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Reference Guide for the Model RT210/RT211 ISDN Routers
Routing IP Over ISDN
The NETGEAR Model RT210 and Model RT211 ISDN routers are able to connect to other
networks using ISDN just as if the network were attached with a leased line. When data is to be
sent to the remote network, the router automatically dials the ISDN number of the router with the
router that is advertising the destination IP network address.
The router is able to match the ISDN telephone number to a given IP address by checking the
remote router path associated with the destination address and then inspecting the path to
determine the ISDN number to be dialed. During initial setup of the router, the user calls any
remote units that are to be part of the network. The router automatically learns about those units
and creates its routing address table. This process makes it very easy to set up the router and user
input errors are less likely.
The NETGEAR Model RT210 and Model RT211 ISDN routers also support a feature called
unnumbered links, meaning that you do not have to configure an inter-router network for the path.
With unnumbered links, the number of network addresses needed is reduced. You do not need to
address each link separately with a consequent reduction in the configuration overhead.
Rather than having all possible destinations preprogrammed, the router is able to support multiple
ISDN lines to various destinations that are based on the requirements of the user. Many client/
server-based network protocols broadcast idle or hello frames regularly, making them unsuitable
for cost-effective bridging or routing across ISDN.
While intelligently routing and spoofing the protocols, the routers make cost-effective routing
possible by filtering broadcast and multicast frames. The IP RIP protocol causes routers to send
out their routing tables for update every 30 seconds. Although this is of no consequence for leased
line connections, it is an important consideration for routers connected over ISDN links. The ISDN
links then become active every 30 seconds to transmit this overhead. The router implements an
advanced spoofing solution known as piggyback update to overcome this problem. Because
changes to the network occur rarely and the routing table updates need not be sent every 30
seconds, the routers transfer the updates only when the link becomes active with real data.
Normally routes learned from routing updates (IP RIP) are aged and removed from the routing
table if an update is not received within 180 seconds. However, when routing updates are received
only when an ISDN data call is connected, it is likely that there would be insufficient calls made to
ensure that routes were not removed after the 180 second time-out had expired. The NETGEAR
Model RT210 and Model RT211 ISDN routers make ISDN routes semipermanent so that they can
be updated, but so that they do not expire as a result of timeout.
TCP/IP Routing
B-5
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Troubleshooting
