The example in Figure 1 configures R1 to enable OSPF MD5 authentication on all interfaces. Notice the informational messages stating that the OSPF neighbor adjacencies with R2 and R3 have changed to the Down state, because R2 and R3 have not yet been configured to support MD5 authentication.
As an alternative to globally enabling MD5 authentication, the example in Figure 2 demonstrates how to configure R1 to enable OSPF MD5 authentication on a per-interface basis. Again, notice how the OSPF neighbor adjacencies have changed to the Down state.
Use the Syntax Checker in Figure 3 to enable OSPF MD5 authentication globally on R2 and per interface on R3.
Again, informational messages appear. The first message is because the neighbor adjacency with R1 has been re-established. However, the adjacency with R3 has transitioned to the Down state, because R3 is still not configured
After R3 is configured, all neighbor adjacencies have been re-established.