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3.2 | ![]() |
Default Routing | |
3.2.2 | ![]() |
Configuring static default routes |
Creating an
ip route
to 0.0.0.0/0 is the simplest way to configure a default route. This is
done using the following syntax:
To the Cisco IOS, network 0.0.0.0 /0 has special meaning as the gateway of last resort. All destination addresses match this route because a mask of all 0s requires none of the 32 bits in an address to be an exact match. A route to 0.0.0.0 /0 is often called a quad-zero route. Manually configuring 0.0.0.0/0 routes on every router might suffice in a simple network. Routers may need to dynamically exchange default routes in more complex situations. The exchange of default information works differently depending on the routing protocol being used and can create severe problems when improperly configured. Default routes typically point outside the network. Therefore, default routes are noticeable when they fail. Static routes to 0.0.0.0/0 are automatically propagated to other routers in RIP updates. The only way to stop this automatic update is to use a route filter. A route filter configuration option is discussed in Module 8, Route Optimization. In IOS releases 12.1 and later, RIP does not propagate a static default route automatically. When using RIP and IOS 12.1, the RIP process must be manually configured to advertise the static default route by issuing the network 0.0.0.0 command. Alternately, the
default-information originate
or the redistribute static
command may be used to configure static default route propagation.
Regardless of the IOS version, the OSPF requires the
default-information originate
command to propagate the static default routes. The
following example illustrates this configuration for RIP. Figure
Using the default-information originate command, an administrator can statically configure a single RIP router with a 0.0.0.0/0 route, and that default route will be propagated to other routers. The default-information originate command can also be used with OSPF to achieve the same effect.
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