Routing is the process of selecting the route that data, in the form of IP packets, must take to reach its destination. Routing can be as simple as delivering packets to another host on the same network (direct routing) or it may involve forwarding packets to routers on its way to the destination network. When a router forwards a packet to a local host (that is, a host on the same network), routing is direct; if the packet must be forwarded to one or more routers to reach its destination, the route is indirect.
This section helps you understand the basics of IP routing and how routing works in Cisco TCP/IP Suite.
Routing information for indirect routes is propagated in three ways:
Static routing is a table-driven system of IP address and router address pairs. The routing table provides quick and easy routing information, but it does not respond automatically to changing environments.
Default routes are used when a host has no specific route for the destination host or network in its routing table. If the data cannot be delivered directly (or if the routing table has no entry for the destination host or network), the data is forwarded to the default router.
Dynamic routing uses router discovery or routing protocols such as RIP (Routing Information Protocol) to share routing information with other routers and hosts.
Cisco TCP/IP Suite supports static, default, and dynamic routing, and router discovery.
Routing tables store information about the routes that hosts can use to reach other hosts on the network or Internet. Routing tables can be static or dynamic:
All routing information in the routing table is based on local addresses only. The routing table is designed to supply the next hop address (which is always local) for data bound for other networks. The routing table never contains information about routers beyond the local network segment, nor does it contain information about how to reach individual host addresses (although it can contain host-specific entries that specify which local router to use when data is bound for a specific host address). Routers always forward data to networks until the destination network is the local network. When the data arrives at the destination network, it is directly forwarded to the appropriate host.
Host-specific routes are special routing table entries that specify which router to use when data is bound for a specific remote host. Host-specific routes are frequently used to test new routers or to implement network security.
Router discovery is a method of finding a router when no default route entry exists in the routing table. When booting, a host using router discovery broadcasts a message asking for available routers. The available routers reply with messages indicating their addresses. The host adds the information to its routing table.
In order to use router discovery, the routers must have Router Discovery enabled. See your router's documentation for information on enabling router discovery, if this is your responsibility.
RIP (Routing Information Protocol) is a dynamic routing protocol. Routers using RIP broadcast their routing tables to other routers on the network at regular intervals. Machines on the network, including other routers, receive these broadcasts and update their routing tables as necessary. If a particular router fails to broadcast its routing table after a specified period of time, the other machines on the network remove that router from their routing tables, thus updating their routing tables dynamically.
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