Lucent Technologies PortMaster - Manual (Page 65)

Configuring Global Settings 3-27
Setting Static Routes
For example, suppose the address of Ether0 is 172.16.1.1 with a 255.255.255.0 subnet
mask (a class B address subnetted on 24 bits) and the destination of ptp1 is 192.168.9.65
with a 255.255.255.240 subnet mask (a class C address subnetted on 28 bits). If routing
broadcast is on, the PortMaster routing broadcast on Ether0 claims a route to the entire
192.168.9.0 network. Additionally, the broadcast on ptp1 claims a route to 172.16.0.0.
Sometimes, however, you want the PortMaster to collapse routes to some bit boundary,
other than the network boundary. In this case, you can use the static netmask table.
However, RIP supports only host and network routes, because it has no provision to
include a netmask. Therefore, if you set a static netmask in the netmask table, the
PortMaster collapses the route to that boundary instead, and broadcasts a host route
with that value. Other PortMaster routers with the same static netmask table entry
convert the host route back into a subnet route when they receive the RIP packet.
This work-around works only if all the products involved are from PortMaster products,
with the following two exceptions:
• If you use a netmask table entry of 255.255.255.255. In this case, the routes
broadcast as host routes really are host routes, so non-PortMaster routers can use
them. Keep in mind that not all routers accept host routes.
• If the non-PortMaster router can convert host routes into subnet routes through
some mechanism of its own.
Uses for Static Netmasks
The most common use for the static netmask table is to split a single class C network
into eight 30-host subnets for use in assigned pools. Subnetting allows each PortMaster
to broadcast a route to the subnet instead of claiming a route to the entire class C
network. An example of that use is provided below.
The next most common use for the static netmask table is to allow dial-in users to use
specified IP addresses across multiple PortMasters in situations where assigned IP
addresses are not sufficient. This use can result in very large routing tables and is not
recommended except where no other alternative is possible.
The netmask table can be accessed only through the command line interface. To add a
static netmask, use the add netmask command. To delete a static netmask, use the
delete netmask command. The show table netmask command shows both dynamic
netmasks and static netmasks, marking them accordingly.