Default routes of firewalls

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 described as follows:
To put it in a simple way, default routes are used only when packets to be forwarded do not match any routing entry in a routing table. In a routing table, a default route is the route to network 0.0.0.0 (with the mask 0.0.0.0). You can run the display ip routing-table command to check whether a default route is configured.
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.

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