OSPFv3 LSA Types

OSPFv3 LSA Types

Each LSA begins with a standard 20-byte LSA header. Each LSA describes a piece of OSPF routing domain. All LSAs are flooded throughout the OSPF routing domain. The flooding is reliable, ensuring all routers have the same collection of LSAs. This collection of LSAs is called link-state database (LSDB). From the LSDB, each router constructs the shortest-path tree with itself as the root. This yields a routing table.

LSA Header:

This header contains enough information to uniquely identify each LSA. The LS Type, Link State ID and the Advertising Router field are used to uniquely identify an LSA.

Different instances of the same LSA could be present. The most recent instance could be identified using LS Age, LS Sequence number and LS Checksum fields present in the LSA Header.

LS Age: Time in seconds since the LSA was originated.

LS Type: Indicates the function performed by the LSA.

Link State ID: Together with LS Type and Advertising Router, uniquely identifies the LSA in the LSDB

Advertising Router: The Router ID of the router that originated the LSA

LS Sequence Number: detects old or duplicate LSA

LS Checksum: Complete checkcum of the LSA including the LSA Header but excluding the LS Age field

length: The length in bytes of the LSA including 20-bytes for LSA Header

LS Type:

The LS Type field indicates the function performed by the LSA. The high-order 3 bits encode generic properties of the LSA, while low-order bits indicates the LSA's specific functionality.

U- indicates how a router should handle unknown LSA.

0= treat the LSA as if it had link-local flooding scope

1= Store and flood the LSA

S2 and S1- indicate flooding scope of the LSA

LSA Function Code- defines LSA's specific functionality.

Router LSA:

Each OSPF router originates Router LSAs indicating the state and cost of the router's interfaces to the area. Router LSAs are flooded throughout the single area only.

A router may originate one or more Router LSAs, distinguished by their Link State IDs. The receiving router concatenates the Router LSAs if it receives more than one Router LSA from a single router.

The Router LSA indicates if the router is an ASBR or an ABR or if it is one end-point of a virtual link. These LSAs have no address information.

Network LSA:

Network LSAs are originated by the DR for a broadcast or NBMA network in the area which supports two or more routers. The LSA describes all routers connected to the link, including the DR. The LSA's Link State ID field is set to the Interface ID that the DR has been using in Hello packets. No address information is carried in the Network LSA.

Inter-Area Prefix LSA:

These LSAs are IPv6 equivalent of IPv4's Type-3 Summary LSAs. These LSAs are originated by the ABR to specify IPv6 prefixes that belong to other areas. A separate LSA is originated for each address prefix.

For Stub areas, the Inter-area Prefix LSA is used to describe a default route. The prefix length of the default route is set to 0.

Inter-Area Router LSA:

These LSAs are IPv6 equivalent of IPv4's Type-4 Summary LSAs. Originated by the ABR, the Inter-Area Router LSA describes the route to the ASBR. Each LSA describes a route to a single router.

AS-External LSA:

These LSAs are IPv6 equivalent of IPv4's Type-5 External LSAs. These LSAs are originated by ASBRs describing the destinations external to the AS. Each LSA describe a route to a single IPv6 prefix external to the AS.

AS-External LSAs can be used to describe a default route. Default routes are used when no specific route exists for a destination.

Link LSA:

A router originates a separate Link LSA for each link it is attached to. These LSAs have link-local flooding scope and are never flooded beyond a link that they are associated with. These LSAs have three purposes-

- notify the link-local address of the router's interface to the routers attached to the link

- inform other routers attached to the link of the list of IPv6 prefixes to associate with the link

- allow the router to assert the collection of Option bits to associate with the Network LSA that will be originated for the link

The Link-State ID is set to the Interface ID of link of the originating router.

Intra-Area Prefix LSA:

A router uses Intra-Area Prefix LSA to advertise IPv6 prefixes that are associated with

a) the router itself (in IPv4, this was carried in Router LSA)

b) an attached stub network segment (in IPv4, this was carried in Router LSA)

c) an attached transit network segment (in IPv4, this was carried in Network LSA)

A router can originate multiple Intra-Area Prefix LSAs for each router or transit network; each LSA is distinguished by its Link State ID.

Options field:

The 24-bit Options field is included in Hello and DBD packets, and Router, Network and Inter-area Router LSAs. It enables OSPF routers to support optional capabilities, and to communicate their capabilities to other OSPF routers in the network.

Further reading:

1. RFC 2740: OSPF for IPv6 http://www.faqs.org/ftp/rfc/pdf/rfc2740.txt.pdf

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