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2.3 | |
Single Area OSPF Configuration | |
| 2.3.6 | |
OSPF, propagating a default route |
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OSPF routing ensures loop-free paths to every network in the domain.
To reach networks outside the domain, either OSPF must know about the
network or OSPF must have a default route. To have an entry for every
network in the world would require enormous resources for each router.
A practical alternative is to add a default route to the OSPF router
connected to the outside network. This route can be redistributed to
each router in the AS through normal OSPF updates.
A configured default route is used by a router to generate a gateway of last resort. The static default route configuration syntax uses the network 0.0.0.0 address and a subnet mask 0.0.0.0:
This is referred to as the quad-zero route, and any network address is matched using the following rule. The network gateway is determined by ANDing the packet destination with the subnet mask. The following configuration statement will propagate this route to all the routers in a normal OSPF area:
All routers in the OSPF area will learn a default route provided that the interface of the border router to the default gateway is active.
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