3Com 4007, Руководство по внедрению, Страница: 164 / 684

164 C
HAPTER
9: B
RIDGE
-W
IDE AND
B
RIDGE
P
ORT
P
ARAMETERS
Forwarding, Filtering,
and Flooding
A bridge filters, floods, or forwards frames by comparing:
■
The frame’s destination address to the source addresses in the bridge’s
address table.
■
The destination bridge port (if known) to the port on which the frame
was received.
The bridge compares the frame’s 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 frame was received, the bridge forwards the frame to
the destination bridge port.
■
If the destination bridge port is the same as the port on which the
frame was received, the bridge filters (discards) the frame.
■
If the destination address is not known to the bridge, the bridge
forwards the frame to all active bridge ports other than the bridge
port on which the frame was received. This process is called
flooding .
Other factors such as VLANs also affect how a bridge processes frames.
Loop Detection and
Network Resiliency
To operate most efficiently, your network topology should have only one
active path between any two bridging devices at any given time. When a
bridge attaches to any single LAN with more than one active path, the
network topology now has a
loop .
When the bridge receives the same frame from multiple ports within a
short period of time, a loop can cause a bridge to continually question
where the source of a given frame is located. As a result, the bridge
forwards and multiplies the same frame continually, which clogs up LAN
bandwidth and challenges the processing capabilities of all devices on the
LAN. This phenomenon of congestion, which can sometimes be so severe
as to bring down network devices and network service, is called a
broadcast storm .