Cisco CCNA Certification Training:

Troubleshooting With Default Static Routes

By Chris Bryant, CCIE #12933

Part of earning your CCNA certification and becoming a world-class network admin is knowing how to keep your network running when things go wrong. When you're working with real-world networks, you've got to know some "quick fixes" that allow the end users to keep working while you troubleshoot. One such fix that works well in real networks (and that couldn't hurt on the CCNA exam) is a default static route.

Here's the routing table we'll use in this tutorial:

Gateway of last resort is not set

172.17.0.0/16 is variably subnetted, 2 subnets, 2 masks

C 172.17.13.0/24 is directly connected, Serial1

C 172.17.21.0/30 is directly connected, BRI0

Packets destined for 20.1.1.0 /24, or any network that doesn't have a match in that small routing table, will be dropped by this router.

A "regular" static route to 20.1.1.0 /24 would work, but what if that routing table is so small because a routing protocol is suddenly working incorrectly? What if you need one single route to act as a route of last resort, a route that will handle all packets that don't have any match in the routing table? That's where default static routes come in. If we wanted all traffic with no more-specific match in the routing table to exit the local router via the BRI interface, the ip route command would look like this:

R1(config)#ip route 0.0.0.0 0.0.0.0 BRI0

After adding this default static route, the routing table looks as follows:

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks

C 172.12.13.0/24 is directly connected, Serial1

C 172.12.21.0/30 is directly connected, BRI0

S* 0.0.0.0/0 is directly connected, BRI0

The S* indicates a default static route, and the exit interface is the BRI interface. Any packets that do not have a match in the routing table will be sent out that interface. Of course, you have to take the ISDN costs into account if you configure this, but that's a subject for another time!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com