OSPF LSA Format

This section describes the format of OSPF LSA.

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). Open Shortest Path First (OSPF) encapsulates routing information into LSAs for transmission. Commonly used LSAs include:

Router-LSAs (Type 1)
Network-LSAs (Type 2)
Summary-LSAs, including network-summary-LSAs (Type 3) and ASBR-summary-LSAs (Type 4)
AS-external-LSAs (Type 5)

LSA Header Format

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

Figure 1 LSA header

Table 1 describes LSA header fields.

**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.
Options	8 bits	The values are as follows: E: Type 5 LSAs are flooded. MC: IP multicast packets are forwarded. N/P: Type 7 LSAs are processed. DC: On-demand links are processed.
LS type	8 bits	Type of the LSA. The values are as follows: Type1: Router-LSA Type2: Network-LSA Type3: Network-summary-LSA Type4: ASBR-summary-LSA Type5: AS-external-LSA Type7: NSSA-LSA
Link State ID	32 bits	This field together with the LS type field describes an LSA in an AS.
Advertising Router	32 bits	Router ID of the router that generates the LSA.
LS sequence number	32 bits	Sequence number of the LSA. Neighbors 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 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 describes router-LSA fields.

**Table 2** Router-LSA fields
Field	Length	Description
Link State ID	32 bits	Router ID of the router that generates the LSA.
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.
# links	16 bits	Number of links and interfaces described in the LSA, including all links and interfaces in the area to which the router belongs.
Link ID	32 bits	Object to which the router is connected. This field has different meanings for different link types: 1: router ID 2: interface IP address of the designated router (DR) 3: network segment or subnet number 4: router ID of the neighbor on a virtual link
Link Data	32 bits	Link data. This field has different meanings for different link types: 1: interface index 3: subnet mask 2 and 4: interface address of the router
Type	8 bits	Type of the router link. The values are as follows: 1: The router is connected to another router in point-to-point (P2P) mode. 2: The router is connected to a transport network. 3: The router is connected to a stub network. 4: The router is connected to another router over a virtual link.
# TOS	8 bits	Number of types of service (ToSs).
metric	16 bits	Cost of the link.
TOS	8 bits	Type of service.
TOS metric	16 bits	Metric for the specified ToS.

Network-LSA

A network-LSA describes the link status 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 describes network-LSA fields.

**Table 3** Network-LSA fields
Field	Length	Description
Link State ID	32 bits	Interface IP address of the DR
Network Mask	32 bits	Mask of the broadcast or NBMA network
Attached Router	32 bits	Router IDs of all routers on the broadcast or NBMA network, including the router ID of the DR

Summary-LSA

A network-summary-LSA describes routes on a network segment in an area. The routes are advertised to other areas.

An ASBR-summary-LSA describes routes to the ASBR in an area. The routes are advertised to all areas except the area to which the ASBR belongs.

The two types of summary-LSAs have the same format and are generated by an ABR. Figure 4 shows the format of a summary-LSA.

Figure 4 Format of a summary-LSA

Table 4 describes network-summary-LSA fields.

**Table 4** Network-summary-LSA fields
Field	Length	Description
Link State ID	32 bits	Advertised network address
Network Mask	32 bits	Mask of the broadcast or NBMA network
metric	24 bits	Cost of the route to the destination address
TOS	8 bits	Type of service
TOS metric	24 bits	Metric for the specified ToS

When a default route is advertised, both the Link State ID and Network Mask fields are set to 0.0.0.0.

Table 5 describes ASBR-summary-LSA fields.

**Table 5** ASBR-summary-LSA fields
Field	Length	Description
Link State ID	32 bits	Router ID of the ASBR
Network Mask	32 bits	Set to 0.0.0.0
metric	24 bits	Cost of the route to the destination address
TOS	8 bits	Type of service
TOS metric	24 bits	Metric for the specified ToS

AS-External-LSA

An AS-external-LSA describes AS external routes. AS-external-LSAs are generated by an ASBR. Among the five types of LSAs, only AS-external-LSAs can be advertised to all areas except stub areas and not-so-stubby areas (NSSAs). Figure 5 shows the format of an AS-external-LSA.

Figure 5 Format of an AS-external-LSA

Table 6 describes AS-external-LSA fields.

**Table 6** AS-external-LSA fields
Field	Length	Description
Link State ID	32 bits	Advertised network address.
Network Mask	32 bits	Mask of the advertised destination address.
E	1 bit	Type of the external route. The values are as follows: 0: Type 1 external route 1: Type 2 external route
metric	24 bit	Cost of the route to the destination address.
Forwarding Address	32 bits	Packets destined for the advertised destination address are forwarded to the address specified by this field.
External Route Tag	32 bits	Tag added to the external route. This field can be used to manage external routes. OSPF itself does not use this field.
TOS	8 bits	Type of service.
TOS metric	24 bits	Metric for the specified ToS.

When AS-external-LSAs are used to advertise default routes, both the Link State ID and Network Mask fields are set to 0.0.0.0.