In most cases, using an exit interface will be more efficient during the
routing table process for resolving the static route. Figure
shows a sample
network with three routers.
Figure
shows a
static route being configured and installed in the routing table.
The
static route that was created uses an intermediate IP address of 172.16.2.2. As
long as the intermediate IP address can be resolved in the routing table, it
does not have to be the actual next-hop routers interface. The static network
route 172.16.3.0 in the example, must finally be resolved to a route in the
routing table that has an exit interface.
Figure
shows the
routing table of RouterA after the static route was installed.
Notice
that the static route that was configured does not contain an exit interface.
Instead, the static routing table entry contains the intermediate address that
was used when configuring the static route.
Whenever the routing table
process needs to use the static route entry for the 172.16.3.0/24 network, it
will also need to resolve the intermediate address, 172.16.2.2. This is called
a recursive lookup.
Because this routing table entry for the 172.16.3.0/24 route does not
contain an exit interface, but instead the intermediate address 172.16.2.2, it
cannot use this entry alone to forward the packets. The routing table process
uses the intermediate address of 172.16.2.2 and does another lookup (recursive
lookup) in the routing table to find a route for this 172.16.2.0 network.
The routing table process finds the directly connected network entry for
172.16.2.0.
C 172.16.2.0 is directly connected,
Serial0/0
Because this entry does have an exit
interface, Serial0/0, the routing table process can use this route to forward
the packets for 172.16.3.0/24. Remember that it took two routing table lookups
to route packets using the static route entry 172.16.3.0/24. The first routing
table lookup was for the 172.16.3.0 network, the destination IP address of the
packet, and the second routing table lookup was for the exit interface of the
intermediate address used in the entry.
