The Cisco IOS provides congestion control on WAN links by adding compression on serial
interfaces. This can ease the WAN bandwidth bottleneck problems by using less bandwidth on
the link. Along with using the different queuing methods discussed earlier in this chapter, one
of the more effective methods of WAN optimization is compression of the data traveling across
the WAN link.
Software compression can significantly affect router CPU performance, and the Cisco rule of
thumb is that the router’s CPU load must not exceed 65 percent when running software compression.
If it does exceed this limit, it would be better to disable any compression running.
Cisco equipment supports the following types of compression:
TCP header compression
Payload compression
Link compression
Microsoft Point-to-Point Compression (MPPC)
By default, Cisco routers transmit data across serial links in an uncompressed format, but by
using Cisco serial compression techniques, you can make more efficient use of your available
bandwidth. It’s true that any compression method will cause overhead on the router’s CPU, but
the benefits of compression on slower links can outweigh that disadvantage.
Figure 30.7 shows the three types of compression used in a Cisco internetworking environment.
FIGURE 3 0 . 7 Cisco serial compression methods
Link compression
Header
compression
Payload compression
PPP, HDLC,
X.25, Frame
Relay, or ATM
header
IP/TCP
header Payload
The compression methods are as follows:
TCP header compression Cisco uses the Van Jacobson algorithm to compress the headers of
IP packets before sending them out onto WAN links.
Payload compression This approach compresses the data but leaves the header intact. Because
the packet’s header isn’t changed, it can be switched through a network. This method is the one
generally used for switching services such as X.25, Switched Multimegabit Data Service (SMDS),
Frame Relay, and ATM.
Link compression This method is a combination of both header and payload compression,
and the data will be encapsulated in either PPP or LAPB. Because of this encapsulation, link
compression allows for transport protocol independence.
Microsoft Point-to-Point Compression (MPPC) protocol This is defined in RFC 2118 and
enables Cisco routers to exchange compressed data with Microsoft clients. You would configure
MPPC when exchanging data with a host using MPPC across a WAN link. The MPPC is not
discussed further in this section.
The Cisco compression methods are discussed in more detail next.