3Com LANPLEX 2500, Руководство по эксплуатации

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5-2 C
HAPTER
5: T
RANSPARENT
B
RIDGING
How a Bridge
Learns Addresses
Bridges learn addresses so that they can make intelligent decisions about
which packets to forward from one bridge port to another. A bridge
automatically learns addresses by listening on the network. For a bridge to
learn the address of a station on the network, that station must transmit a
packet. Each bridge maintains a dynamic table, called the address table,
which contains all learned source addresses.
When a bridge receives a packet, it looks up the packet’s source address in
the address table, and does one of the following:
■
If the source address is known to the bridge, then the bridge updates the
source address entry in the address table and verifies the port on which the
packet was received.
■
If the source address is not known to the bridge, then the bridge stores the
packet’s source address in the address table, along with the port on which
the packet was received. See Figure 5-1.
Figure 5-1 Learning Source Addresses
Packet
Source address
00308e3d0042
Address Table
00803e003900
008030e03d20
00803e003d20
00803e001520
00803e342000
00803e003420
00308e000017
00308e3d0042
00803e003d20
Address Port
port 1
port 3
port 4
port 5
port 1
port 7
port 8
port 2
port 4