ELSA Cable manual Local routing, Interaction of static and dynamic tables

Operating modes and functions 35

automatically incremented when the corresponding amount of time has elapsed. The distance is set to '16' after 3.5 minutes (route not reachable) and the route is deleted after 5.5 minutes.

Now if the router receives an IP/RIP packet, it must decide whether or not to incorporate the route contained into its dynamic table. This is done as follows:

KThe route is incorporated if it is not yet listed in the table (as long as there is enough space in the table).

KThe route exists in the table with a time of '5' or '6'. The new route is then used if it indicates the same or a better distance.

KThe route exists in the table with a time of '7' to '10' and thus has the distance '16'. The new route will always be used.

KThe route exists in the table. The new route comes from the same router which notified this route, but has a worse distance than the previous entry. If a device notifies the degradation of its own static routing table in this way, the cable modem will take this into account and include the poorer entry in its dynamic table.

The interaction of static and dynamic tables

The router uses the static and dynamic tables to calculate the actual IP routing table it uses to determine the path for data packets. In doing so, it includes the routes from the dynamic table which it does not know itself or which indicate a shorter distance than its own (static) route with the routes from its own static table.

Local routing

You know the following behavior of a workstation within a local network: The computer searches for a router to assist with transmitting a data packet to an IP address which is not on its own LAN. This router is usually notified to the operating system by its property of being the default router or gateway. It is often only possible to enter one default router which is supposed to be able to reach all the IP addresses which are unknown to the workstation computer if there are several routers in a network. Occasionally, however, this default router cannot reach the destination network itself but does know another router which can find this destination.

How else can you assist the workstation computer?

By default, the router sends the computer a response with the address of the router which knows the route to the destination network (this response is known as an ICMP redirect). The workstation computer then accepts this address and sends the data packet straight to the other router.

Certain computers, however, do not know how to handle ICMP redirects. To ensure that the data packets reach their destination anyway, use local routing (in ELSA LANconfig in the 'TCP/IP' configuration section on the 'Router' tab or in the /Setup/IP Router

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