KB.15.15

When OSPF is enabled and multiple, equal-cost, next-hop routes are available for

traffic destinations on different subnets, this feature, by default, enables load-sharing

among up to four next-hop routes.

1 - 4 : Specifies the maximum number of equal-cost next-hop paths the router

allows.

Default: 4; range: 2–4

The no form of the command disables this load-sharing so that only one route in a

group of multiple, equal-cost, next-hop routes is used for traffic that could otherwise

be load-shared across multiple routes.

For example, in Figure 43 (page 204), the next-hop routers "B", "C", and "D" are

available for equal-cost load-sharing of eligible traffic. Disabling IP load-sharing

means that router "A" selects only one next-hop router for traffic that is actually

eligible for load-sharing through different next-hop routers.

Default: Enabled with four equal-cost, next-hop routes allowed.

NOTE: This command enables or disables load-sharing for both IPv4 (OSPFv2)

and IPv6 (OSPFv3) operation. For more information, see the IPv6 Configuration

Guide for your switch.

In the default configuration, IP load-sharing is enabled by default. However, it has

no effect unless IP routing and OSPF are enabled.

Viewing the current IP load-sharing configuration

Use the show running command to view the currently active IP load-sharing configuration, and

show config to view the IP load-sharing configuration in the startup-config file. (While in its

default configuration, IP load-sharing does not appear in the command output.)

If IP load sharing is configured with non-default settings (disabled or configured for either two or

three equal-cost next-hop paths), the current settings are displayed in the command output.

Figure 36 Displaying a non-default IP load-sharing configuration

Overview of OSPF

OSPF is a link-state routing protocol applied to routers grouped into OSPF areas identified by the

routing configuration on each routing switch. The protocol uses LSAs transmitted by each router to

update neighboring routers regarding its interfaces and the routes available through those interfaces.

Each routing switch in an area also maintains a link-state database (LSDB) that describes the area

topology. (All routers in a given OSPF area have identical LSDBs.) The routing switches used to

connect areas to each other flood summary link LSAs and external link LSAs to neighboring OSPF

areas to update them regarding available routes. Through this means, each OSPF router determines

Overview of OSPF 189