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. Remember,
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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