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When a LSA is re-advertised (flooded) to another router will the readvertising router modify the "Advertising Router-id" field to its own router id ?

Eg:

Routerzero---NetworkA---RouterA-----RtrB-----RtrC------RtrD

Say in above topology RouterA originated a LSA with NetworkA information in its Router-LSA and this is advertised to RtrB. Now when RtrB re-advertises this LSA to RtrC will it modify the "Advertising Router-id" field in the LSA to its own(RtrB 's router-id) and send to RtrC?

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1 Answer 1

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Router ID must not be modified inside the same area

RtrB must not modify the Advertising RouterID in RouterA's LSA.

Quoting RFC 2328 - OSPFv2: Section 12.1.5 "Advertising Router":

12.1.5.  Advertising Router

        This field specifies the OSPF Router ID of the LSA's
        originator.  For router-LSAs, this field is identical to the
        Link State ID field.  Network-LSAs are originated by the
        network's Designated Router.  Summary-LSAs originated by
        area border routers.  AS-external-LSAs are originated by AS
        boundary routers.

Specifically, For router-LSAs, this field is identical to the Link State ID field forbids modification of RouterA's LSA.

Detecting Self-originated LSAs

The fact that LSA Router-IDs must be unchanged when flooded is reinforced in Section 13.4: Receiving Self-Originated LSAs:

13.4.  Receiving self-originated LSAs

    It is a common occurrence for a router to receive self-
    originated LSAs via the flooding procedure. A self-originated
    LSA is detected when either 1) the LSA's Advertising Router is
    equal to the router's own Router ID or 2) the LSA is a network-
    LSA and its Link State ID is equal to one of the router's own IP
    interface addresses.

A router knows it received a copy of its own LSAs when the Router ID inside the LSA is the same as its own Router ID; detecting self-originated LSAs within the same area would be impossible if Router IDs were modified in-transit.

Router ID may change at the Area Border Router

Your diagram doesn't mention a different area, and it implies this problem is within the same area when it mentions that RtrB re-advertises RouterA's LSA. However, if RtrB was an ABR, and RtrC was in a different area than RouterA, then the Router ID for NetworkA must have a different Router ID, because the ABR (RtrB) must originate a Type 3 Summary LSA for NetworkA when it sees RouterA's Type 1 Router LSA.

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