background image
TCP Load Distribution 341
Load distribution occurs, for example, when a large hardware company has multiple mirrored
servers on their internal web site and has advertised through DNS that to access their server,
you must attach to 122.17.17.128. In reality, however, the server is addressed as 122.17.17.1,
122.17.17.2, and so on. In this fashion, as each request comes in, it would be sent in a rotary or
round-robin fashion to each of the mirrored servers. Figure 11-5 shows an example of this
configuration.
Cisco offers the Local Director software product, which can accomplish the same load
distribution, but in a much more resilient fashion. The use of NAT for load distribution can be
likened to a "poor-man's" solution. However, given a small shop, this feature can provide a cost
effective solution.
Figure 11-5
TCP Load Distribution Using NAT
The ubiquitous use of the private space and the proficiency of NAT and PAT have greatly
reduced the short supply of address space available on the Internet. This has not stopped the
development of IP Version 6 (or IP Next Generation IPNG), but has slowed the implementation
of it dramatically.
SRV
A
SRV
B
SRV
C
SRV
D
122.7.7.1
122.7.7.2
122.7.7.3
Virtual host
122.7.7.7
Outside IP address
advertised as
122.7.7.7
122.7.7.4
NAT
router