Configuring OSPF authentication on a virtual link
OSPF supports the same two methods of authentication for virtual links as it does for VLANs and subnets in an area—password and MD5. In the default configuration, OSPF authentication is disabled. Only one method of authentication can be active on a virtual link at a time, and if one method is configured on a virtual link, configuring the alternative method on the same link automatically replaces the first method with the second. Both ends of a virtual link must use the same authentication method (none, password, or MD5 key chain) and related credentials.(Any interfaces that share a VLAN or subnet with the interface used on an ABR for a virtual link, including intermediate routing switches, must be configured with the same OSPF authentication.)