Ethernet Interface (10BASE-T or 10/100BASE-T) 253The aggregate of VPIs/VCIs assigned to the Ethernet connection of the PathBuilderS330/S310 switch is referred to as a bridge. Since the switch reads and storesMAC addresses and associated VPIs/VCIs as described above, the bridge is called alearning bridge. Each learning bridge of the PathBuilder S330/S310 switch has aseparate 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 agingtimer in the switch, which you set, purges entries from the forwarding tables afterthey have been in the tables for the aged time. The bridges must then relearnpurged addresses. This controls the size of the forwarding tables, particularly inlarge networks.In the switch, each bridge also has a static table associated with it. You can enterMAC addresses and corresponding VPIs/VCIs into the static table that will not beaffected by the aging timer. These entries might be for LAN units that are relativelypermanent and are accessed through the ATM network frequently, such as servers.In the example shown in Figure 180, there is only one transmission path: the T1/nx T1 (or E1/n x E1) between the two PathBuilder S330 switches, so cells leavingSwitch A on VPI/VCI 1/33 arrive at Switch B on VPI/VCI 1/33. Also if theT1/nxT1 (orE1/n x E1) trunk were switched through a standard telephone switching system tointerconnect many PathBuilder S330 switches, each switch would have to havethe same VPI/VCI designations assigned. The ATM switch removes this restriction.Figure 181 shows several PathBuilder S330/S31 switches in an ATM-switchednetwork. The switch maps one VPI/VCI into another based on the destination ofthe packets in the cell. It learns the complete topology of the network to do so.You can independently configure each PathBuilder S330/S310 switch connectedto the ATM switch.Figure 181 Virtual Circuits in a Switched NetworkPathBuilder S330/S310 APathBuilder S330/S310 CPathBuilder S330/S310 BATMSwitchVPI/VCI 2/6VPI/VCI 1/3VPI/VCI 3/23
