Specifications

Spanning Tree Operation
9-10 Ethernet Bridge Operation
SAM checks the incoming BPDU from ANN and it reflects a different root
ID. In the process of checking the incoming data against that which is
current at that port, SAM realizes that it has the higher priority root ID
(lower number) and does not forward ANN’s BPDU through port 2; it
continues to send its own.
ANN checks the incoming BPDU from SAM and senses that the BPDU
carries a higher priority BPDU (lower number) than its own. ANN now
stops transmitting its own BPDUs and begins to modify and retransmit
those received from SAM. These retransmissions by ANN are transmitted
out of port 2, but not port 1. This procedure continues until the only
BPDUs being generated are originating at the root bridge.
Before we jump to the root bridge, however, let’s continue the BPDU trail
to the finish. ANN relinquished its bid to become root and agreed that
SAM is better qualified. ANN propagates SAM’s BPDUs to any other LAN
that it is directly connected to. Here are ANN’s new BPDUs:
You should recognize the BPDU coming in on port 1 (SAM’s BPDU). Now
look at the BPDU coming out of port 2. It indicates that SAM is the root
bridge (root ID) and the Bridge Identifier indicates that this BPDU was
transmitted from ANN. Notice that the root Path Cost is updated to
100–meaning it costs 100 to get to the root through ANN.
BPDU IN (Port 1) BPDU OUT (Port 2)
Root ID 80-00-00-00-1d-23-56-a2 80-00-00-00-1d-23-56-a2
Path Cost 0 100
Bridge ID 80-00-00-00-1d-23-56-a2 80-00-00-00-1d-56-d4-f4
Port ID 8001 8002