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| 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.