OSPFv3 LSA Format

Each router in an autonomous system (AS) generates one or more types of link state advertisements (LSAs), depending on the router's type. Multiple LSAs form a link state database (LSDB). OSPFv3 encapsulates routing information into LSAs for transmission. Commonly used LSAs include:

Router-LSA (Type 1)
Network-LSA (Type 2)
Inter-Area-Prefix-LSA (Type 3)
Inter-Area-Router-LSA (Type 4)
AS-external-LSA (Type 5)
NSSA LSA (Type 7)
Link-LSA (Type 8)
Intra-Area-Prefix-LSA (Type 9)

LSA Header Format

All LSAs have the same header. Figure 1 shows an LSA header.

Figure 1 LSA header

**Table 1** LSA header fields
Field	Length	Description
LS age	16 bits	Time that elapses after the LSA is generated, in seconds. The value of this field continually increases regardless of whether the LSA is transmitted over a link or saved in an LSDB.
LS type	16 bits	Type of the LSA. The values are as follows: Type 1: Router-LSA. Type 2: Network-LSA. Type 3: Inter-Area-Prefix-LSA. Type 4: Inter-Area-Router-LSA. Type 5: AS-external-LSA. Type 7: NSSA-LSA. Type 8: Link-LSA. Type 9: Intra-Area-Prefix-LSA.
Link State ID	32 bits	This field together with the LS type field describes an LSA in an area.
Advertising Router	32 bits	Router ID of the router that generates the LSA.
LS sequence number	32 bits	Sequence number of the LSA. routers can use this field to identify the latest LSA.
LS checksum	16 bits	Checksum of all fields except the LS age field.
Length	16 bits	Length of the LSA including the LSA header, in bytes.

Router-LSA

A router-LSA (Type 1) describes the link status and cost of a router. Router-LSAs are generated by a router and advertised within the area to which the router belongs. Figure 2 shows the format of a router-LSA.

Figure 2 Format of a router-LSA

**Table 2** Router-LSA fields
Field	Length	Description
Nt (NSSA translation)	1 bit	If the router that generates the LSA is an NSSA border router, this field is set to 1. In other cases, this field is set to 0. When this field is set to 1, the router unconditionally translates NSSA-LSAs into AS-external-LSAs.
x	1 bit	This field is deprecated.
V (Virtual Link)	1 bit	If the router that generates the LSA is located at one end of a virtual link, this field is set to 1. In other cases, this field is set to 0.
E (External)	1 bit	If the router that generates the LSA is an autonomous system boundary router (ASBR), this field is set to 1. In other cases, this field is set to 0.
B (Border)	1 bit	If the router that generates the LSA is an area border router (ABR), this field is set to 1. In other cases, this field is set to 0.
Options	24 bits	The optional capabilities supported by the router.
Type	8 bits	Type of the router link. The values are as follows: 1: Connected to another router in point-to-point (P2P) mode. 2: Connected to a transport network. 3: Reserved. 4: Virtual link.
metric	16 bits	Cost of the link.
Interface ID	32 bits	The Interface ID assigned to the interface.
Neighbor Interface ID	32 bits	Neighbor's interface ID. For transit links, the value is the interface ID of the DR. For links of other types, the value is the interface ID of the neighboring device.
Neighbor Router ID	32 bits	Router ID of the neighbor. For transit links, the value is the router ID of the DR. For links of other types, the value is the router ID of the neighboring device.

Network-LSA

A network-LSA (Type 2) records the router IDs of all routers on the local network segment. Network-LSAs are generated by a DR on a broadcast or non-broadcast multiple access (NBMA) network and advertised within the area to which the DR belongs. Figure 3 shows the format of a network-LSA.

Figure 3 Format of a network-LSA

**Table 3** Network-LSA fields
Field	Length	Description
Options	24 bits	The optional capabilities supported by the router.
Attached Router	32 bits	Router IDs of all routers on the same network, including the router ID of the DR

Inter-Area-Prefix-LSA

An inter-area-prefix-LSA (Type 3) describes routes on a network segment in an area. It is generated by the ABR. The routes are advertised to other areas.

Figure 4 shows the format of an inter-area-prefix-LSA.

Figure 4 Format of an inter-area-prefix-LSA

**Table 4** Inter-area-prefix-LSA fields
Field	Length	Description
PrefixLength	8 bits	Length of the prefix.
PrefixOption	8 bits	Prefix-related capability option, indicating the length in the packet.
Address Prefix	32 bits	Address prefix.

Inter-Area-Router-LSA

An inter-area-router-LSA (Type 4) describes routes to the ASBR in other areas. It is generated by the ABR. The routes are advertised to all related areas except the area that the ASBR belongs to.

Figure 5 shows the format of an inter-area-router-LSA.

Figure 5 Format of an inter-area-router-LSA

**Table 5** Inter-area-router-LSA fields
Field	Length	Description
Destination Router ID	32 bits	The router ID of the router described by the LSA.

AS-External-LSA

An AS-external-LSA describes a route to a destination outside the AS and is generated by an ASBR.

Figure 6 shows the format of an AS-external-LSA.

Figure 6 Format of an AS-external-LSA

**Table 6** AS-external-LSA fields
Field	Length	Description
E	1 bit	Type of external metric. If this field is 1, the specified metric is a Type 2 external metric. If this field is 0, the specified metric is a Type 1 external metric.
F	1 bit	Whether a Forwarding Address has been included in the LSA. If this field is 1, a Forwarding Address has been included in the LSA. If this field is 0, no Forwarding Address is included in the LSA.
T	1 bit	Whether an External Route Tag has been included in the LSA. If this field is 1, an External Route Tag has been included in the LSA. If this field is 0, no External Route Tag is included in the LSA.
Referenced LS Type	16 bits	Referenced LS type. If this value is not 0, an LSA with this LS type is to be associated with this LSA (see Referenced Link State ID below).
Forwarding Address	128 bits	A fully qualified global IPv6 address.
External Route Tag	32 bits	External route tag, which can be used to communicate additional information between ASBRs.
Referenced Link State ID	32 bits	Referenced link state ID.

NSSA-LSA

NSSA-LSAs are originated by ASBRs within an NSSA and describe routes to destinations external to the AS.

Figure 7 shows the format of an NSSA-LSA.

Figure 7 Format of an NSSA-LSA

Link-LSA

Each router generates a link LSA for each link. A link LSA describes the link-local address and IPv6 address prefix associated with the link and the link option set in the network LSA. It is transmitted only on the link.

Figure 8 shows the format of a link-LSA.

Figure 8 Format of a Link-LSA

**Table 7** Link-LSA fields
Field	Length	Description
Rtr Priority	8 bits	Router priority of the interface.
Options	24 bits	Set of options that may be set in the network-LSA generated by the DR on broadcast or NBMA links.
Link-local Interface Address	128 bits	The originating router's link-local interface address on the link.
Number of prefixes	32 bits	Number of IPv6 address prefixes contained in the LSA.

Intra-Area-Prefix-LSA

Each router and DR generates one or such LSAs and transmits them in the local area.

An LSA generated on a router describes the IPv6 address prefix associated with the router LSA.
Such LSAs generated by a DR describe the IPv6 address prefixes associated with network LSAs.

Figure 9 shows the format of an intra-area-prefix-LSA.

Figure 9 Format of an intra-area-prefix-LSA

**Table 8** Intra-area-prefix-LSA fields
Field	Length	Description
Referenced LS Type	16 bits	Router-LSA or network-LSA with which the IPv6 address prefixes should be associated. If Referenced LS Type is 0x2001, the IPv6 prefixes are associated with a router-LSA. If Referenced LS Type is 0x2002, the IPv6 prefixes are associated with a network-LSA.
Referenced Link State ID	32 bits	Referenced link state ID. If Referenced LS Type is 0x2001, Referenced Link State ID should be 0. If Referenced LS Type is 0x2002, Referenced Link State ID should be the interface ID of the link's DR.
Referenced Advertising Router	32 bits	ID of the referenced advertising router. If Referenced LS Type is 0x2001, Referenced Advertising Router should be the originating router's router ID. If Referenced LS Type is 0x2002, Referenced Advertising Router should be the DR's router ID.