Bay Networks Nautica 200 Reference Manual page 27

Understanding the Technology
The Nautica Series also supports a feature called unnumbered
links. This means that you do not have to configure an
inter-router network for the path. This reduces the number of
network addresses needed, because you no longer need to address
each link separately, with a consequent reduction in the
configuration overhead. Hence the Nautica Series is able to
support multiple ISDN lines to various destinations, based on the
needs of the user, rather than having all the possible destinations
pre-programmed, as is the case with other routers.
Many client/server-based network protocols broadcast Idle or
Hello frames regularly, making them unsuitable for cost-effective
bridging or routing across ISDN. The Nautica Series solves this
by filtering broadcast and multicast frames, while intelligently
routing and spoofing protocols.
The IP RIP protocol causes routers to send out their updated
routing tables every 30 seconds. While this does not matter for
leased line connections, it is an important consideration for
routers connected over ISDN links, because the ISDN links then
become active every 30 seconds to transmit this overhead.
Nautica series routers have developed an advanced spoofing
solution to overcome this problem. It works on the basis that
changes to the network occur rarely, so routing table updates need
not be sent every 30 seconds, but can be transferred with real data
when the link becomes active. This is known as a piggyback
update.
Normally routes learned from routing updates (IP RIP and IPX
RIP) are aged and removed from the table if no update is received
in 180 seconds. However, when routing updates are only received
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 Nautica Series
overcomes this by making ISDN routes semi-permanent, so that
they can be updated, but never expire due to time-out.
117237-C Rev. A 1-7