3Com CoreBuilder 3500, Руководство по внедрению, Страница: 117 / 592

Key Bridging Concepts 117
The bridge compares the destination address to the addresses in the
address table and does one of the following:
■
If the destination address is known to the bridge, the bridge identifies
the port on which the destination address is located.
■
If the destination bridge port is different from the bridge port on
which the packet was received, the bridge forwards the packet to
the destination bridge port.
■
If the destination bridge port is the same as the port on which the
packet was received, the bridge filters (discards) the packet.
■
If the destination address is not known to the bridge, the bridge
forwards the packet to all active bridge ports other than the bridge
port on which the packet was received. This process is called
flooding .
Spanning Tree
Protocol
A bridge maintains connectivity between LANs with assistance from the
Spanning Tree Protocol (STP), which is specified in the IEEE 802.1D MAC
Bridges standard.
When a bridge attaches to any single LAN with more than one path, this
results in a
loop in the network topology. Because the bridge receives the
same packet from multiple ports within a short period of time, a loop can
cause a bridge to continually question where the source of a given packet
is located. As a result, the bridge forwards and multiplies the same packet
continually, which clogs up the LAN bandwidth and eventually affects the
bridge’s processing capability.
A backup or redundant path remains a valuable concept nevertheless.
STP balances both concerns by allowing redundant paths to exist but
keeps them inactive until they are needed.
STP uses an algorithm which compares the values in a few different
parameters to determine all possible paths and then map out a loopless
network topology which ensures that only one active path exists between
every pair of LANs. STP keeps one bridge port active and puts redundant
bridge ports in the
blocking state. A port in the blocking state neither
forwards nor receives data packets. See Figure 19.
After STP logically eliminates the redundant paths, the network
configuration stabilizes. Thereafter, if one or more of the bridges or
communication paths in the stable topology fail, STP recognizes the
changed configuration and, within a few seconds, activates redundant
links to ensure network connectivity is maintained.