Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.7.03 - Page 275

Configuring forwarding parameters 7 The following commands configure static IP routes to the same destination, but with different metrics. The route with the lowest metric is used by default. The other routes are backups in case the first route becomes unavailable. The device uses the route with the lowest metric if the route is available. BigIron RX(config)# ip route 192.128.2.69 255.255.255.0 209.157.22.1 BigIron RX(config)# ip route 192.128.2.69 255.255.255.0 192.111.10.1 2 BigIron RX(config)# ip route 192.128.2.69 255.255.255.0 201.1.1.1 3 In this example, each static route has a different metric. The metric is not specified for the first route, so the default (1) is used. A metric is specified for the second and third static IP routes. The second route has a metric of two and the third route has a metric of 3. Thus, the second route is used only of the first route (which has a metric of 1) becomes unavailable. Likewise, the third route is used only if the first and second routes (which have lower metrics) are both unavailable. For complete syntax information, refer to "Configuring a static IP route" on page 199. Configuring standard static IP routes and interface or null static routes to the same destination You can configure a null0 or interface-based static route to a destination and also configure a normal static route to the same destination, so long as the route metrics are different. When the device has multiple routes to the same destination, the device always prefers the route with the lowest metric. Generally, when you configure a static route to a destination network, you assign the route a low metric so that the device prefers the static route over other routes to the destination. This feature is especially useful for the following configurations. These are not the only allowed configurations but they are typical uses of this enhancement: • When you want to ensure that if a given destination network is unavailable, the device drops (forwards to the null interface) traffic for that network instead of using alternate paths to route the traffic. In this case, assign the normal static route to the destination network a lower metric than the null route. • When you want to use a specific interface by default to route traffic to a given destination network, but want to allow the device to use other interfaces to reach the destination network if the path that uses the default interface becomes unavailable. In this case, give the interface route a lower metric than the normal static route. NOTE You cannot add a null or interface-based static route to a network if there is already a static route of any type with the same metric you specify for the null or interface-based route. Figure 11 shows an example of two static routes configured for the same destination network. One of the routes is a standard static route and has a metric of 1. The other static route is a null route and has a higher metric than the standard static route. The device always prefers the static route with the lower metric. In this example, the device always uses the standard static route for traffic to destination network 192.168.7.0/24, unless that route becomes unavailable, in which case the device sends traffic to the null route instead. BigIron RX Series Configuration Guide 203 53-1001986-01