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Unformatted text preview: re hop counts. The hop count for all directly connected interfaces
is 1. Consider the routers and networks shown in Figure 10.4. The four dashed lines we show
are broadcast RIP messages. Figure 10.4 Example routers and networks.
Router Rl advertises a route to N2 with a hop count of 1 by sending a broadcast on Nl. (It
makes no sense to advertise a route to Nl in the broadcast sent on Nl.) It also advertises a
route to Nl with a hop count of 1 by sending a broadcast on N2. Similarly, R2 advertises a
route to N2 with a metric of 1, and a route to N3 with a metric of 1.
If an adjacent router advertises a route to another network with a hop count of 1, then our
metric for that network is 2, since we have to send a packet to that router to get to the
network. In our example, the metric to Nl for R2 is 2, as is the metric to N3 for Rl.
As each router sends its routing tables to its neighbors, a route can be determined to each
network within the AS. If there are multiple paths within the AS from a router to a network,
the router selects the path with the smallest hop count and ignores the other paths.
The hop count is limited to 15, meaning RIP can be used only within an AS where the
maximum number of hops between hosts is 15. The special metric of 16 indicates that no file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (5 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols route exists to the IP address.
As simple as this sounds, there are pitfalls. First, RIP has no knowledge of subnet addressing.
If the normal 16-bit host ID of a class B address is nonzero, for example, RIP can't tell if the
nonzero portion is a subnet ID or if the IP address is a complete host address. Some
implementations use the subnet mask of the interface through which the RIP information
arrived, which isn't always correct.
Next, RIP takes a long time to stabilize after the failure of a router or a link. The time is
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