Default routes are special routes. Generally, administrators can manually configure default static routes. Default routes can also be generated through dynamic routing protocols such as OSPF and IS-IS.
Default routes are used only when packets to be forwarded have no matching routing entry in a routing table. In the routing table, a default route is the route to the network 0.0.0.0 (with the mask also being 0.0.0.0). You can check whether the default route is configured by using the display ip routing-table command.
If the destination address of a packet does not match any entry in the routing table, the packet is sent through a default route. If no default route exists and the destination address of the packet does not match any entry in the routing table, the packet is discarded. An Internet Control Message Protocol (ICMP) packet is then sent, informing the originating host that the destination host or network is unreachable.