OSPF Multi-Area Configuration

This topic is to discuss the following lesson:

Hi Rene,
Hope you are doing well :slight_smile:
My question is, In ospf database/Topology table we find type-1 LSA,on that it shows Link ID and ADV. Router ID both are same why ??

br//zaman

Hi Rene,
I want to know how OSPF Prevent Inter-Area Loop at ABR and ASBR end ?? How its works if two ABR /ASBR Scenario ? How ABR do function ? I think a details write up needed regarding looping prevention issue in your clear text . Appreciate your write up on it .Thx

br//zaman

Hello Mohammad

The ADV Router ID heading shows the router ID of the advertising router, that is the router from which this LSA was received.

Now the Link State ID heading is a little more tricky. According to the OSPF RFC, when a router is packaging all of its local, Type-1 Router LSAs into a single OSPF packet, and adds an LSA Header to the front of those LSAs, the “Link State ID” field in that header is always the Router-id of that router. Now…within each, individual LSA there is another field called the “Link ID” field. For each Router LSA it is THIS field that is under the Link State ID heading. This value may change depending on whether that Router LSA is describing a transit network, stub network, point-to-point network, or virtual-link.

Now in the examples given by Rene, these are the same because the Router ID happens to be the loopback interface which also happens to be used as the ID for the specific link. THis is why they are the same, however, this is not always the case. This is especially true when you configure a router ID using the router-id x.x.x.x command in the OSPF configuration.

I hope this has been helpful!

Laz

Hello Mohammad

I’ll let @ReneMolenaar respond to this one. I’ll let him know and he can tell us if/when inter-area loop prevention for OSPF will be included in a future lesson.

Laz

Hi Laz,
Thanks for your valuable reply on complex issue as always :slight_smile:
Could you please reply on Looping issue for the intermediate time ?? It will very much helpful to keep my study continue .Many thanks

br//zaman

Hello Mohammad

INE has an excellent article on this issue and it can be found here.

I hope this has been helpful!

Laz

Hi Laz ,
Thans dear :slight_smile:
I have read the doc before but some issue still confusing .Could you please give me clear concept of ABR,ASBR JOB while injecting LSA-3 & LSA-5 to other Area . Actually a bit confusion for two ABR , ASBR scenario . Could you please help me on two ABR,ASBR scenario .Many Thx

br//zaman

Hello Mohammad

@ReneMolenaar has an excellent lesson on LSA types that may help you in the scinarios you are describing above.

If however this does not cover you, please indicate a more specific example so that we can take a look.

I hope this has been helpful!

Laz

Hi Laz ,
Many Thx laz…:slight_smile:
I have read this lesson before . I need to know about how ospf prevent looping issue by using ABR . Suppose two ABR here . One ABR injecting LSA-3/LSA-5 and what will happend when other ABR receiving those LSA-3/LSA-5 and vice versa .Thx

br//zaman

br//
zaman