Operating modes and functions
ELSA LANCOM DSL/10 Office
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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 can you assist the workstation computer now?
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-
module/Local Routing On
menu). This is how you tell the router in your device
to send the data packet to the other, responsible router itself. The router will then no
longer send any ICMP redirects.
This may seem to be a good idea in principle, but local routing should still only be used
as a last resort, since this function leads to doubling of the number of data packets being
sent to the destination network required. The data is first sent to the default router and
is then sent on from here to the router which is actually responsible in the local network.
Dynamic routing with IP RIP
In addition to the static routing table ELSA routers also have a dynamic routing table
containing up to 128 entries. Unlike the static table, you do not fill this out yourself, but
leave it to be dealt with by the router itself. It uses the Routing Information Protocol (RIP)
for this purpose. This protocol is used by all devices with RIP to exchange information
regarding the reachable routes.
What information is propagated by IP RIP?
A router uses the IP RIP information to inform the other routers in the network of the
routes it finds in its own static table. The following entries are ignored in this process:
K
Rejected routes with the '0.0.0.0' router setting.
K
Routes referring to on other routers in the local network.
