Configuring IP
December 2005
© Foundry Networks, Inc.
16 - 37
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, see “Configuring a Static IP Route” on page 16-34.
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 16.3 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.
Содержание FastIron Edge Switch X424
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