2 Static and Default Routing

How routers select routes

A router forwards packets using its routing table. Each entry identifies a destination network and how to reach it, usually through a next-hop router or an exit interface. If multiple entries match a destination, the router uses longest-prefix match: the most specific matching route wins.

Configure static routes

A static route is entered manually and remains in the configuration until it is changed or removed. Static routes are useful in small or stable networks, for specific paths, or as backup routes, but they do not automatically adapt to topology changes.

Configure a static route

On Cisco IOS, the general command specifies the destination network, subnet mask, and next-hop address:

For example, R1 and R2 are connected over 10.0.12.0/30. R1 uses 10.0.12.1 on the link and connects to LAN 192.168.1.0/24; R2 uses 10.0.12.2 and connects to LAN 192.168.2.0/24. Each router needs a route to the LAN behind the other router:

The next hop must be reachable through an active interface. A route on only one router is not enough for two-way communication; the destination network also needs a return path.

To remove a static route, negate the command by adding no:

Configure a default route

A default route is used when no more-specific route matches the destination. It is also called the gateway of last resort. On Cisco IOS, the IPv4 default route is written as 0.0.0.0/0:

For example, an edge router can send otherwise-unmatched traffic to its upstream router:

A default route does not replace routes needed to reach internal networks. Configure it only where forwarding unknown destinations to that next hop is appropriate, and ensure the next hop can return traffic.

Verify routes and connectivity

Check interface status and addresses, inspect the routing table, and test the path with commands such as:

In the routing table, C identifies a connected route and S identifies a static route. A static default route may appear as S*; the asterisk marks it as a candidate default. Confirm that the expected route is present and points to the intended next hop. A successful ping to the next-hop address is a useful first test, but it does not by itself prove that the final destination or its return path is working.

Troubleshoot unreachable destinations

When a destination is unreachable, check the route hop by hop:

1. Check interface state and addressing. The outgoing interface should be up, and the link and LAN addresses and masks should be correct.

2. Check the route’s destination and mask. A mistyped network or subnet mask can send traffic toward the wrong route or prevent the route from matching.

3. Check next-hop reachability. The router must have a working path to the configured next hop. If the next hop cannot be resolved, the static route may not be installed in the active routing table.

4. Check the return path. The far-end router needs a route back to the source network; otherwise, replies cannot return.

5. Check which route matches. A more-specific route takes precedence over a default route, even if the default route is configured correctly.

6. Test in stages. Ping the local gateway, then the next hop, then the remote router interface, and finally a host. Use traceroute to identify where forwarding stops.

Static routes manually specify paths to destination networks. A default route forwards traffic when no more-specific route matches. For end-to-end connectivity, verify the routing table, interfaces, next-hop reachability, and the route back to the source.