Specifications

Ethernet Bridge Operation 9-13
Spanning Tree Operation
The information that both the bridges have in relation to LAN B is as
follows:
The algorithm to determine who breaks the identified data loop is in the
following order:
a. lowest root path cost
b. highest priority designated bridge ID
c. highest priority designated port ID
d. highest priority port ID
Each bridge compares the incoming BPDU data up against its current port
parameters, and determines the active topology based upon the algorithm
above.
1. The root path cost is the first parameter checked. Both ANN and
EVENIN offer a cost of 100 from LAN B to the root so this is a
deadlock.
2. The designated bridge parameter is the next parameter checked:
- ANN’s designated bridge is itself: 80-00-00-00-1d-56-d4-f4
- EVENIN’s designated bridge is itself: 80-00-00-00-1d-4f-94-a1
Here, it appears that we have broken the tie. The bridge that holds the
highest priority designated bridge ID will win out. EVENIN sees an
inferior bridge ID coming in compared to its current parameters, so it will
go to the FORWARDING state. ANN sees a superior bridge ID coming in
so it will go to the BLOCKING State. EVENIN becomes the designated
bridge to LAN B.
Port Parameters
ANNs
Port 2
EVENINs
Port 1
Port ID 8002 8001
Designated Root SAM SAM
Designated Bridge ANN EVENIN
Designated Port 8002 8001
Root Path Cost 100 100