OSPFv3 Fundamentals

Running on IPv6, OSPFv3 is an independent routing protocol that is developed on the basis of OSPFv2.

OSPFv3 and OSPFv2 are the same in terms of the working principles of the Hello packet, state machine, link-state database (LSDB), flooding, and route calculation.
OSPFv3 packets are encapsulated into IPv6 packets and can be transmitted in unicast or multicast mode.

OSPFv3 Packet Types

Packet Type	Function
Hello packet	Hello packets are sent periodically to discover and maintain OSPFv3 neighbor relationships.
Database Description (DD) packet	DD packets contain the summary of the local LSDB and are exchanged between two OSPFv3 routers to update the LSDBs.
Link State Request (LSR) packet	LSR packets are sent to the neighbor to request the required LSAs. An OSPFv3 device sends LSR packets to its neighbor only after they exchange DD packets.
Link State Update (LSU) packet	LSU packets carry the LSAs required by neighbors.
Link State Acknowledgment (LSAck) packet	LSAck packets acknowledge the receipt of an LSA.

LSA Types

OSPFv3 encapsulates routing information into LSAs for transmission. Table 1 describes LSAs and their functions.

**Table 1** LSAs and their functions
LSA Type	LSA Function
Router-LSA (Type 1)	A device generates a router-LSA for the area in which each OSPFv3 interface resides, and advertises the LSA within the area. This LSA describes the link status and cost of the device.
Network-LSA (Type 2)	Describes the states of local links. Such LSAs are generated by a designated router (DR) and advertised in the area to which the DR belongs.
Inter-Area-Prefix-LSA (Type 3)	Describes routes to a specific network segment in an area. Inter-Area-Prefix-LSAs are generated on the Area Border Router (ABR) and sent to related areas.
Inter-Area-Router-LSA (Type 4)	Describes routes to an Autonomous System Boundary Router (ASBR). Inter-Area-Router-LSAs are generated by an ABR and advertised to all related areas except the area to which the ASBR belongs.
AS-external-LSA (Type 5)	Generated by ASBRs, and flooded to all areas, excluding stub areas and NSSAs. Such LSAs describe routes beyond the local AS.
NSSA LSA (Type7)	Describes routes to a destination outside the AS. It is generated by an ASBR and advertised in NSSAs only.
Link-LSA (Type 8)	Each device generates a link-LSA for each link. The link-LSA describes the link-local address and IPv6 prefix address of the link, and provides the link options to be set in the network-LSA. The link-LSA is transmitted only on the link.
Intra-Area-Prefix-LSA (Type 9)	Each device and DR generate one or more such LSAs and transmit them in the areas to which the device and DR belong. Such LSAs generated by a device describe the IPv6 address prefixes associated with router LSAs. An LSA generated on a DR describes the IPv6 prefix address associated with the network LSA.

Router Types

Figure 1 Router types

**Table 2** Router types and descriptions
Router Type	Description
Internal router	All interfaces on an internal router belong to the same OSPFv3 area.
Area border router (ABR)	An ABR belongs to two or more areas, one of which must be the backbone area. An ABR is used to connect the backbone area and non-backbone areas. It can be physically or logically connected to the backbone area.
Backbone router	At least one interface on a backbone router belongs to the backbone area. Internal routers in Area 0 and all ABRs are backbone routers.
AS boundary router (ASBR)	An ASBR exchanges routing information with other ASs. An ASBR does not necessarily reside on the border of an AS. It can be an internal router or an ABR. An OSPFv3 device that has imported external routing information will become an ASBR.

OSPFv3 Route Types

Inter-area routes and intra-area routes describe the network structure of an AS. External routes describe how to select a route to the destination outside an AS. OSPFv3 classifies the imported AS external routes into Type 1 routes and Type 2 routes.

Table 3 describes the routes in descending order of priority.

**Table 3** Types of OSPFv3 routes
Route Type	Description
Intra Area	Indicates routes within an area.
Inter Area	Indicates routes between areas.
Type 1 external route	Such routes offer higher reliability, and their costs are approximately the same as those of AS internal routes and are comparable with the costs of routes generated by OSPFv3. Cost of a Type 1 external route = Cost of the route from a local router to an ASBR + Cost of the route from the ASBR to the destination of the Type 1 external route
Type 2 external route	Type 2 external routes have low reliability, and therefore OSPFv3 considers the cost of the route from an ASBR to the destination of a Type 2 external route to be much greater than the cost of any internal route to the ASBR. Cost of a Type 2 external route = Cost of the route from the ASBR to the destination of the Type 2 external route

Area

When a large number of routers run OSPFv3, LSDBs become very large and require a large amount of storage space. Large LSDBs also complicate shortest path first (SPF) computation and are computationally intensive for the routers. Network expansion causes the network topology to change, which results in route flapping and frequent OSPFv3 packet transmission. When a large number of OSPFv3 packets are transmitted on the network, bandwidth usage efficiency decreases. Each change in the network topology causes all routers on the network to recalculate routes.

OSPFv3 prevents frequent LSDB updates and improves network utilization by partitioning an AS into different areas. routers can be logically allocated to different groups (areas), and each group is identified by an area ID. A router, not a link, resides at the border of an area. A network segment or link can belong only to one area. An area must be specified for each OSPFv3 interface.

OSPFv3 areas include common areas, stub areas, and NSSAs. Table 4 describes these in more detail.

**Table 4** Area types
Area Type	Function	Notes
Common area	OSPFv3 areas are common areas by default. Common areas include standard areas and backbone areas. 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.	In the backbone area, all devices must be connected. All non-backbone areas must be connected to the backbone area.
Stub area	A stub area is a non-backbone area with only one area border router (ABR) and generally resides at the border of an AS. The 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 router 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 stubby area allows only intra-area routes and ABR-advertised Type 3 default routes to be advertised within the area. The totally stubby area does not allow AS external routes or inter-area routes to be advertised.	The backbone area cannot be configured as a stub area. An ASBR cannot exist in a stub area. Therefore, AS external routes cannot be advertised within the stub area.
NSSA	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.

Network Types Supported by OSPFv3

As shown in Table 5, OSPFv3 classifies networks into four types based on the types of link layer protocols.

**Table 5** Types of OSPFv3 networks
Network Type	Description
Broadcast	OSPFv3 considers networks with Ethernet or Fiber Distributed Data Interface (FDDI) as the link layer protocol as broadcast networks by default. On this type of network: Hello packets, LSU packets, and LSAck packets are usually transmitted in multicast mode. FF02::5 is an IPv6 multicast address reserved for an OSPFv3 device. FF02::6 is an IPv6 multicast address reserved for an OSPFv3 DR or backup designated router (BDR). DD and LSR packets are transmitted in unicast mode.
Non-broadcast Multiple Access (NBMA)	OSPFv3 considers networks with X.25 as the link layer protocol as NBMA networks by default. On an NBMA network, protocol packets, such as Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are sent in unicast mode.
Point-to-Multipoint (P2MP)	No network is a P2MP network by default, no matter what type of link layer protocol is used on the network. A non-fully meshed NBMA network can be changed to a P2MP network. On this type of network: Hello packets are transmitted in multicast mode using the multicast address FF02::5. Other types of protocol packets, such as DD packets, LSR packets, LSU packets, and LSAck packets are sent in unicast mode.
Point-to-point (P2P)	OSPFv3 considers networks with PPP, or LAPB as the link layer protocol to be P2P networks by default. On a P2P network, protocol packets, such as Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are sent in multicast mode using the multicast address FF02::5.

Stub Area

Stub areas are specific areas where ABRs do not flood received AS external routes. In stub areas, routers maintain fewer routing entries and less routing information than the routers in other areas.

Configuring a stub area is optional. Not every area can be configured as a stub area, because a stub area is usually a non-backbone area with only one ABR and is located at the AS border.

To ensure the reachability of the routes to destinations outside an AS, the ABR in the stub area generates a default route and advertises the route to the non-ABRs in the same stub area.

Note the following points when configuring a stub area:

The backbone area cannot be configured as a stub area.
If an area needs to be configured as a stub area, all the devices in the area must be configured as stub devices using the stub command.
No ASBRs are allowed in the area to be configured as a stub area because AS external routes cannot be transmitted in the stub area.

NSSA

Stub areas cannot import or transmit external routes, which prevents a large number of external routes from consuming the bandwidth and storage resources of routers in the Stub areas. If you need to import external routes to an area and prevent these routes from consuming resources, configure the area as a not-so-stubby area (NSSA).

NSSAs are a special type of OSPFv3 area. There are many similarities between NSSAs and stub areas. Different from stub areas, NSSAs can import AS external routes and advertise them within the entire OSPFv3 AS, without learning external routes from other areas on the OSPFv3 network.

To advertise external routes imported by an NSSA to other areas on the OSPFv3 network, a translator must translate Type 7 LSAs into Type 5 LSAs.

The propagate bit (P-bit) is used to notify a translator whether Type 7 LSAs need to be translated.
By default, the translator is the ABR with the largest router ID in the NSSA.
The P-bit is not set for Type 7 LSAs generated by an ABR.

OSPFv3 Route Summarization

Routes with the same IPv6 prefix can be summarized into one route. On a large-scale OSPFv3 network, route lookup may slow down because of the large size of the routing table. To reduce the routing table size and simplify management, configure route summarization. With route summarization, if a link connected to a device within an IPv6 address range that has been summarized alternates between Up and Down, the link status change is not advertised to the devices beyond the IPv6 address range. Therefore, route summarization prevents route flapping on the network and improves network stability.

OSPFv3 route summarization is classified as follows:

Route summarization on an ABR

An ABR can summarize routes with the same prefix into one route and advertise the summarized route to other areas.

When sending routing information to other areas, the ABR generates Type 3 LSAs based on the IPv6 address prefix. If consecutive IPv6 address prefixes exist in an area and summarization is enabled on the ABR, these IPv6 address prefixes are summarized into one address prefix. For multiple LSAs with the same prefix, the ABR sends only one summary LSA. The LSAs that belong to the summary network segment are not transmitted separately.
Route summarization on an ASBR

An ASBR can summarize imported routes with the same prefix into one route and then advertise only this summary route to other areas.

With route summarization, an ASBR summarizes imported Type 5 LSAs within the summarized address range. After route summarization, the ASBR does not generate a separate Type 5 LSA for each specific prefix within the configured range. Instead, the ASBR generates a Type 5 LSA for only the summary prefix. In an NSSA, an ASBR summarizes multiple imported Type 7 LSAs within the summary address range into one Type 7 LSA.

OSPFv3 Multi-process

OSPFv3 supports multi-process. Multiple OSPFv3 processes can independently run on the same device. Route exchange between different OSPFv3 processes is similar to that between different routing protocols.