3Com S330 manual Virtual Circuits in a Switched Network

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

VPI/VCI 1/3

ATM

Switch

VPI/VCI 3/23

PathBuilder S330/S310 C

PathBuilder S330/S310 B

VPI/VCI 2/6