Using only an exit-interface instead of an intermediate address causes a
problem. Because the network is multiaccess, there most likely are multiple
devices, receivers, and so on, sharing this network.
Figure
shows a network
between RouterA and RouterB that is a multiaccess, Fast Ethernet link.
A
static route using only an intermediate address could be configured. However,
this will cause a recursive routing table lookup. Figure
shows
both the static route command and its installment in the routing table. When
packets need to be routed for 172.16.1.0, the recursive route lookup happens
first for the 172.16.1.0 network, and then for the intermediate address, with
the lookup of 172.16.2.0 network.
To avoid the recursive route lookup,
the solution is to use both an intermediate address and an exit interface.
Figure
shows the
recommended way to configure a static route in this case, and the routing table
entry. When using both the intermediate address and the exit interface, only a
single lookup is needed in the routing table lookup process.
The
standard rule-of-thumb when configuring static routes is to use an exit
interface over point-to-point and exit interfaces with the intermediate address
over multiaccess networks. This will avoid the recursive route lookups caused
by static routing entries that only contain an intermediate address.