Installation and Getting Started Guide

Figure 6.7 shows an example of IP load sharing cache entries for network-based IP load sharing. The network in this example is the same as the network in Figure 6.5 and Figure 6.6. Notice that the cache contains one entry for each destination network, instead of a separate entry for each destination host. Based on the cache entries, traffic for all hosts (H1, H2, and H3) on network N1 uses the path through R2.

IP Forwarding Cache

Network-Based Load Sharing

Destination Network

Next-Hop

192.168.1.0 (N1)

192.168.6.2 (R2)

192.168.2.0 (N2)

192.168.6.2 (R3)

192.168.3.0 (N3)

192.168.5.1 (R2)

 

 

R1 is configured with four IP load sharing paths, and has two paths to networks N1 - N3, attached to R4.

The cache entries in this example are based on the assumption that

R1 receives traffic for hosts in N1 - N3 in that order.

Once a packet for a host on N1 is received, the cache entry applies to all hosts on N1. The same applies for N2 and N3.

192.168.1.170 H1

192.168.1.234 H2

192.168.1.218

H3

 

 

 

 

 

192.168.2.175

192.168.2.193

192.168.2.155

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

192.168.6.2

R2

 

192.168.7.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H4

 

 

H5

 

 

H6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

192.168.1.1 (N1)

192.168.6.1

 

192.168.7.2

 

 

192.168.2.1 (N2)

R1

 

R4

 

 

192.168.3.1 (N3)

192.168.5.2

 

192.168.4.1

 

 

 

 

H7

192.168.5.1

R3

192.168.4.2

 

 

192.168.3.209

H8

192.168.3.159

H9

192.168.3.111

Figure 6.7

Network-based IP load sharing – basic example

 

Notice that network-based load sharing does not use a simple round-robin method. The path rotation starts with

path 2, then proceeds in ascending numerical order through the remaining paths and ends with path 1. In Figure

6.7, the first cache entry uses path 2 instead of path 1. The algorithm evenly distributes the load among the

available paths, but starts with the second path instead of the first path.

 

For

optimal results, set the maximum number of paths to a value at least as high as the maximum number of

 

equal-cost paths your network typically contains. For example, if the routing switch you are configuring for IP load

sharing has six next-hop routers, set the

 

 

 

 

maximum paths value to six. See “

Load Sharing Paths” on page 6-59.

Changing the Maximum Number of

 

 

 

NOTE:

If the setting for the maximum number of paths is lower than the actual number of equal-cost paths, the

software does not use all the paths for load sharing.

 

 

 

 

 

The

 

 

 

 

network-based IP load sharing mechanism selects a path based on the following calculation, which involves

the maximum number of paths allowed on the routing switch and the number of equal-cost paths available to the

destination network.

 

 

M modulo P + 1 = S

 

 

where:

 

 

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