45 FAST FORWARDING CONFIGURATIONWhen configuring fast forwarding, go to these sections for the information youare interested in:■ “Introduction to Fast Forwarding” on page 653■ “Configuring Fast Forwarding” on page 654■ “Displaying and Maintaining Fast Forwarding” on page 654Introduction to FastForwardingForwarding efficiency is a key index of the performance of a router. In an ordinaryforwarding process, when a router receives a packet, it copies the packet from theinterface memory to the CPU. Then, the CPU searches the routing table for routesmatching the destination address to fix the best route and encapsulate the packetinto a proper link layer frame. Finally, the link layer frame is copied to the outputqueue through direct memory access (DMA) for forwarding. The system bus willbe involved twice in this process and the forwarding of each packet will repeat thisprocess.Fast forwarding employs cache and the data-flow-based technology to handlepackets. The data on the Internet is generally based on data flow, which is aspecific application between two hosts, for example, the operation of using FTP totransfer a file. A data flow is usually described by five tuples (source IP address,source port number, destination IP address, destination port number, and protocolnumber). When the first packet is forwarded by means of searching the routingtable, corresponding routing information is generated in the cache so that thesubsequent packets in the flow can be forwarded by means of searching the cachedirectly. As a result, fast forwarding reduces the IP packet queue and routing timeand enhances forwarding throughput of IP packets. In addition, because therouting table is already optimized in the cache, the searching speed is especiallyhigh.Fast forwarding is currently implemented:■ On all kinds of high-speed interfaces (including sub-interfaces), such asEthernet, synchronous PPP, Frame Relay and HDLC interfaces.■ On PPP MP links.■ On IPHC compression or VJ compression enabled PPP links.■ When a packet filter is configured.■ When an application specific packet filter (ASPF) is configured.■ When network address translation (NAT) is configured.■ When generic routing encapsulation (GRE) is configured.
