Installation and Getting Started Guide

IP Forwarding Cache Host-Based Load Sharing

Destination Host

Next-Hop

192.168.1.170 (H1)

192.168.6.2 (R2)

192.168.1.234 (H2)

192.168.5.1 (R3)

192.168.1.218 (H3)

192.168.6.2 (R2)

192.168.2.175 (H4)

192.168.5.1 (R3)

192.168.2.193 (H5)

192.168.6.2 (R2)

192.168.2.155 (H6)

192.168.5.1 (R3)

192.168.3.209 (H7)

192.168.6.2 (R2)

192.168.3.159 (H8)

192.168.5.1 (R3)

192.168.3.111 (H9)

192.168.5.1 (R2)

R1 is configured with four IP load sharing paths, and has two paths to hosts H1 - H9, attached to R4.

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

R1 receives traffic for hosts in H1 - H9 in that order.

Once a packet for host H1 is received, the cache entry applies to all traffic for H1. Thus, R2 is always used.

192.168.1.170 192.168.1.234 192.168.1.218

 

H1

 

H2

 

H3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

192.168.6.2

R2

192.168.7.1

 

 

 

 

 

 

192.168.2.175

192.168.2.193

192.168.2.155

 

 

 

192.168.1.1

 

 

192.168.6.1

 

192.168.7.2

H4

H5

H6

 

 

 

 

 

 

 

 

 

R1

 

R4

 

 

 

 

 

 

192.168.2.1

 

 

192.168.5.2

 

192.168.4.1

 

 

 

 

 

 

192.168.3.1

 

 

192.168.5.1

R3

192.168.4.2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H7

 

H8

 

 

H9

 

 

 

 

 

 

 

 

 

192.168.3.209

192.168.3.159

192.168.3.111

Figure 6.5 Host-based IP load sharing – basic example

As shown in this example, when the routing switch receives traffic for a destination and the IP route table has multiple equal-cost paths to that destination, the routing switch selects the next equal-cost path (next-hop router) in the rotation and assigns that path to destination. The path rotation is determined by the order in which the IP route table receives the paths.

Since the configuration in this example contains two paths to hosts H1 – H9, the software alternates between the two paths when creating new load sharing cache entries for hosts H1 – H9. So long as the cache entry for a destination remains in the cache, the routing switch always uses the same path for the traffic to the destination. In this example, the routing switch always uses R2 as the next hop for forwarding traffic to H1.

Figure 6.6 shows another example of IP forwarding cache entries for the configuration shown in Figure 6.5. The network and load sharing configurations are the same, but the order in which R1 receives traffic for the host is different. The paths differ due to the order in which the routing switch receives the traffic for the destination hosts.

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