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RIP Route Determination Algorithm and Metric
(Page 3 of 3)
Default Routes
In some cases it is not convenient for every network or host in a large internetwork to be fully specified with its own routing entry. In this case it may be advantageous to specify a default route for the network to use in reaching hosts or networks for which they have no information. The most common example of this is when an autonomous system connects to the public Internet through a single router. Except for that router, the rest of the local network doesn't need to know how to access the Internet.
In RIP, information about a default route is communicated by having routers intended to handle such traffic send information about a “dummy” network with the address 0.0.0.0. This is treated as if it were a regular network when information about routes is propagated on the internetwork using RIP messages, but other devices recognize this special address and understand it means a default route.
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Note: The algorithm description in this topic is the basic one used by RIP. This is modified in certain ways to address some of the problems that can occur in special circumstances due to how the algorithm works. Topics later in this chapter describe these problems and the special features RIP includes to address them.
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Key Concept: Routing information is propagated between routers in RIP using a simple algorithm. On a regular basis, each router sends out RIP messages that specify what networks it can reach, and how many hops to reach them. Other routers directly connected to that one know that they can then reach those networks through that router at a cost of one additional hop. So if router A sends a message saying it can reach network X for a cost of N hops, each other router that connects directly to A can reach network X for a cost of N+1 hops. It will put that information into its routing table, unless it knows of an alternate route through another router that has a lower cost.
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Version 3.0 - Version Date: September 20, 2005
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