troubleshoot networks in the field.
Content
5.2 Troubleshooting Static Routes
5.2.1 Static routes and classful lookups
Routing table maintenance can be more complex for static routes
than dynamic routes. Static routes in the routing table can be
invalid, in other words, they can reference an inactive
exit-interface or an intermediate network address that cannot
be resolved. The static routing process within the IOS must be
able to find invalid static routes and remove them from the
routing table, as well as install new static routes that become
valid or available. 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. It is important to 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. For example, if an interface has gone down, this
function will remove any static routes that were resolved using
this interface. This may be a static route that includes this
interface as its exit interface, or a static route that has an
intermediate address, but eventually that intermediate address
is ultimately resolved via this downed interface. In addition,
static routes may be installed in the routing table when an
interface becomes active or a network is installed in the
routing table. Lab Activity Lab Exercise:
Troubleshooting Problems at the Physical, Data Link, and
Network Layers 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.
Content 5.2 Troubleshooting
Static Routes 5.2.2 Static routes and
intermediate addresses Static routes can be created either
by using an intermediate network address or an exit interface.
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. This is sometimes
referred to as the next-hop IP address, but the IP address
does not have to be the physical next-hop. 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.
Ultimately, 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. One additional route
lookup may not be much of a factor on the performance of the
routing process. However, static routes, which take multiple
recursive lookups to get resolved, could have an impact.
Content 5.2 Troubleshooting Static
Routes 5.2.3 Static route optimization with
serial networks There are ways to avoid recursive table
lookups. Although, there may be times when recursive table
lookups are preferred and configured by the network
administrator. Most of the time, static routes over serial
point-to-point networks can easily avoid recursive route
lookups by using an exit interface instead of the intermediate
or next-hop address. Figure shows an example of creating a
static route on RouterB for RouterA LAN, using an exit
interface instead of an intermediate address. Examine how this
route was installed in the routing table, as shown in Figure .
Instead of using an intermediate address, this route is
resolved with the exit interface, Serial0/0 that was configured
with the static route command. The routing process, using this
single entry, can forward any packets destined for the
172.16.1.0/24 network. No recursive route lookups are needed.
Because only a single routing table lookup is needed, instead
of multiple, recursive lookups, this type of static route will
increase performance of the routing table process. Notice that
the routing table states that this static route is “directly
connected.” S 172.16.1.0 is directly connected, Serial0/0
In the case of a static route, “directly connected” means
that the static route was configured with an exit interface.
This does not mean that it is a directly connected network of
an interface on the router. Like all static routes, the default
administrative distance of this static route is still 1. Only
directly connected interfaces, which have a code of C in the
routing table, can have an administrative distance of 0.
Although it is possible to modify the administrative distance
of static and dynamic routes, they cannot be given the
administrative distance of 0. Likewise, directly connected