OSPF Multi-Area Configuration

Lessons Discussion
43

why all the areas must be connected to area 0 in OSPF?

44

Hello Diksha

Take a look at this NetworkLessons note why a backbone area is necessary for OSPF. If you have any further questions, let us know!

I hope this has been helpful!

Laz

45

Hello.

Why are the loopback advertised with a 32bit mask while having a 24bit mask?

Thanks for the good article.

46

Hello Oussama

Take a look at this NetworkLessons note on the topic of advertising loopback networks on OSPF.

I hope this has been helpful!

Laz

1 Like
47

Hello,
I have made this lab myself in Packet Tracer, and I have the below output in R2 which is different from what’s mentioned in the article:

R2#show ip ospf neighbor 


Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.12.1      1   FULL/BDR        00:00:35    192.168.12.1    GigabitEthernet0/0
4.4.4.4           1   FULL/BDR        00:00:39    192.168.24.4

Note that it shows to me the neighbor ID is 192.168.12.1, and it seems this is the correct one, while for Rene in the article the neighbor ID is 192.168.13.1

Can someone explain this to me please?

48

Hello Siraj

The output shown in the lesson is incorrect. It should indeed show a neighbor ID of 192.168.12.1 as in your output. I will let Rene know to make the correction on the lesson.

Thanks for pointing that out!

Laz

1 Like
49

Hello, The output below is from “OSPF Multi-Area Configuration” lecture can someone explain why there are two “BDR’s”? There should only be one I thought. Thanks.

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.13.1      1   FULL/**BDR**        00:00:34    192.168.13.1    GigabitEthernet0/1
4.4.4.4           1   FULL/**BDR**        00:00:30    192.168.24.4    GigabitEthernet0/2
50

Hello Marcus

Remember that the purpose of a DR/BDR election is to select one OSPF router per broadcast domain to be the central point with which all other OSPF routers will become neighbors. This is the case for all multi-access technologies, where you can have multiple OSPF routers sharing the same network segment.

Now R2 in the lesson is connected to two broadcast domains, the one connected to its Gi0/1 interface, and the one connected to its Gi0/2 interface. Thus, it takes part in two elections. According to the output of the show ip ospf neighbor command, in both cases, it has been elected as the BDR.

If a router has 10 Ethernet interfaces, it will take part in 10 DR/BDR elections because it is connected to 10 network segments. So it may become DR for some, BDR for others, and DROTHER for still others.

I hope this has been helpful!

Laz

51

Yes, This is very helpful. Thank You!

1 Like
52

I recently began working with a new client and I have never seen an OSPF area configured like this area 0.0.0.22. I have always seen them configured at area 0 or area 1, etc. Can someone point me to where this is discussed in more detail?

Thanks!

53

Hello John

What you are seeing is the Area ID indicated in dotted decimal notation, like an IPv4 address. Take a look at this NetworkLessons note on OSPF area notation for more information.

I hope this has been helpful!

Laz

54

Hello @lagapidis its hard to comprehend this concept. can you please explain in details as you guys are good in that ?. Please

56

Hello Team,
Can you please give a brief about the “Checksum” in the OSPF database and how it plays in order to protect the prefixes from corruption ?

57

Hello Sathish

Take a look at these NetworkLessons notes that further describe these concepts of the OSPF database.

If you have any more specific questions, feel free to let us know!

I hope this has been helpful!

Laz

58

Hello Sathish

Checksum that is shown in the OSPF database is the checksum that belongs to the specific LSA. It is used to ensure the integrity of the OSPF LSAs as they traverse the network. It is a 16-bit value (typically represented in hexadecimal) that is calculated for each LSA using the contents of the LSA itself.

The checksum is calculated by the sending router, and is included within the contents of the LSA itself. When the receiving router processes it, it calculates the checksum again and compares it with the value within the LSA. If they are the same, the LSA is deemed uncorrupted and error-free. If it is not, it is discarded.

This checksum should not be confused with the checksum that is included in all OSPF messages. This checksum ensures the integrity of the entire OSPF message that is transmitted over the network, and exists for all OSPF packets.

I hope this has been helpful!

Laz

1 Like
59

Hello Rene! It’s still incorrect. It’s under “Verification” topic. I know this because it just confused me and I had to think it through…then double-checked the comments to see if anyone noticed. Thank you.

60

Hello Carlos

I see in the verification section that Rene states the following:

“R2 has formed neighbor adjacencies with R1 and R4.”

It looks like the original typo was corrected. However, if you see it differently, please specify the specific text (copy and paste it into the post) to indicate the typo so that we can fix it.

Thanks so much!

Laz