RFC 5340 and RFC 1583 each define a route selection rule differently. After OSPFv3 is enabled on a switch, you need to specify a route selection rule based on the switch configuration. By default, a switch complies with the route selection rule defined in RFC 5340. If the neighboring switch complies with the route selection rule defined in RFC 1583, you need to configure the local switch to comply with that defined in RFC 1583. This allows all switches in an OSPFv3 area to comply with the same route selection rule.