A National Consulting Company

The consulting company has a network of a dozen sites using Cisco site routers and HP hubs for the site LAN. You install NNM on an existing HP-UX workstation, configure the necessary router community strings, and prepare to manually direct NNM to discover the network. Unfortunately, the workstation’s subnet mask is not set correctly. NNM assumed that the entire class B network was flat and prepared to discover the entire network. NNM depicted the single subnet as a dense wheel of icons. After correcting the NNM system’s subnet mask, autodiscovery proceeds normally.

The network administrator quickly partitions the map by site and consults with his peers to verify that some specific topology information was consistent with NNM’s discovery and autolayout. While all the routers were configured with the proper subnet masks, the corporate standard appeared to allow computer systems to default their subnet mask to 255.255.0.0. As a result, NNM generated a torrent of configuration alarms, complaining that many devices had an incorrect subnet mask. There wasn’t anything to be done about it from an NNM point of view.

Curiously, the network behaved correctly despite the fact that many devices had no routing table entries. It turns out that the Cisco routers had proxy ARP enabled by default. This kept things working.

You configure NNM to monitor the performance and status of the WAN links of the routers and unmanage any systems you had no control over. This has the pleasant side effect of reducing the configuration alarms relating to bad subnet masks.