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Ethernet Interface 227
The aggregate of VPIs/VCIs assigned to the Ethernet connection of the PathBuilder
S330/S310 is referred to as a bridge. Since the PathBuilder S330/S310 reads and
stores MAC addresses and associated VPIs/VCIs as described above, the bridge is
called a learning bridge. Each learning bridge of the PathBuilder S330/S310 has a
separate forwarding table containing the MAC address to VPI/VCI associations.
When a learned entry is stored in a forwarding table, it is time tagged. An aging
timer in the PathBuilder S330/S310, which you set, purges entries from the
forwarding tables after they have been in the tables for the aged time. The bridges
must then relearn purged addresses. This controls the size of the forwarding
tables, particularly in large networks.
In the PathBuilder S330/S310, each bridge also has a static table associated with it.
You can enter MAC addresses and corresponding VPIs/VCIs into the static table
that will not be affected by the aging timer. These entries might be for LAN units
that are relatively permanent and are accessed through the ATM network
frequently, such as servers.
In the example shown in Figure 164, there is only one transmission path: the T1/n
x T1 (or E1/n x E1) between the two PathBuilder S330s, so cells leaving PathBuilder
S330 A on VPI/VCI 1/33 arrive at PathBuilder S330 B on VPI/VCI 1/33. Also if
theT1/nxT1 (or E1/n x E1) trunk were switched through a standard telephone
switching system to interconnect many PathBuilder S330s, each PathBuilder S330
would have to have the same VPI/VCI designations assigned. The ATM switch
removes this restriction.
Figure 165 shows several PathBuilder S330/S310s in an ATM-switched network.
The switch maps one VPI/VCI into another based on the destination of the packets
in the cell. It learns the complete topology of the network to do so. You can
independently configure each PathBuilder S330/S310 connected to the switch.
Figure 165 Virtual Circuits in a Switched Network
PathBuilder S330/S310 A
PathBuilder S330/S310 C
PathBuilder S330/S310 B
ATM
Switch
VPI/VCI 2/6
VPI/VCI 1/3
VPI/VCI 3/23