Configuration Guide User guide

FastIron Configuration Guide 997
53-1002494-02
Configuring IP parameters – Layer 3 Switches
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 Layer 3 Switch has multiple routes to the same destination, the Layer 3 Switch 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 Layer 3 Switch 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 Layer 3 Switch
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 Layer 3 Switch 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 114 shows an example of two static routes configured for the same destination network. In
this example, 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 Layer 3 Switch
always prefers the static route with the lower metric. In this example, the Layer 3 Switch 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 Layer 3 Switch sends traffic to the null route instead.
FIGURE 114 Standard and null static routes to the same destination network