Specifications

Ethernet Bridge Operation 9-9

Spanning Tree Operation

Now let’s look at one of the other bridges, in this case ANN. Figure 9-4

shows ANN generating BPDUs from all of its ports, considering itself as

root until it ﬁnds out differently.

Figure 9-4. ANN’s Initial BPDUs

The BPDUs generated by ANN are shown below:

Here again, this bridge thinks that it is the root, indicated by the root ID

value within the BPDU, and the cost to the root of 0. It also indicates by the

Bridge ID that ANN is the bridge that has transmitted this BPDU. Again,

the Port Identiﬁer reﬂects the transmitting port of the BPDU on ANN. We

now assume that both SAM and ANN receive each other’s BPDUs from

LAN A.

BPDU (Port 1) BPDU (Port 2)

Root ID 80-00-00-00-1d-56-d4-f4 80-00-00-00-1d-56-d4-f4

Path Cost 0 0

Bridge ID 80-00-00-00-1d-56-d4-f4 80-00-00-00-1d-56-d4-f4

Port ID 8001 8002

LAN A

LAN B

LAN C

ANN JANET

SAM

EVENIN

Port 1

Port 2

1913-34

ANN's BPDUs