1d-23-56-a2
Path Cost 0 0
Bridge
ID
80-00-00-00-1d-23-56-a2 80-00-00-00-1d-23-56-a2
Port ID 8001 8002
LAN A
LAN B
LAN C
ANN JANET
SAM
EVENIN
SAM's BPDUs
Port 1
Port 2
1913-33