Information Technology Reference
In-Depth Information
Static Routes and Floating Static Routes
The surest way to route packets to a dialer interface is with static routing. These routes are manually
entered into the configuration of the router or access server with this command:
ip route prefix mask {address
|
interface} [distance]
•
prefix—IP route prefix for the destination.
•
mask—Prefix mask for the destination.
•
address—IP address of the next hop that can be used to reach the destination network.
•
interface—Network interface to use for outbound traffic.
•
distance—(Optional) An administrative distance. This argument is used in
floating static routes.
Static routes are used in situations in which the dial link is the only connection to the remote site. A static
route has an administrative distance value of 1, which makes it preferred over dynamic routes to the same
destination.
On the other hand, floating static routes—that is, static routes with a predefined administrative
distance—are typically used in backup DDR scenarios, in which a dynamic routing protocol such as RIP
or EIGRP is used to route packets across the primary link. If a normal static route were to be used, its
administrative distance of 1 would make it preferable to either EIGRP (75) or RIP (120), causing packets
to be routed across the dial line even if the primary were up and capable of passing traffic. However, if
the static route is configured with an administrative distance higher than that of any of the dynamic
routing protocols in use on the router, the floating static route will be used only in the absence of a better
route—one with a lower administrative distance.
If backup DDR is being invoked by use of the
backup interface
command, the situation is somewhat
different. Because the dialer interface remains in standby mode while the primary is up, either a static
route or a floating static route may be configured, and the dialer interface will not attempt to connect
until after the primary interface goes down/down.
For a given connection, the number of static (or floating static) routes necessary is a function of the
addressing on the dialer interfaces. In cases in which the two dialer interfaces (one on each of the two
routers) share a common network or subnet, typically only one static route is required, pointing to the
remote local-area network using the address of the remote router's dialer interface as the next-hop
address.
Examples
Example 1:
Dial is the only connection, with numbered interfaces, as shown in Figure 16-5. One route is sufficient.
Figure16-5 Dial Using Numbered Interfaces
E0
E0
BRI0
BRI0
192.168.10.1 /24
10.1.1.1 /24
192.168.254.1
192.168.254.2
Montecito
Goleta
Montecito:
ip route 192.168.10.0 255.255.255.0 172.16.20.2
