OSPF Written Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.


*** Theory ***

  • OSPF hello Multicast address is
  • OSPF DR and BDR Multicast address is
  • DBD – Database Description – Packets used to exchange the link state database. Once done, SPF is performed.
  • Hello packet contains the following (bolded are ones that must match to form a relationship): Router ID, Area ID, Address of originating interface, Authentication Information, Hello Interval, Dead Interval, priority, DR & BDR ID, Neighbour router ID
  • OSPF Networks                – Hello timers etc are different for each network
    • Point to point: two routers, no DR or BDR election. All updates sent to
      • Timers: Hello 10, Dead 40, Wait 40
  • Broadcast (default for Ethernet): Rather then form relationships with every router, each router forms relationship with DR and BDR to avoid insufficient full mesh. DR keeps topology upto date.
    • Timers: Hello 10, Dead 40, Wait 40
  • NBMA (default for point-to-multipoint): Statically define all neighbours using the neighbour ip command only on the hub router and send hello via unicast not multicast. DR must be connected to all neighbours so usually the hub not spokes. Relationships may take around 45 seconds to form!
    • Conf-if: ip ospf priority 255 to make sure hubs is the DR
    • Conf-if: ip ospf priority 0 to make sure spokes do not participate in elections
    • Timers: Hello 30, Dead 120, Wait 120
  • Point to Multipoint: No DR election and messages are multicasted. Treats each link as point to pont, sends hello down each dlci  – Preferred network type.
  • DR/BDR elections
    • Init: lists eachother as neighbours. Stays in init state if core fields don’t match
    • 2way: DR/BDR election
    • ExStart: Exchange of DBD
    • Full state: Proper relationship
      • Conf-if: ip ospf priority 255 to make sure hubs is the DR
      • Conf-if: ip ospf priority 0 to make sure spokes do not participate in elections
      • Area types
        • Area 0 (Back bone) – all areas connect to this
        • Stub area – blocks external routes e.g. RIP etc (blocks type 5 LSA)
        • Totally stub – blocks external routes and routes from other areas as ABR generates default route to all other internal routers (blocks Type 3,4,5 LSA) – blocks inter-area
          • Must only do area 3 stub no-summary on ABR only
  • routers and just only keep intra-area routes
  • NSSA – When a stub has a ASBR (e.g. acquired another company), uses Type 7 LSA
  • Totally stubby not so stubby area – same as NSSA?
  • Router types
    • Internal – inside an area
    • Backbone – Internal router in Area 0
    • Area Border – 2 or more interfaces connected to 2 more areas – Only router that can do summarisation!
    • Autonomous system boundary – 1 or more interfaces connected to external networks
    • LSA Types
      • LSA 1 – Routing update to DR
      • LSA 2 – routing update from DR to all internal in area
      • LSA 3 – Generated by ABR (Summarisation route)
      • LSA 4 – IP Address of ABR
      • LSA 5 – External routes generated by ASBR
      • LSA 7 – generated by NSSA to get to stub routers that generally block Type 5 LSA – Kind of like encapsulation as type 7 and then into type 5 when back in stub area, like a bandage
      • Route tables
        • IA – Inter-Area route
        • External type 1 – metric will increment per hop
        • External type 2 – metric will remain the same per hop
        • Best practice: Set router-id to router name for ease of logs etc
        • Summarisation: area 0 range  – do at ABR to summarise routes from one area to another
          • Summary-address – use this to summarise external routes
          • Tag – tag routes so for example in redistribution you can filter on another router based on tag!
          • Virtual Link
            • Create on the transit router e.g. R1-R6-R7. R6 is the transit area.
            • On the transit router do it on the area directly connected to area 0 e.g. area 1
            • Create with neighbour ID not the ip address
            • Partner virtual link is going to be with the router  connected in area 0 e.g. R1
            • Redistribution
              • Always set an access list with routes you want to redistribute and TAG them
              • Do it both ways or you might not be able to ping etc even if it shows in the routing table
              • When redistributing ospf into another protocol, it good to match it to internal routes only as this will prevent routing loops
              • Default-information redistribute always – advertises a default route to all ospf neighbours. Always will advertise even if that router looses it’s default route
              • Authentication
                • Config-router# Area 0 authentication message digest
                • Config-subif# ip ospf message-digest-key 1 (number of keys) md5 cisco (name)
                  • Becareful of spaces in password when hitting return
                  • Must do this for virtual links too if necessary

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