Default routes are special routes. In most cases, they are configured by administrators. Default routes can also be generated by dynamic routing protocols, such as OSPF and IS-IS.
Default routes are used only when no matching routing entry is available for packet forwarding in the routing table. A default route in the routing table is the route to the network 0.0.0.0 (with mask 0.0.0.0). You can check whether the default route is configured 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 along 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.