OSPF : Few basic questions…

On this blog I would like to discuss few basic details of ospf which everybody needs to know and which is very helpful to build the concepts of OSPF.

For this we are going to use below reference GNS 3 topology.


Question 1 : How and when the information about the ABR and ASBR is transferred to other routers in the area.

Answer :  As we all know that over cisco devices we need to not specify any router as ABR or ASBR. Then the question comes in mind that how other devices know about the presence of the ABR/ASBR and when this information is propagated. Obviously all the information in OSPF is carried with the help of LSA so for this task as well ospf uses LSAs. Ospf process propagate this information by setting different bits in the LSA type 1 or router LSA.

Though I would explain the bits with the help of wireshark screen captures but before that we need to know when those bits are set and LSAs and generated corresponding to it.

a) When we configure any router for two areas(area0 and any other area)  that router become the ABR(Area border router). This is the time when one router LSA would be generated and sent to DR in the network. Subsequently DR forwards the LSA to all the routers in that multiaccess network.


b) On other hand when we configure any router to redistribute the routes of any other routing protocol into ospf then the router sends one Router LSA(LSA type 1) to inform ospf DR  and the info would be sent to all other router in the network by DR.


Question 2:  What are few other important flags/options in ospf hello packet or LSU?



NP option : NSSA propagation bit, is used to notify the other end nssa router to translate the LSA 7 to LSA 5 and forward it to other areas. If this bit is not set then the routing information learnt through NSSA would not be translated into LSA 5 and hence would not be propagated into the other areas. Possible scenarios may be when the NSSA router which is connected with different routing domain is redistributing into both(area 0 and other area), in this case the advertisement sent into area 0 would be having the NP option set and the LSA sent into other area would not have the option set. Also in case the ASBR is also the ABR and hence doing summarization.In this case as well the NP bit would not be set.

Demand Circuit option: OSPF introduces the demand circuit which gives the feature to suppress the hello and periodic update while having the adjacency up at the same time. Only triggered updates are sent once the demand circuit is configured. This is done to save the bandwidth and resources on the device and in network.

L option : This option is newly introduced, this tells if the packet contains LLS TLV or not. LLS header is used for NSF(non stop forwarding), I would explain this is my upcoming bolg , for now you may also read NSF on cisco site .

E Option : This is external routing capability option should be set on all Backbone LSAs,non-stub LSAs and AS-external-LSAs. All router LSAs, network and summary for stub area  would have reset this option.

Question 3: Does all the routers connected to other areas would be considered as ABR ?

Answer : No, all the routers which has the link into other areas(other than area 0) can not be termed as ABR. The router which has its link into the area 0 only would be by default the ABR router.

Question 4 : How can we make the router as ABR even if it does not have any link in area 0(backbone area)?

Answer : We can create a loopback and advertise it into area 0.

Question 5 : What is the route preference order in ospf?

Answer :  Below is the path selection rule in ospf.

Intra-Area route denoted by (O)
Inter-Area route denoted by (O IA)
External Type 1 route denoted by (E1)
External Type 2 route denoted by (E2)
NSSA Type 1 route denoted by  (N1)
NSSA Type 2 route denoted by  (N2)

More details about question 3 and 4 would be explained in upcoming blogs with the help of GNS3 labs.

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