OSPF IMPORTANT POINTS

    • Order of OSPF Path Selection Process is given below. The below sequence is preferred regardless of metric or administrative distance.
    1. Intra-Area (O)
    2. Inter-Area (O IA)
    3. External Type 1 (E1)
    4. External Type 2 (E2)
    5. NSSA Type 1 (N1)
    6. NSSA Type 2 (N2)
    • DR/BDR are elected per shared network in an Area.
    • Default seed metric while redistributing any IPG prefix in OSPF is 20, while redistributing BGP prefix in OSPF is 1.
    • Filtering OSPF prefixes using distribute-list (along with route-maps) and distance command only blocks the prefix from entering into local RIB, but cannot stop LSAs propagation into the OSPF database.
    • OSPF filtering using filter-list, ‘area range not-adv’ command, ‘summary-address not-adv’ command, ‘ip ospf database-filter all out’ command and ‘neighbor database-filter all out’ command can filter LSAs from OSPF database.
    • In an NSSA, “default-information originate” command cannot be used to generate a default route, since it generates Type-5 LSA, which is not permitted in an NSSA area.
    • In an NSSA, ABR with the highest router-id does the LSA 7 to 5 conversion.
    • The internal OSPF prefixes can only be summarized on ABRs whereas the external (redistributed) routes can only be summarized on ASBRs.
    • The only time that OSPF will form adjacencies between neighbors that are not on the same subnet is when the neighbors are connected through point-to-point links using “ip unnumbered” command.
    • To establish OSPF neighbor adjacency, Hello/Dead timers, MTU (otherwise use ‘ip ospf mtu-ignore’) must match and also a unique Router-ID is required.
    • Default route in OSPF can only be advertised using “default-information originate [always]” command. we cannot redistribute a static default route (ip route 0.0.0.0 0.0.0.0…) into OSPF database. Same is true for ISIS also.