
Configuring IP Routers and Interfaces
2-23
Configuring Router Discovery
Before a host can send IP datagrams beyond its directly attached subnet, the host
must discover the address of at least one operational router on that subnet. Router
Discovery is an extension of the Internet Control Message Protocol (ICMP) that
enables hosts attached to multicast or broadcast networks to discover the IP
addresses of their neighboring routers.
Routers configured with Router Discovery periodically multicast or broadcast a
router advertisement from each of their interfaces, announcing the IP address or
addresses of that interface. Hosts discover the addresses of their neighboring
routers by listening for these advertisements. Hosts will use the router with the
highest preference level as a gateway.
Specifying the Size of IP Forwarding Tables
To keep to a minimum the amount of time that it spends looking up routes, IP
creates and maintains a cache of frequently used routes — called a forwarding
table — for each IP interface.
A forwarding table is a first-in-first-out (FIFO) buffer. When a datagram arrives on
an IP interface for forwarding, IP performs the following steps:
1. IP searches the forwarding table associated with the interface for the
destination network.
2. If the search is successful, IP dispatches the datagram to the interface noted in
the table entry.
3. If the search is unsuccessful, IP consults the routing table to get the same
information, dispatches the datagram to the appropriate interface, and caches
the information in the appropriate forwarding table — either by appending
information to the table (if the table is not full) or by overwriting the oldest,
first-in table entry (if the table is full).
For instructions on using Site Manager to configure IP Router Discovery, see “Configuring
Router Discovery” on page 2-86.
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