Скачать или смотреть How to configure OSPF default route ?

Default Routing - Default Routing is something called gateway of last resort, when a specific route to a particular network does not exist, a router will drop all the packets destined to that specific network. Router forward packets using a default route when there is no specific routes that match a packet’s destination IP address in the routing table. The default route can be identified by all zeros in both the network and subnet mask (0.0.0.0 0.0.0.0). It is the least specific route possible. Default route are also use to connect ISP site or head site.

Routing protocols like RIPv2, EIGRP, and OSPF can advertise default routes, each router chooses the best default route to list as that router’s gateway of last resort.
Default Routing and configuration
https://www.internetworks.in/2018/11/...
for ccna/ccnp
https://www.internetworks.in/
for ccie labs
https://ccie.internetworks.in/
for MPLS
https://mpls.internetworks.in/