3Com LANPLEX 2500, Руководство по эксплуатации

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5-10 C
HAPTER
5: T
RANSPARENT
B
RIDGING
Figure 5-6 shows a bridged network with its Spanning Tree elements.
Figure 5-6 Root and Designated Bridges and Ports in a Spanning Tree Topology
CBPDU’s contents
The specific information that bridges receive from the CBPDU allows them
to calculate a Spanning Tree topology:
■
Root ID — The identification of the bridge assumed to be the root
■
Cost — The cost of the least-cost path to the root from the transmitting
bridge. One of the determining factors in cost is the speed of the bridge’s
network interface. In this case, the faster the speed, the lower the cost.
■
Transmitting bridge ID — The identification of the bridge transmitting this
CBPDU. The bridge ID consists of the bridge address and the bridge priority
■
Port identifier — The port priority plus the number of the port from which
the transmitting bridge sent a CBPDU. It is only used in the Spanning Tree
calculation if the root IDs, transmitting bridge IDs, and costs (when
compared) are equal. In other words, the port identifier is a tiebreaker in
which the lowest port identifier takes priority. This field is useful primarily for
selecting the preferred port when two ports of a bridge are attached to the
same LAN or when two routes are available from the bridge to the root
bridge.
Comparing CBPDUs
Here are some examples showing how the best CBPDU is determined by
the bridge. The root ID is the most important determining factor. If the root
ID fields are equal, then the cost is compared. The last determining factor is
LAN 1
LAN 2
LAN 3
LAN 4
Root bridge
Designated
bridge for
LAN 3
Designated
bridge for LANs
1, 2, and 4
Bridge
Bridge
Blocked path
Root port
Designated port
Root
Designated
ports
Designated port
P3
P2
P1
P1
P2
P2
P1