4.4. Link State Advertisements
For IPv6, the OSPF LSA header has changed slightly, with the LS type field expanding and the Options field being moved into the body of appropriate LSAs. Also, the formats of some LSAs have changed somewhat (namely, router-LSAs, network-LSAs, AS-external-LSAs, and NSSA-LSAs), while the names of other LSAs have been changed (type 3 and 4 summary-LSAs are now inter-area-prefix-LSAs and inter-area-router-LSAs respectively) and additional LSAs have been added (link-LSAs and intra-area-prefix-LSAs). Type of Service (TOS) has been removed from the OSPFv2 specification [OSPFV2] and is not encoded within OSPF for IPv6's LSAs.
These changes will be described in detail in the following subsections.
4.4.1. The LSA Header
In both IPv4 and IPv6, all OSPF LSAs begin with a standard 20-byte LSA header. However, the contents of this 20-byte header have changed in IPv6. The LS age, Advertising Router, LS Sequence Number, LS checksum, and length fields within the LSA header remain unchanged, as documented in Sections 12.1.1, 12.1.5, 12.1.6, 12.1.7, and A.4.1 of [OSPFV2], respectively. However, the following fields have changed for IPv6:
Options
The Options field has been removed from the standard 20-byte LSA header and moved into the body of router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs. The size of the Options field has increased from 8 to 24 bits, and some of the bit definitions have changed (see Appendix A.2). Additionally, a separate PrefixOptions field, 8 bits in length, is attached to each prefix advertised within the body of an LSA.
LS type
The size of the LS type field has increased from 8 to 16 bits, with high-order bit encoding the handling of unknown types and the next two bits encoding flooding scope. See Appendix A.4.2.1 for the current coding of the LS type field.
Link State ID
The Link State ID remains at 32 bits in length. However, except for network-LSAs and link-LSAs, the Link State ID has shed any addressing semantics. For example, an IPv6 router originating multiple AS-external-LSAs could start by assigning the first a Link State ID of 0.0.0.1, the second a Link State ID of 0.0.0.2, and so on. Instead of the IPv4 behavior of encoding the network number within the AS-external-LSA's Link State ID, the IPv6 Link State ID simply serves as a way to differentiate multiple LSAs originated by the same router.
For network-LSAs, the Link State ID is set to the Designated Router's Interface ID on the link. When a router originates a link-LSA for a given link, its Link State ID is set equal to the router's Interface ID on the link.