When the routing table process checks for a resolvable static route using an
intermediate address, this check is always done in classful mode. This is
regardless of whether or not the ip classless command is
used. If the intermediate address cannot be resolved in the routing table in
classful mode, the static route will be deleted.
Figure
shows the output
of show ip route, when all links are up. Figure
shows
the output of debug ip routing when the 172.16.2.0/24
network goes down. The new routing table is shown in Figure
. Because Serial
0/1 is down, the directly connected network 172.16.2.0/24 is deleted from the
routing table.
The static route for 172.16.1.0/24 uses the now deleted 172.16.2.0/24
network to resolve its exit interface of 172.16.2.1. The subnet 172.16.2.0/24
has been removed in the routing table from under the parent classful network of
172.16.0.0. Notice however, that there is still another subnet, 172.16.3.0/24,
which is below that same parent route.
The static route
for 172.16.1.0/24 will not use the default route, 0.0.0.0/0 to resolve its next
hop address of 172.16.2.1. Instead, this static route will also be removed from
the routing table as shown in Figure
. Like
the 172.16.2.0/24 subnet, any packets destined for the 172.16.1.0/24 subnet
will be dropped.
The static route was removed because the routing table
uses the classful mode for resolving intermediate (next hop) addresses. There
is a reason for using classful mode for resolving intermediate addresses of
static routes. If classless mode was used and a default route was present,
backup static routes with higher administrative distances would never be
installed in the routing table if the primary static route failed. This is
because any static route, even one that references a nonexistent intermediate
address, could be resolved using the default route.
Remember that the
Cisco IOS Software stores all static routes, whether or not they are installed
in the routing table. The Cisco routing table process invokes a static route
function every 60 seconds which checks the routing table to install or remove
any static routes according to the dynamically changing routing table.
Lab
Exercise: Troubleshooting Problems at the Physical, Data Link, and Network
Layers I
After completing this lab, the student will be able to follow a
logical troubleshooting process to define, isolate, and correct problems
outlined in a trouble ticket.