Specifications

Spanning Tree Operation
9-12 Ethernet Bridge Operation
Data Loop Resolution
The center of our discussion here will be within EVENIN. It received
BPDUs from both of its ports. A BPDU from SAM came in on port 2, and a
BPDU from ANN came in on port 1. With the reception of these two
frames from different ports, each identifying SAM as the root, EVENIN
realizes that there is a data loop present.
Looking at this objectively, we can see that the root bridge is directly
connected to both LANs A and C so the root bridge is the designated
bridge for both of these LANs. However, the designated bridge for LAN B
is still undecided.
The BPDU transmitted from EVENIN onto LAN B and the BPDU
transmitted from ANN onto LAN B are shown next:
At this point we see that SAM is considered to be the root bridge by all
other bridges (root ID). There is no conflict concerning which bridge is the
designated bridge for LANs A and C, as the root is directly connected to
these two LANs. However we have two bridges trying to service LAN B:
ANN and EVENIN. This problem is resolved by the Bridge Entities
residing in both of these bridges.
Each bridge entity maintains a list of current port parameters for each
bridge port. The bridge entity receives BPDUs coming in from a port and
compare the BPDU information against the current port parameters. In the
case of a data loop, the bridge entity makes a decision as to whether it
should put the receiving port in the BLOCKING state or FORWARDING
state. It is this periodic dialogue that eventually settles our Bridged Local
Area Network in a single Spanning Tree.
In this scenario ANN retransmits SAM’s BPDUs onto LAN B and EVENIN
retransmits SAM’s BPDUs onto LAN B.
EVENINs BPDU OUT
(port 1) to ANN
ANNs BPDU OUT
(port 2) to EVENIN
Root ID 80-00-00-1d-4f-94-a1 80-00-00-00-1d-23-56-a2
Path
Cost
100 100
Bridge
ID
80-00-00-00-1d-4f-94-a1 80-00-00-00-1d-56-d4-f4
port ID 8001 8001