OSPF Packet Format

On the picture in the link below, I have print screened the packet format of a router LSA that has 3 links (neighbors) off of it. I understand all of the fields except the "Link State ID" field at the top. What is this field for, what does it do?



  • Hi,

    The "Link State ID" is part of the trio that uniquely identifies an LSA.

    • Link State ID
    • Link State Type
    • Advertising Router

    Its source can be different depending on the LSType associated.

    For LSType 1 (Router LSA): LSID is the Router ID

    For LSType 2 (network LSA):IP address of the DR interface (for multi-access interfaces)

    For LSType 3 (Network summary LSA): The destination network IP address

    For LSType 4 (ASBR Summary LSA): The Router ID (RID) of the ASBR

    For LSType 5 (External LSA): The destination network IP address


    The RFC2328 (https://www.ietf.org/rfc/rfc2328.txt) section 12.1.4 would be a starting point for more details.

  • You've got that wrong I think. The Link Type determines the Link ID (not link state ID, located once within this entire packet at the top). So a type 1 LSA always has a link id of the neighbor's router-ID. And each of the 3 links at the bottom use that structure. However, I don't know what only the top link state ID is.

  • Link ID is different from Link State ID.

    For each LSA, you would always have a Link State ID. That Link State ID must always existe and its source its different depending on the Link State Type (as shown above on my previous message).

    The Link ID, which you see 3 times on the image you provided is included on LSA type 1 (router LSA) and describes the router's attached networks of different types (p2p, broadcast, stub, etc..)

    An LSA can only have one link state ID. So would not make sense to have many link ID fields.

    Excerpts from the RFC2328:

    12.1.  The LSA Header

    The LSA header contains the LS type, Link State ID and
    Advertising Router fields
    . The combination of these three
    fields uniquely identifies the LSA.

    12.1.4.  Link State ID

    This field identifies the piece of the routing domain that
    is being described by the LSA. Depending on the LSA's LS
    type, the Link State ID takes on the values listed ...
                .... (router, network, summary, external ...)

    12.4.1.  Router-LSAs

    A router originates a router-LSA for each area that it
    belongs to. Such an LSA describes the collected states of
    the router's links to the area. .....
             Each link is typed according to the kind of attached network.
    Each link is also labelled with its Link ID
    Hope it helps
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