Principle of OSPFv3

LSR packets are sent to the neighbor to request the required LSAs.

An OSPFv3 router sends LSR packets to its neighbor only after they exchange DD packets.

Each router or DR generates one or more intra-area prefix LSAs and transmits it in the local area.

• An LSA generated on a router describes the IPv6 address prefix associated with the router LSA.
• An LSA generated on a DR describes the IPv6 address prefix associated with the network LSA.

An ABR can belong to two or more areas, but one of the areas must be a backbone area.

An ABR is used to connect the backbone area and the non-backbone areas. It can be physically or logically connected to the backbone area.

At least one interface on a backbone router belongs to the backbone area.

All ABRs and internal routers in Area 0, therefore, are backbone routers.

A router that exchanges routing information with other ASs is called an ASBR.

An ASBR may not locate on the boundary of an AS. It can be an internal router or an ABR. If an OSPFv3 router imports the external routing information, the router is an ASBR.

Because of the high reliability of Type 1 external routes, the calculated cost of external routes is equal to that of AS internal routes, and can be compared with the cost of OSPFv3 routes.

That is, the cost of a Type1 external route equals the cost of the route from the router to the corresponding ASBR plus the cost of the route from the ASBR to the destination address.

Because of the low reliability of Type2 external routes, the cost of the route from the ASBR to a destination outside the AS is considered far greater than the cost of any internal path to an ASBR.

Therefore, OSPFv3 only takes the cost of the route from the ASBR to a destination outside the AS into account when calculating route costs. That is, the cost of a Type2 external route equals the cost of the route from the ASBR to the destination of the route.

By default, OSPFv3 areas are defined as common areas. Common areas include:

• Standard area: transmits intra-area, inter-area, and external routes.
• Backbone area: connects to all other OSPFv3 areas and transmits inter-area routes. The backbone area is represented by area 0. Routes between non-backbone areas must be forwarded through the backbone area.

• The backbone area must have all its devices connected.
• All non-backbone areas must remain connected to the backbone area.

A stub area is a non-backbone area with only one ABR and generally resides at the border of an AS. The area border router (ABR) in a stub area does not transmit received AS external routes, which significantly decreases the number of entries in the routing table on the ABR and the amount of routing information to be transmitted. To ensure the reachability of AS external routes, the ABR in the stub area generates a default route and advertises the route to non-ABRs in the stub area.

A totally stub area allows only intra-area routes and ABR-advertised Type 3 link state advertisements (LSAs) carrying a default route to be advertised within the area.

• The backbone area cannot be configured as a stub area.
• An autonomous system boundary router (ASBR) cannot exist in a stub area. Therefore, AS external routes cannot be advertised within the stub area.
• A virtual link cannot pass through a stub area.

An NSSA is similar to a stub area. An NSSA does not advertise Type 5 LSAs but can import AS external routes. ASBRs in an NSSA generate Type 7 LSAs to carry the information about the AS external routes. The Type 7 LSAs are advertised only within the NSSA. When the Type 7 LSAs reach an ABR in the NSSA, the ABR translates the Type 7 LSAs into Type 5 LSAs and floods them to the entire AS.

A totally NSSA area allows only intra-area routes to be advertised within the area.

• ABRs in an NSSA advertise Type 3 LSAs carrying a default route within the NSSA. All inter-area routes are advertised by ABRs.
• A virtual link cannot pass through an NSSA.

If the link layer protocol is Ethernet or FDDI, OSPFv3 defaults the network type to broadcast.

In this type of networks, the following situations occur:

• Hello messages, LSU packets, and LSAck packets are transmitted in multicast mode (FF02::5 is the reserved IPv6 multicast address of the OSPFv3 router; FF02::6 is the reserved IPv6 multicast address of the OSPFv3 DR or BDR).
• DD packets and LSR packets are transmitted in unicast mode.

If the link layer protocol is frame relay, ATM, or X.25, OSPFv3 defaults the network type to NBMA.

In this type of networks, protocol packets such as Hello messages, DD packets, LSR packets, LSU packets, and LSAck packets, are transmitted in unicast mode.

Regardless of the link layer protocol, OSPFv3 does not default the network type to P2MP. A P2MP network must be forcibly changed from other network types. The common practice is to change a non-fully connected NBMA to a P2MP network.

In this type of networks, the following situations occur:

• Hello messages are transmitted in multicast mode with the multicast address as FF02::5.
• Other protocol packets, including DD packets, LSR packets, LSU packets, and LSAck packets, are transmitted in unicast mode.

If the link layer protocol is PPP, or LAPB, OSPFv3 defaults the network type to P2P.

In this type of network, the protocol packets, including Hello messages, DD packets, LSR packets, LSU packets, and LSAck packets, are transmitted to the multicast address FF02::5.