8 Replies Latest reply: Jul 3, 2019 1:14 AM by bobhatbobhat RSS

    OSPF network type

    nammon

      Hi all,

      Does 2 OSPF form a full adjacency if one has the connected interface's network type different from the other?

      I have a topology:

      (loopback 1.1.1.1/32)R1(f0/0) ---- (f0/0)R2(loopback 2.2.2.2/32)

      Config OSPF on both, but change network type of f0/0 on R1 into point-to-point.

      I see that;

      R1 and R2 can form FULL/- adjacency relationship.

      R2 claims to be DR and see R1 as BDR.

      R1 still see R2 as a neighbor connected via point-to-point link.

       

      R1 and R2 don't learn routes advertised by each other.

       

      In CCNA route (page 504), I see that 2 OSPF router only form neighbor relationship only if 3 parameters match with each others.

      Capture.PNG

      In Hello packet, there is no field represent network type. How can received router determine which network type interface of neighbor are?

        • 1. Re: OSPF network type
          ciscodaze1
          • 2. Re: OSPF network type
            nammon

            I've seen, I assume that if we adjust Hello timer and Dead timer, 2 OSPF router can show FULL neighbor relationship as the result of "show ip ospf neighbor".

            I see both sent/received LSA to/from each other but it couldn't calculate routes to networks which included in those LSA.

            I don't know what did it check to act like that?

            • 3. Re: OSPF network type
              ciscodaze1

              Right, with incompatible network types you may still see the ospf connection as a neighbor, but lsa's will not be exchanged, so routes will not be created.

               

              Broadcast to Non-Broadcast (adjust hello/dead timers)

              Point-to-Point to Point-to-Multipoint (adjust hello/dead timers)

              • 4. Re: OSPF network type
                nammon

                In my lab, one was configured network type as Broadcast, other was configured network type as p2p.

                LSAs still exchange between two routers but they can't calculate routes to networks which included in those LSA.

                • 5. Re: OSPF network type
                  ciscodaze1

                  p2p network type does not understand the election process, which is what the broadcast interface is doing...this will not work.

                  • 6. Re: OSPF network type
                    bobhatbobhat

                    This issues is discussed by Nick Russo in his session "Troubleshooting OSPF - BRKRST-3310" at CiscoLive in Orlando 2018.  He starts talking about your particular issue around 00:29 minutes.

                    • 7. Re: OSPF network type
                      Samer

                      You might see it in updates:

                       

                      I'm attaching the packet capture file, packet 27 LS update

                       

                      this is after changing network type.

                      • 8. Re: OSPF network type
                        bobhatbobhat

                        Ch. 6 "IP routing on Cisco IOS, IOS XE and IOS XR"

                         

                        "OSPF Adjacency with Different OSPF Network Types A common misconception is that the OSPF network types must match to form an adjacency and exchange routes. OSPF neighbor adjacency requires that the hello timers and usage of a DR must match. This means that two routers can form an adjacency if one has a non-broadcast OSPF network type, and the other neighbor has a broadcast OSPF network type. Alternatively, a router can form an adjacency between P2P OSPF network type and point-to-multipoint OSPF network type. In both instances, the hello timers must be changed on at least one node for the timers to match.

                         

                        Note If you use a network type that uses a DR with a network type that does not use a DR, the adjacency may still form, but routes will not install correctly because each type uses a different type of LSA for route advertisement."

                         

                         

                        Edgeworth, Bradley; Foss, Aaron; Rios, Ramiro Garza (2014-12-19). IP Routing on Cisco IOS, IOS XE, and IOS XR (Networking Technology) (Kindle Locations 5814-5818). Pearson Education. Kindle Edition.