Installing and Administering LAN/9000 Software

Chapter 6 165
Network Addressing
Configuring Gateways on Variable-Length Subnets
Hosts A, B and C:
ROUTE_DESTINATION[0]=”default”
ROUTE_GATEWAY[0]=”192.6.12.132”
ROUTE_COUNT[0]=”1”
Host D (Site gateway):
ROUTE_DESTINATION[0]=”net 192.6.12.64”
ROUTE_MASK[0]=”255.255.255.192”
ROUTE_GATEWAY[0]=”192.6.12.129”
ROUTE_COUNT[0]=”1”
ROUTE_DESTINATION[1]=”net 192.6.12.192”
ROUTE_MASK[1]=”255.255.255.224”
ROUTE_GATEWAY[1]=”192.6.12.130”
ROUTE_COUNT[1]=”1”
ROUTE_DESTINATION[2]=”net 192.6.12.32”
ROUTE_MASK[2]=”255.255.255.240”
ROUTE_GATEWAY[2]=”192.6.12.34”
ROUTE_COUNT[2]=”1”
ROUTE_DESTINATION[3]=”default”
ROUTE_GATEWAY[3]=”192.6.20.1”
ROUTE_COUNT[3]=”0”
If you add a new subnetwork to the Facility LAN at a later time, you will
need to add only an appropriate routing entry on Host D. It will not be
necessary to configure the other subnet gateways A, B, and C.
With this configuration, each subnet gateway (Hosts A, B, and C) will
initially route messages for a system outside its subnet to Host D. The
subnet gateway, however, will learn of the more direct routes
automatically when Host D redirects the messages to one of the other
subnet gateways. Subsequent messages for the destination system will
be routed directly to the appropriate subnet gateway.
For example, referring to Figure 6-15, suppose messages are sent from
system A1 (192.6.12.67) to system B1 (192.6.12.34). The first message
will actually be routed to Host D (through Host A). Host D then will
redirect the message through Host B. At the same time, Host D will
notify Host A that Host B is a more direct route for messages to system
B1. Subsequent messages to system B1 will be routed directly from Host
A to Host B.
Redirected routes are called dynamic routes. You can see these dynamic
routes by executing the command netstat -rv on Host A. Dynamic
routes are indicated in the display by a D flag.