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Improving the availability of network services at a site requires more than simply installing a second router. We started with an examination of how end systems on the network could best find a functional router when there is more than one to choose from. Although there are many protocols available that can serve better than a static default gateway assignment, the usual choice will be Cisco's Hot Standby Router Protocol (if Cisco routers are being used) or Virtual Router Redundancy Protocol (if other brands of routers are used). Another very popular approach, although it is slow to recover from failures unless tuned, is passive RIP.

We then diverged to look at the impact on dial backup when the dial backup link is on one router and the link being backed up is on another. While backup interface can no longer be used, we found that both dialer watch and dial-on-demand could easily adapt to the modified environment.

We finished our investigation by looking at the challenge of minimizing the impact of LAN failures that split a single LAN into disconnected segments. This mode of failure should be extremely rare in a well-designed, highly redundant switched LAN. However, when it does occur, the impact on IP and IPX communications is severe. While nothing can be done to help IPX—other than installing two or more independent LANs and multihoming all critical systems—non-standard assignment of IP subnetwork addresses can be combined with overlapping subnetwork masks to greatly reduce the impact on singly connected IP end systems. We showed a configuration in which the only IP communications disrupted would be those between local systems on opposite sides of the break in the LAN.


  1. While dialer watch can also be used with Cisco's IGRP, today we would only design a network based on IGRP for backwards compatibility with an existing IGRP implementation.
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