Page 359

324

Chapter 6

Troubleshooting TCP/IP Connectivity

How do you configure a router to send packets to a neighbor without a

route? That is where the gateway of last resort comes in. A gateway of last
resort tells the router that if it doesn't have a route to a given network, it
should send the packet out the specified interface, or default gateway.

The purpose of a default gateway is somewhat of a last-ditch effort to for-

ward a packet. Look at Figure 6.9. In this example, Router A receives a
packet from Host A that is destined for network 10.1.2.0. The problem is
that Router A does not have a route for 10.1.2.0. The only chance of get-
ting the packet forwarded to network 10.1.2.0 is to send it to Router B and
hope that Router B has a route to network 10.1.2.0. Router A considers
Router B as its default gateway, so it sends the packet to Router B. For this
example, assume that Router B does have the route and sends the packet on
its way.

F I G U R E 6 . 9

Default gateways

TCP/IP hosts also have default gateways set. If the default gateway for a

router or a host is configured improperly, data will not be routed. Default
gateways are used on TCP/IP hosts so they do not have to keep individual
route tables. All hosts need to point to a router on the same network to be
used as the default gateway.

When the default gateway is not working properly, whether it is on a host

or on a router, the problem is probably caused by incorrect configuration.

www.sybex.com