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4. Implementation Details

When going from IPv4 to IPv6, the basic OSPF mechanisms remain unchanged from those documented in [OSPFV2]. These mechanisms are briefly outlined in Section 4 of [OSPFV2]. Both IPv6 and IPv4 have a link-state database composed of LSAs and synchronized between adjacent routers. Initial synchronization is performed through the Database Exchange process, which includes the exchange of Database Description, Link State Request, and Link State Update packets. Thereafter, database synchronization is maintained via flooding, utilizing Link State Update and Link State Acknowledgment packets. Both IPv6 and IPv4 use OSPF Hello packets to discover and maintain neighbor relationships, as well as to elect Designated Routers and Backup Designated Routers on broadcast and NBMA links. The decision as to which neighbor relationships become adjacencies, and the basic ideas behind inter-area routing, importing external information in AS-external-LSAs, and the various routing calculations are also the same.

In particular, the following IPv4 OSPF functionality described in [OSPFV2] remains completely unchanged for IPv6:

  • Both IPv4 and IPv6 use OSPF packet types described in Section 4.3 of [OSPFV2], namely: Hello, Database Description, Link State Request, Link State Update, and Link State Acknowledgment packets. While in some cases (e.g., Hello packets) their format has changed somewhat, the functions of the various packet types remain the same.

  • The system requirements for an OSPF implementation remain unchanged, although OSPF for IPv6 requires an IPv6 protocol stack (from the network layer on down) since it runs directly over the IPv6 network layer.

  • The discovery and maintenance of neighbor relationships, and the selection and establishment of adjacencies, remain the same. This includes election of the Designated Router and Backup Designated Router on broadcast and NBMA links. These mechanisms are described in Sections 7, 7.1, 7.2, 7.3, 7.4, and 7.5 of [OSPFV2].

  • The link types (or equivalently, interface types) supported by OSPF remain unchanged, namely: point-to-point, broadcast, NBMA, point-to-multipoint, and virtual links.

  • The interface state machine, including the list of OSPF interface states and events, and the Designated Router and Backup Designated Router election algorithm remain unchanged. These are described in Sections 9.1, 9.2, 9.3, and 9.4 of [OSPFV2].

  • The neighbor state machine, including the list of OSPF neighbor states and events, remains unchanged. The neighbor state machine is described in Sections 10.1, 10.2, 10.3, and 10.4 of [OSPFV2].

  • Aging of the link-state database, as well as flushing LSAs from the routing domain through the premature aging process, remains unchanged from the description in Sections 14 and 14.1 of [OSPFV2].

However, some OSPF protocol mechanisms have changed as previously described in Section 2 herein. These changes are explained in detail in the following subsections, making references to the appropriate sections of [OSPFV2].

The following subsections provide a recipe for turning an IPv4 OSPF implementation into an IPv6 OSPF implementation.

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