Workshop

If something changes, the routers communicate it directly; because they do a quick check on each other
fairly often (usually 10 seconds), a dead router is noticed and the routing tables are updated quickly.
Good stuff!
There’s not really a lot that goes wrong with routing protocols that aren’t domino-effect problems—that
is, problems caused ultimately by a bad hub port, cable, or card on the network, or by somebody
changing a configuration. Still, it’s good to be acquainted with the basics so that you can read the
configuration from your router and get the basic gist of what’s going on.
All modern servers—whether NT, UNIX, or NetWare—can be configured to be simple and cheap routers.
Of course, they need to be multihomed in order to be connected to more than one network.
I use this sometimes to swap out routers. If I suspect that a network problem might be router related, it’s a
lot cheaper to temporarily put in a PC with UNIX on it acting as a router than it is to purchase a new
$10,000 router just to rule out a router problem.
Router Rumble!
Be careful if you start to experiment with routers. Unlike servers, which are typically self-contained
entities, routers that participate in routing protocols on your network can seriously damage your
connectivity if they’re misconfigured.
I’ve seen a situation where somebody plugged a router into network A that was configured to connect
network A with network B. This was because the new router was intended to replace the router that
currently served network B.
Unfortunately, the new router’s second interface was configured as being on network B, and though it
wasn’t plugged into network B proper, it started to advertise via routing protocols that it knew the best
route to network B. This caused confusion in the routing tables; some packets started to go to this
router, only to discover that there was an entrance, but no exit! The legitimate router for network B
was doing what it was supposed to do, but it was being usurped by the new router.
Here’s the lesson: Don’t plug a new router into a production network until it’s time; instead, test on a
disconnected network. Test hubs are cheap enough—putting together a couple of test segments
shouldn’t cost more than $100.
I’ve also seen a situation where a RIP router for network C was accidentally plugged into network A.
Because RIP routes are broadcast rather than going address to address, the router was able to tell all the
routers on that segment that it knew the best way to network C. Again, all of a sudden, folks couldn’t