9.7 Summary

9.7 Summary

The operation of IP routing is fundamental to a system running TCP/IP, be it a host or router. The routing table entries are simple: up to 5 flag bits, a destination IP address (host, network, or default), a next -hop router IP address (for an indirect route) or a local interface IP address (for a direct route), and a pointer to a local interface to use. Host entries have priority over network entries, which have priority over default entries.

A search of this routing table is made for every IP datagram that the system generates or forwards, and can be updated by either a routing daemon or ICMP redirects. By default a system should never forward a datagram unless it has specifically been configured to do so. Static routes can be entered using the route command, and the newer ICMP router discovery messages can be used to initialize and dynamically update default entries. Hosts can start with a simple routing table that is updated dynamically by ICMP redirects from its default router.

Our discussion in this chapter has focused on how a single system uses its routing table. In the next chapter we examine how routers exchange routing information with each other.



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

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