OSPF : Quick Facts and Important other Facts

OSPF : Open shortest path first is one of the link state routing protocol and widely used in the enterprise network. It has been seen that Network designer's and administrators first choice is OSPF when they are deploying the network on the large scale. I knew some of you guys are very much interesting to know the important facts of OSPF but apart from OSPF, IS-IS is a very excellent routing protocol as well. I don't knew some of the people says they don't like IS-IS, may be they don't much know about IS-IS protocol or may be they have some less understanding about IS-IS protocol.

When i am talking about IS-IS , It doesn't states about any country. It's states about the protocol named Intermediate system-intermediate systems. May be sometimes people hate this protocol due to the name. Lets not talk about this protocol and i will cover this in another post.

If we are talking about the OSPF protocol, some of the quick facts you should know

Quick Facts

  • OSPF: Link state Protocol
  • AD: 110
  • It's a layer 3 Protocol and protocol number is 89
  • worked on Dijkistra algorithm and calculates the path on metrics.
  • Generally used metric in OSPF is Cost. The formula to calculate the cost is reference bandwidth divided by interface bandwidth.
  • There are LSA ( Link state Advertisements ) working in the OSPF and these LSAs are LSA1, LSA2, LSA3, LSA4. LSA5, LSA6 ( Multicast ), LSA7, LSA8, LSA9-11.We will discuss all the LSAs in detail in another post soon.
  • OSPF area types - Backbone, Standard, Stub, Totally Stubby, NSSA Area. I will explain this one in another post.
  • Using Multicast address for sharing information and these multicast addresses are and
  • The concept of DR/BDR is there. I will discuss the same in another post as well.
  • OSPF network types are Point to point network, Broadcast Network, Non-broadcast multi-access Network and Point to Multi-Point network. 
  • OSPF neighbouring states are Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading and Full. I will discuss this as well in another post.
  • OSPF router types are Backbone Router, Internal Router, Area Border Router ( ABR ) and Autonomous  System Border Router ( ASBR ).
  • OSPF Packet types are Hello, DBD (Database description ), LSA ( Link state advertisement ) . LSU ( Link state update ), LSAck ( Link state Acknowledgment )
Fig 1.1 - Sample OSPF Topology

Facts you may know

  • The IP header of an OSPF packet specifies protocol number 89.
  • To establish OSPF neighbor adjacency, hello/dead timers, MTU else you need to use "ip ospf mtu-ignore" should match and you should also required unique router-id is also required.
  • Please keep in mind that routers in stub area can only be adjacent with the routers in stubs or totally stubby area. Routers in NSSA can only be adjacent with the routers in NSSA or totally NSSA.
  • The another you should take note of that the OSPF sees secondary networks as stub networks and cannot make adjacencies over secondary addresses. OSPF will advertise a secondary network or subnet only if it is also running on the primary network or subnet and OSPF routes of secondary addresses must be in same area as the primary address to be advertised. To understand routes from a neighbor connected to the secondary network, another routing protocol such as RIP should be running and redistributed into OSPF. May be we can have another solution like to create dot1q sub interfaces.
  • Another interesting fact that OSPF will form adjacencies between neighbors that are not on the same subnet is when the neighbors are connected through point-to-point (P2P) links using "ip unnumbered"command.
  • With related to the above statement you should remember that the primary interface and IP unnumbered interface will have OSPF enabled if a network statement matches the IP address of the primary interface.
  • Some of the guys have myth about OSPF as external routing, let me tell you that an OSPF external route cannot use another OSPF external route as its next hop.
  • If you are talking about Inside an area, OSPF uses Link State logic, but between areas OSPF acts much like a Distance Vector (DV) protocol.So you mean in an area Its Link state routing protocol but 2 areas will work as distance vector if i summarise the statement again For example, the advertisement of a Type 3 LSA from one area to another hides the topology in the original area from the second area, just listing a destination subnet, metric (cost), and the ABR through which the subnet can be reached—all DV concepts.
  • This is important guys that only broadcast and non-broadcast network elect DR/BDR based on priority or router-id (in case of a tie in the priority).
  • In related to the above fact, we have other kinds of network as well and these are P2P and non-broadcast network, DR/BDR must have layer 2 connectivity to all other routers in the same area.
  • Let's have a look on the OSPF network types,with OSPF network types broadcast and non-broadcast, next hop values are not modified when updates are transmitted across an NBMA media. Both point-to-multipoint and point-to-multipoint non-broadcast network type update the next-hop value of routes learned on partially meshed networks to the directly connected neighbor, and advertise the network as a set of endpoints instead of a transit network.
  • Another major misunderstanding, let me clear you on this as well that OSPF network point-to-point is the default option for point-to-point interfaces such as HDLC, PPP, or point-to-point NBMA subinterfaces.
  • Make sure you should understand that only broadcast and non-broadcast network type elects DR/BDR, they are compatible with each other, but they are not compatible with any other network types.
  • OSPF cost can be modified using some of the manual commands you enter in the routers and they are like interface bandwidth command, interface ip ospf cost command, process auto-cost reference-bandwidth command, and neighbor cost command on point-to-multipoint non-broadcast network.
  • It is important to know that only OSPF point-to-multipoint and point-to-multipoint non-broadcast network types support OSPF cost value on a per neighbor basis. On point-to-multipoint broadcast networks, if the neighbor command is used, a cost to that neighbor must be specified.  But on point-to-multipoint non-broadcast networks, the neighbor command must be used to identify neighbors, assigning a cost to a neighbor is optional.
  • If you are using internal OSPF routes then you should know that the internal OSPF routes can only be summarized on ABRs whereas the external (redistributed) routes can only be summarized on ASBRs.
  •  Did you hear about the command area default-cost command, well area default-cost command is used to specify a cost for the default summary route that is sent into a stub area or NSSA.
  • Let's move to NSSA area type in OSPF, In NSSA, ABR with the highest router-id does the LSA 7 to 5 conversion.
  • The another fact in NSSA is that, default-information originate command cannot be used, since it generates Type-5 LSA, which is prohibited in NSSA area.
  • With respect to the above fact of NSSA area, NSSA ASBR can generate a default only when it has a default route in its routing table whereas NSSA ABR can generate a default route with or without a default route in its own routing table. So you can now have the difference in ABR and ASBR in NSSA area.
  • Let me touched upon some of the light on Virtual links, Virtual links are not allowed in the stubby area or NSSA. In this case OSPF can be tunneled over a stub area using GRE tunnel it must be connected to Area 0
  • let's talk about the authentication in OSPF virtual link, If the authentication is wrong on the virtual-link, the virtual-link interface will not go down immediately. As the virtual-link does not support periodic hellos, clear ip ospf process command should be issued if the authentication is enabled on the virtual link.
  • Take a note of it that the virtual link will not come up if the only interface to reach the other end of the virtual link has a cost that is maximised till 65535.
  • If you are doing redistribution from to BGP to OSPF, the router-id must be identical, in OSPF and in BGP.
  • For the concept of OSPF filtering. It uses distribute-list, route-map and distance commands can only block route from entering into local RIB, but cannot stop LSAs propagation into the OSPF database.
  • In respect to the above fact about OSPF filtering, the OSPF filtering using area filter-list prefix, area range not-adv, summary-address not-adv, ip ospf database-filter all out, or neighbor database-filter all out  commands can filter LSAs from OSPF database in OSPF domain.
  • It should be keep in mind that If the  area range and area filter-list prefix out both commands are configured for an area, then type 3 LSAs that correspond to the area range are sent to all other areas, only if at least one prefix in the area range matches an entry in the prefix list.
  • Many of you are confused but it is the fact that OSPF defaults to cost 20 when redistributing from an IGP, and 1 when redistributing from BGP.
  • The command neighbor database-filter all out only works on point-to-multipoint network types.
  • If we are talking about the distribute-list out command  which is configured on an ASBR, then the ASBR generates Type 5 external LSAs only for those networks that are explicitly permitted in the distribute list.
  • OSPF demand circuit sets “do not age” flag on all LSAs learned and will only send updates when there is a change in the OSPF topology. The command must be configured in a point-to-point link and is needed only on one side. If the router is part of a point-to-multipoint topology, only the multipoint end must be configured with this command.
  • The main difference between flooding reduction (ip ospf flood-reduction ) and demand circuits (ip ospf demand-circuit) is that former suppresses only periodic LSA refreshes.It does not suppress periodic hello packets. Thus, the flooding reduction feature does not impair the detection of a neighbor router going down.
  • OSPF stub router (max-metric router-lsa) advertises all non self-originated routes/LSAs with maximum metric.
  • When  you are going to configure redistribute maximum-prefix command , the redistribution limit does not apply to default routes or prefixes that are generated as a result of Type-7 to Type-5 translation.

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