11-6
C
HAPTER
11: E
THERNET
LAN
S
AND
B
RIDGES
IN
THE
7600 C
ARD
Transparent Bridges
The
transparent bridge
is a device that connects to two or more LANs and
provides a selective data packet transfer capability between them which
allows them to work in a bridged-LAN environment. The bridged-LAN
can support more stations and a greater amount of traffic than can a
simple LAN and, at the same time, it eliminates the cable length
restriction between stations.
Bridge ports
A LAN is attached to a bridge through a
bridge port
which handles traffic
between the LAN and the bridge. Each bridge port has a unique address.
The bridge ports are numbered internally by the bridge. In the 7600 Card,
bridge ports are numbered differently from the Ethernet ports to which
the Ethernet LANs are physically attached. LANs are assigned to bridges
by associating physical port numbers to bridge port numbers when
setting up the device. Figure 11-4 below shows LANs connected to a
bridge to form a bridged-LAN environment.
Figure 11-3
Forming a bridged-LAN environment
Store-and-Forward
The basic idea of the transparent bridge, as specified in the IEEE 802.1d
bridging standard, is that it listens to every packet transmitted by the
LANs to which it is attached, storing each received packet until it can be
transmitted on the LANs other than the one on which it was received.
Since the entire packet is stored in the bridge and retransmitted only
when the receiving LANs are idle, the cable length restriction needed to
avoid collisions in a single LAN can be exceeded between stations in the
bridged LAN. In addition, the total number of stations in the bridged LAN
can exceed the permissible number in a single LAN because each LAN is
treated separately by the bridge.
abtthgde.book Page 6 Tuesday, June 23, 1998 10:29 AM
