Month: September 2017

OSPF Default Routing

  • Create a default route into a specific area
  • Create a default route into the entire OSPF domain
  • Default-information originate
    • Creates a default route into OSPF, external Type 2 route, using a Type 5 LSA with metric 20
    • default-information originate (always) metric 30 metric-type 1 route-map NAME

      • Default metric 20
      • Default metric-type 2
        • metric-type 1 cost can be changed
        • metric-type 2 cost can’t be changed
    • The metric-type defines LSA is Type 1 External or Type 2 External (default)
    • Route-map is used for tracking networks. If Route-map condition is met, default route is advertized.

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OSPF Route Filtering and Summarization

OSPF Route filtering between areas

  • Filtering prevents the creation of LSAs.
  • LSDB’s have to be the same for all routers ifnot SPF logic will fail
  • OSPF can filter the originiation of LSA between areas
  • Type 3 LSAs are filtered prior to origination ABR
      • With multiple ABRs filtering should be done on both.
      • ospf# area number prefix-list prefix name in|out
        • In: IOS filters routes comming in to that area
        • out: IOS filters routes comming out of that area
    ip prefix-list NAME seq 5 deny 10.10.10.0/24 le 32
ip prefix-list NAME seq 10 permit 0.0.0.0/0 le 32

R1#(config-router)#area 1 filter-list prefix NAME in
R1#show ip ospf database summary self-originate

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OSPF Metric Calculations & Tuning

    • Analyse the LSDB to find all possible routes to reach the subnet.
    • Per route, add the interface cost for all outgoing interfaces in that route.
    • Pick the route with the lowest cost.
      • Cost is derived from the egress interface bandwith.

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OSPF Message Types

Five types of OSPF Messages:

  • Hello
  • Database Description (DD or DBD)
  • Link-State Request
  • Link-State Update
  • Link-State Acknowledgement

OSPF LSDB and LSA

OSPF Link-State Database (LSDB)

  • Each router stores it’s LSA in it’s LSDB.
  • Each router within the same area should have the same LSDB information.
  • SPF algortithm for each area it is connected to.
  • The router thinks it’s the root of the tree and draws branches via SPF
  • LSAs in OSPF LSDB are pieces that the SPF process uses.
  • OSPF poisons LSA by setting max-age (3600).

OSPF types of Link State Advertisement (LSA)

  • Type 1: Router LSA
  • Type 2: Network LSA
  • Type 3: Summary LSA
  • Type 4: Summary ASBR LSA
  • Type 5: Autonomous system external LSA
  • Type 6: Multicast OSPF LSA
  • Type 7: Not-so-stubby area LSA
  • Type 8: External attribute LSA for BGP

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