The Proper Use Of Default Static Routes

By: Chris Bryant

Earning your Cisco CCNA certification means knowing the details of Cisco routing, and that includes knowing when Cisco routing terms don't quite mean what they sound like they mean.  For example, the general meaning of "default" is a setting that is used unless you or I change it. On the other hand, a default route is a route taken by packets that have no other route they can take.  Let's take a look at how a default static route is configured and used on a Cisco router.

Here's our current routing table:

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets

C       1.1.1.1 is directly connected, Loopback0

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

If we have packets destined for the network 15.1.1.0 /24, the packets will be dropped by this router.  There's no match in that routing table for that network and the gateway of last resort is not set. 

We could configure a static route to the 15.1.1.0 /24 network, but instead we'll use a default static route.  The hardest part of configuring that route type is getting used to the odd syntax!  As with any other static route, we can use the IP address of the next-hop router or the local router's exit interface.

Here, we'll send any traffic with no more-specific match in the routing table out the local router's Serial1 interface.

R1(config)#ip route 0.0.0.0 0.0.0.0 serial1

Let's take a look at the routing table now.

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     1.0.0.0/32 is subnetted, 1 subnets

C       1.1.1.1 is directly connected, Loopback0

     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, Serial1

A gateway of last resort has now been successfully configured, and the S* means that last route in the table is a static default route.  RememberFree Articles, the default route is not the route that all packets will take - it's the route packets use if there is no other possible match for their destination in the routing table.

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