OSPF LSA HEADER

All LSA types begin with this Header.

The header is used by itself, ie just this header and nothing else , in DBD and LSA-Ack

Three fields Uniquely identify Each LSA

1) Type

2) Link state ID

3) Advertising Router.

Three fields uniquely identify most recent instances of LSA

1) Age

2) Sequence 

3) Checksum

Age is the time, in seconds, since the LSA was originated. As the LSA is flooded, the ageis incremented by InfTransDelay seconds at each router interface it exits. The age is also incremented in seconds as it resides in a link-state database.
Options is described in the section "Options Field." In the LSA header, the Options field specifies the optional capabilities supported by the portion of the OSPF domain described by the LSA.
Type is the LSA type. Type code 1 – Router LSA, 2 – Network LSA, 3 – Nw summary LSA, 4- ASBR summary LSA, 5 – AS external LSA, 7 – NSSA external LSA
Link State ID identifies the portion of the OSPF domain being described by the LSA. The specific usage of this field varies according to the LSA type; the descriptions of each LSA include a description of how the LSA uses this field.
Advertising Router is the router ID of the router that originated the LSA.
Sequence Number is incremented each time a new instance of the LSA is originated, so that other routers can identify the most recent instance of the LSA.

Options Field Explanation

DN: DN is used with MPLS-based layer 3 VPNs. When a route is learned from a customer network via OSPF,is advertised across the RFC 2547 VPN using Multiprotocol BGP, and then is advertised back to a customer network via OSPF, a loop can occur in which the OSPF route is redistributed back to the VPN provider network in BGP. The DN bit prevents this looping. When the DN bit is set in a type 3, 5, or 7 LSA, the receiving router cannot use that LSA in its OSPF route calculations.

N is used only in Hello packets

N-bit = 1 Iindicate support for NSSA External LSAs.

N-bit = 0, the router will not accept or send these LSAs

Neighboring routers with mismatched N-bits will not become adjacent; this restriction ensures that all routers in an area support NSSA capabilities equally

If N-bit= 1, the E-bit = 0

E is set when the originating router is capable of accepting AS External LSAs. It will be set to 1 in all AS.External LSAs and in all LSAs originated in the backbone and nonstub areas. E-bit = 0 in all LSAs originated within a stub area. Additionally, the bit is used in the Hello packet to indicate an interface's capability of sending and receiving type 5 LSAs. Neighboring routers with mismatched E-bits will not become adjacent; this restriction ensures that all routers in an area support stub capabilities equally.


P is used only in NSSA External LSA headers. (For this reason, the N- and P-bit can use the same position. This bit tells the ABR of a not-so-stubby area to translate type 7 LSAs into type 5 LSAs. P bit is automatically set for all the prefixes that are redistributed.