Foundry Networks Switch and Router Installation And Configuration Manual page 498

Foundry Switch and Router Installation and Configuration Guide
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)
192.168.6.2 (R2)
192.168.3.209 (H7)
192.168.5.1 (R3)
192.168.3.111 (H9)
192.168.6.2 (R2)
192.168.1.234 (H2)
192.168.5.1 (R3)
192.168.2.193 (H5)
192.168.5.1 (R2)
192.168.3.159 (H8)
192.168.6.1
R1
192.168.5.2
Figure 15.6 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 Layer 3 Switches and is the default.
When the Layer 3 Switch receives traffic for a device on a destination network for which the IP route table has
multiple equal-cost paths, the Layer 3 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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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.
R2
192.168.6.2 192.168.7.1
192.168.7.2
R3
192.168.5.1 192.168.4.2
192.168.1.170
H1
192.168.2.175
192.168.1.1
H4
R4
192.168.2.1
192.168.4.1
192.168.3.1
H7
192.168.3.209
192.168.1.234 192.168.1.218
H2 H3
192.168.2.193 192.168.2.155
H5
H6
H8 H9
192.168.3.159 192.168.3.111
December 2000