OSPF Point-To-Point – Common Questions and Answers

ospfpoint-to-pointprotocol-theoryrfcrouting

I've consulted the spec https://www.ietf.org/rfc/rfc2328.txt and have googled to the end of the internet but I'm confused on OSPF Point-To-Point.

If I have the following:

  Link State ID: 4.4.4.4
  Advertising Router: 4.4.4.4

And then in my Router-LSA Update I have the following:

 (Link ID) Neighboring Router ID: 2.2.2.2
 (Link Data) Router Interface address: 10.0.1.1
 Number of TOS metrics: 0
 TOS 0 Metrics: 64

I know this means that OSPF Router 4.4.4.4 has a point-to-point connection to 2.2.2.2 via the interface 10.0.1.1 However, my question is as follows: Does the 10.0.1.1. interface reside on the 4.4.4.4 router or the 2.2.2.2 router?

Best Answer

What you are seeing is the neighbor Router ID and interface IP address. This is explained in the RFC on pages 12 and 13, particularly the point-to-point link of figure 1a:

                                              **FROM**

                                       *      |RT1|RT2|
            +---+Ia    +---+           *   ------------
            |RT1|------|RT2|           T   RT1|   | X |
            +---+    Ib+---+           O   RT2| X |   |
                                       *    Ia|   | X |
                                       *    Ib| X |   |

                 Physical point-to-point networks

A router will know its own interface IP addresses, but it needs to know the interface IP address of its neighbor. If it knows the interface IP address of its neighbor, then it knows which interface to use to reach that neighbor.

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