Ethernet Interface 223n The SAR will then assemble the cells belonging to the connections specified forit.n When the cells are assembled, the CPU is given confirmation, and the bridgefunction of the CPU examines the packet header, removes the encapsulation,and—after learning the address and updating the bridge table—forwards thepacket to the proper destination.n The pointer of the packet to be forwarded is placed in the transmit queue ofthe Ethernet port.Bridge Operation The PathBuilder S330/S310 supports ATM permanent virtual circuits (PVCs) foraccess to the ATM network. All the PVC’s configured for a port will form a virtualBridged Ethernet network to all other ports at the other end of the PVC.The bridge operation is equivalent to a multiport bridge. It works as follows:n The bridge learns and builds forwarding tables for every PVC that is tied to theEthernet port.n When a packet is received on the port or any PVC tied to that port, the SourceMAC address is learned and kept in the forwarding table until the aging timerexpires. You can use the Bridge Configuration menu to add static forwardingaddresses that the bridge will not delete after the aging timer expires. See“Configuring the Bridge” in Chapter 4, for details.n When the packet arrives, the bridge looks up the destination MAC address todetermine the destination of the packet from the forwarding tables. If adestination is found, the packet is forwarded to the correct destination. If adestination is not found, the bridge broadcasts or floods the packet on allPVC’s that are tied to the Ethernet port. Flooding is performed by forwardingthe packet on all destinations tied to the port.The PathBuilder S330/S310 performs LLC-based multiplexing. Ethernet frames arebridged as per RFC1483. Frames with NLPID of 0x00 are discarded as being invalidframes. The PathBuilder S330/S310 does not support frames bigger than Ethernetmaximum frame size (1518) or IEEE802.3 frame size (1492) bytes (+ framing bytes)coming off the ATM network.The following subsections provide further detail about bridging operations.FilteringThe first bridging operation determines if the packet is to be processed andtransmitted across the bridge or filtered out based on the list of protocols andaddresses input as part of the PathBuilder S330/S310 configuration. Filtering givesyou control over who communicates with whom in the network. The CPU readsthe header of each packet to determine the protocol, source address, anddestination address and then looks in the list to see if the packet should be passedthrough the bridge. If it is to be filtered out, it is cleared from memory.
