This topic is to discuss the following lesson:
Hi Rene,
Hope you are doing well 
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
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
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…
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
Just wondering if you have a lab that taught in the lessons? Currently, I’m doing OSPF Multi-Area Configuration for CCNA and I wish I can import the same lab and practice in my GNS3 or packet tracer.
Hello Tila
The lesson here includes the configurations of the devices which should make it easy for you to recreate the lab in GNS3. Take a look at the configs, and follow along in the video to employ the configuration step by step.
I hope this has been helpful!
Laz
Why does OSPF require all traffic between non-backbone areas to pass through a backbone area (area 0)?
Hello Rakhi
This is just a matter of design. OSPF is a link state routing protocol, which means that each router participating in OSPF must have a complete topology of the whole network including all of the participating OSPF routers. If an OSPF area grows too large, it will use many resources (CPU, memory etc) on the routers and it will slow down in its convergence time.
In order to make OSPF scalable to much larger networks, the concept of areas was introduced. As OSPF was being developed, much research went into its multi-area design. Based on the tests that engineers performed, they found that they can ensure a satisfactory convergence time only if all non-backbone areas are directly connected to area 0.
It is for this reason that one of the prerequisites of the creation of non-backbone OSPF areas is that they must be directly connected to the backbone area 0.
As you may know, it is possible to “trick” an area into believing it is directly connected to the backbone area even if it isn’t This is done using a virtual link, and you can find out more information about that here:
The virtual link was introduced only as a temporary measure and should not be used as a permanent solution. It is often useful when companies merge, and their networks must be interconnected. Often such mergings require temporary network topologies that may result in non-backbone networks not connecting directly to the backbone. Until more permanent network changes can be made, a virtual link can be a “quick fix” temporary solution.
So to summarize, OSPF requires non-backbone areas to be directly connected to area 0 in order to ensure the best possible performance of OSPF, and this is based on the fundamental design of the protocol itself.
I hope this has been helpful!
Laz
Hi,
I was going through the OSPF Multi-Area Configuration topic and found one line information which needs to be corrected. I am pasting this line “R3 has formed neighbor adjacencies with R1 and R4.” but according to the mentioned topology R2 should be mentioned instead of R3. Kindly have a look into the same and correct me if I am wrong.
Thank you Saurabh, that’s a typo indeed. I just fixed it.
Rene
Hi Laz,
OSPF Router with ID (192.168.24.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.24.2 192.168.24.2 629 0x80000008 0x003b1d 1
192.168.12.1 192.168.12.1 669 0x80000008 0x00165d 1
Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
192.168.24.2 192.168.24.2 102 0x80000007 0x0050ef 1
192.168.24.4 192.168.24.4 1617 0x80000007 0x0020f9 2
I am referring to the output of router 2 “Sh ip ospf database” . If I see the router link states of Area 0 and Area 2 that it shows, I am having the below questions. Please clarify.
- As per router LSA definition it states that it stays within the area . In this LSA we will find a list of all directly connected links of this router.So as per this statement it should show area 0 directly connected link only right ? why it is showing 192.168.24.2 in the link ID column and Adv router column for area 0 ? I am thinking 192.168.24.2 is configured for Area 2 and it should not come in the router LSA for Area 0 . Please explain
If I think that, in the link ID column it displays the router ID of the Area 0 in both the directly connected routers, then it is right as we see 192.168.24.2 in the Link ID of the area 0 .192.168.24.2 is the router ID of the router 2. But one question pops up here ,Router ID is any interface IP that has highest IP address providing we have not configured any loopback in router 1 and router 2. So if this is the case how come in router 1 , 192.168.12.1 is chosen as router ID ? It should be 192.168.13.1 right as it is the highest IP in this router 1 ? Please explain.
Hello Dakshinamurthy
It took me a while to get my head around it, but I finally understood. 
It does indeed look very strange to have these LINK IDs and ADV Router values for Area 0.
I suggest you take a look at this post which describes both of these columns and the values that they contain:
If you have further questions, just let us know!
I hope this has been helpful!
Laz
HI Rene/Laz,
Is got correct to assume that LSA4 is generated by ABR, if there are multiple routers in between ASBR and ABR the LSA initially originated by ASBR flips one of its bits to indicate that it is a ASBR however since this is being circulated with in an area it is considered as an Router LSA until it reaches ABR, which converts this from type 1 LSA to type 4 LSA and then it is flooded to the adjacent areas. Is this correct if not can you kindly explain how they work.
Thanks and Regards,
Teja
Hello Teja
That is exactly correct. The ASBR creates a Router LSA (Type 1 LSA) and it remains a Type 1 LSA within the area. Once the ABR receives this, it sees the flipped bit, recognizes that it comes from an ASBR, and generates the Type 4 LSA.
You can see an explanation of this in the following lesson:
I hope this has been helpful!
Laz
1 Like