Introduction To Fast Forwarding - 3Com MSR 50 Series Configuration Manual

45
Introduction to Fast
Forwarding
F F
AST ORWARDING
When configuring fast forwarding, go to these sections for the information you
are interested in:
"Introduction to Fast Forwarding" on page 653
■
"Configuring Fast Forwarding" on page 654
■
"Displaying and Maintaining Fast Forwarding" on page 654
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Forwarding efficiency is a key index of the performance of a router. In an ordinary
forwarding process, when a router receives a packet, it copies the packet from the
interface memory to the CPU. Then, the CPU searches the routing table for routes
matching the destination address to fix the best route and encapsulate the packet
into a proper link layer frame. Finally, the link layer frame is copied to the output
queue through direct memory access (DMA) for forwarding. The system bus will
be involved twice in this process and the forwarding of each packet will repeat this
process.
Fast forwarding employs cache and the data-flow-based technology to handle
packets. The data on the Internet is generally based on data flow, which is a
specific application between two hosts, for example, the operation of using FTP to
transfer a file. A data flow is usually described by five tuples (source IP address,
source port number, destination IP address, destination port number, and protocol
number). When the first packet is forwarded by means of searching the routing
table, corresponding routing information is generated in the cache so that the
subsequent packets in the flow can be forwarded by means of searching the cache
directly. As a result, fast forwarding reduces the IP packet queue and routing time
and enhances forwarding throughput of IP packets. In addition, because the
routing table is already optimized in the cache, the searching speed is especially
high.
Fast forwarding is currently implemented:
On all kinds of high-speed interfaces (including sub-interfaces), such as
■
Ethernet, synchronous PPP, Frame Relay and HDLC interfaces.
On PPP MP links.
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On IPHC compression or VJ compression enabled PPP links.
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When a packet filter is configured.
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When an application specific packet filter (ASPF) is configured.
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When network address translation (NAT) is configured.
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When generic routing encapsulation (GRE) is configured.
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C
ONFIGURATION