8.4.3 Precautions when using load balancing

1. The switch selects one of 16 paths based on the packet's hash value. For this reason, packets are not necessarily distributed equally among the paths to a given destination network.

2. Because the switch applies no weighting to individual paths, line speed is not taken into account when distributing packets. However, you can give greater weighting to faster lines by using a multihomed connection.In this case, you must use a redundant configuration to guard against failures.

3. If the switch attempts to send packets in a manner that continuously exceeds the bandwidth of a path selected on the basis of hash values, these packets will be discarded without being re-allocated to another path.

4. Note the following when using the traceroute command to check the paths selected for load balancing:

   • Although the response to the traceroute command bears the IP address of the interface that received the command in its source field, the response does not necessarily originate from that interface.

   • If the traceroute command is received by a multihomed interface, it cannot determine the address to which the neighboring device sent the command.The interface responds by using one of the addresses in the multihomed environment as the source IP address.

5. When using load balancing, relay performance may decline dramatically if traffic is concentrated on a specific route (gateway). In such a case, configure a static ARP entry for all gateways.

6. When multipaths for a BGP4 route include a null interface as a result of next hop resolution performed on the basis of an IGP route that specifies a null interface, the switch is unable to use this route to relay packets. In such a case, use the BGP configuration command bgp nexthop to ensure that the IGP route that specifies the null interface is not used for next hop resolution of the BGP4 route.

   If a next hop to which the Switch is not directly connected is included in the multipath static route, and that next hop is resolved in a route in which a null interface is used as a next hop, that route cannot be used for forwarding.

7. You cannot change the maximum number of multipath routes after the Switch has begun operation. To change this setting, modify the maximum number of multipaths in the configuration for each unicast routing protocol (static routing, OSPF, BGP4), and then restart the switch.

8. Although you can configure a static route in which multiple VRFs exist in the next hop, the multipath of the created route consists of only one VRF.

   The path is selected from among the next hops in the same VRF as the next hop with the highest weight value. [SL-L3A]

9. The same multipath route may result in multiple multipath routes when the route is changed. If a multipath route is switched due to an error or other reason, the newly registered multipath route retains the original multipath route, and the route is registered. Therefore, the system temporarily consumes the total number of multipath routes.

   Use the number of multipath routes with sufficient room so that the capacity limit is not exceeded when a route change occurs.