3Com 3C63311 Reference Guide - Page 235

Ethernet Interface 223 n The SAR will then assemble the cells belonging to the connections specified for it. n When the cells are assembled, the CPU is given confirmation, and the bridge function of the CPU examines the packet header, removes the encapsulation, and-after learning the address and updating the bridge table-forwards the packet to the proper destination. n The pointer of the packet to be forwarded is placed in the transmit queue of the Ethernet port. Bridge Operation The PathBuilder S330/S310 supports ATM permanent virtual circuits (PVCs) for access to the ATM network. All the PVC's configured for a port will form a virtual Bridged 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 the Ethernet port. n When a packet is received on the port or any PVC tied to that port, the Source MAC address is learned and kept in the forwarding table until the aging timer expires. You can use the Bridge Configuration menu to add static forwarding addresses 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 to determine the destination of the packet from the forwarding tables. If a destination is found, the packet is forwarded to the correct destination. If a destination is not found, the bridge broadcasts or floods the packet on all PVC's that are tied to the Ethernet port. Flooding is performed by forwarding the packet on all destinations tied to the port. The PathBuilder S330/S310 performs LLC-based multiplexing. Ethernet frames are bridged as per RFC1483. Frames with NLPID of 0x00 are discarded as being invalid frames. The PathBuilder S330/S310 does not support frames bigger than Ethernet maximum 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. Filtering The first bridging operation determines if the packet is to be processed and transmitted across the bridge or filtered out based on the list of protocols and addresses input as part of the PathBuilder S330/S310 configuration. Filtering gives you control over who communicates with whom in the network. The CPU reads the header of each packet to determine the protocol, source address, and destination address and then looks in the list to see if the packet should be passed through the bridge. If it is to be filtered out, it is cleared from memory.