Configuring IP

IP Forwarding Cache Host-Based Load Sharing

Destination Host

Next-Hop

192.168.2.175 (H4)

192.168.6.2 (R2)

192.168.1.170 (H1)

192.168.5.1 (R3)

192.168.1.218 (H3)

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)

192.168.3.111 (H9)

192.168.5.1 (R3)

192.168.1.234 (H2)

192.168.6.2 (R2)

192.168.2.193 (H5)

192.168.5.1 (R3)

192.168.3.159 (H8)

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 the following order: H4, H1, H3, H6, H7, H9, H2, H5, and H8.

Once a packet for host H4 is received, the cache entry applies to all traffic for H4. 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 6.6 Host-based IP load sharing – additional example

Network-Based IP Load Sharing

Network-based load sharing distributes traffic across multiple equal-cost paths based on the destination network. This method of load sharing optimizes system resources by aggregating the forwarding cache entries used for load sharing. Host-based load sharing contains a separate cache entry for each destination host, whereas network-based load sharing contains a single entry for each destination network.

The network-based load sharing method is available only on chassis routing switches and is the default.

When the routing switch receives traffic for a device on a destination network for which the IP route table has multiple equal-cost paths, the routing switch checks the IP forwarding cache for a forwarding entry to the destination network:

•If the IP forwarding cache contains a forwarding entry for the destination network, the device uses the entry to forward the traffic.

•If the IP forwarding cache does not contain a forwarding entry for the destination network, the software selects the next path in the rotation (the path after the one the software used for the previous load sharing selection). The software then creates an IP forwarding cache entry that associates the destination network address with the selected path. IP forwarding cache entries for network-based load sharing do not age out. Once the software creates a cache entry for a destination network, traffic for all hosts on the network uses the same path. The cache entries remain in effect until the state of one of the paths changes or the software is reloaded.

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