Tag Archives: ospf

OSPF RFC1583 vs RFC2328 compatibility

Best path selection:
RFC1583 – best path based solely on cost.
RFC2328 – Intra-area paths that use non-backbone areas are always the most preferred.

Can lead to the loops:

Summary metric:
RFC1583 – lowest metric of the componet routes
RFC2328 – highest metric of the components.

OSPF RFC1587 vs RFC3101 NSSA compatibility.

  1. Path selection
    a. Any type 5 LSA.
    b. A type-7 LSA with the P-bit set and the forwarding
    address non-zero.
    c. Any other type-7 LSA
    1. A Type-7 LSA with the P-bit set.
    2. A Type-5 LSA.
    3. The LSA with the higher router ID.
  2. The P-bit default is now defined as cleared.
  3. A new area configuration parameter, NSSATranslatorRole, is defined in Appendix D. It specifies whether or not an NSSA router will unconditionally translate Type-7 LSAs to Type-5 LSAs when acting as an NSSA border router.



Change the cost of the default route advertised into a stub or NSSA area.

area 1 default-cost 123


Type7 to Type5 by ABR with highest RID.
Forwarding-address not ZERO in Type7 and Type7 to Type5. Recurse to Type3 and not Type4.
The ABR connected to the NSSA takes the type 7 LSAs and converts them into type 5 LSAs, which makes it an ASBR as well. Therefore this ABR doesn’t generate a type 4 LSAs for itself. The type 4 LSA is rather generated by the other ABRs connected to other areas.
Per the NSSA area RFC, the use of FA is mandatory with these LSAs. The reason is that there is only one 7-to-5 translating ABR and this might result in suboptimal routing without the use of FA.
FA SHOULD be reachable via OSPF.

(config-router)#area 2 nssa translate type7 ?
  always       Always translate LSAs on this ABR
  suppress-fa  Suppress forwarding address in translated LSAs

area 1 nssa default-information-originate – generates Type7 default with metric 1

default-information originate [always] – generates Type5 default with metric 1


Optional subnets. Metric 1 for BGP, 20 for others.

  default-metric         Set metric of redistributed routes



Hidden commands

sh ip ospf delete
show ip ospf maxage-list