OSPF Hello Packets Deep Dive

This is a post which is part of my learning journey on OSPF…I will be going through the details of the different OSPF packet (Message) type and what are the components of each packet type…This will be best explained with the used of wireshark….To have an overview, the 5 OSPF Message Types includes the following:

1. Hello
2. DBD or Database Description
3. Link State Request
4. Link State Update
5. Link State Acknowledgment

Below will be my lab for this post…

OSPF Configurations:

R1#sh run | sec ospf
    router ospf 1
    network 1.1.1.1 0.0.0.0 area 1
    network 192.168.12.0 0.0.0.255 area 0

R2#sh run | sec ospf
    router ospf 1
    network 2.2.2.2 0.0.0.0 area 2
    network 192.168.12.0 0.0.0.255 area 0

The moment, we have enabled the network statement under the OSPF process on R1, the first
OSPF Hello Packet will be sent to the multicast address 224.0.0.5

The OSPF header would have the following:
       1. OSPF Version
       2. Message Type
       3. Packet Length
       4. OSPF Router
       5. Area
       6. Checksum
       7. Authentication Type ( Plan Text, MD5 or No Authentications)
       8. Authenticated Data

Below are the information’s included on the Hello Packet. As we have notice the Hello Interval is
10 seconds and the Dead Intervals is 40 seconds ( 4 times the Hello Interval). Take note that the
following parameters have to be the same in order for the OSPF Neighbors will formed:

       1. Network Mask
       2. Hello Interval
       3. Areas
       4. Dead Interval

The wireshark capture below shows the Hello Packets send to the multicast address 224.0.0.5 of
both R1 and R2 the moments, I have enabled the network command under the OSPF process.

Let’s take a look at R2 by this time…

As observed aboved R2 has the required parameters to formed the neigbor with R1.. This includes

– Same Network Mask as with R1

– Same Hello interval as with R1

– Same Area as R1

– Same Dead Interval as R1

Now, look at R2… It does shows the Active Neighbor which is R1.

The next OSPF Message Type is the Database Description ( DBD)… This will only appear once the
OSPF neighbors were formed.

   DBD keeps all the LSA’s of all the routers within the area. As we only have one OSPF neighbor, on
   my wireshark capture below, I only have a LSA Type 1 or Router LSA. Important thing to note with\    DBD message are as follows:
      1.   LSA Type
2. Link State ID
3. Advertising router
       4. Sequence Number
   5. Checksum
   6. Length

The next OSPF Message is the LSR.. The picture below means that R1 is requesting for a newer
LSA from R2.

In response to the LSR, R2 is sending an LSU…

Finally, the last OSPF Message is the LS Acknowledgment or LS- Acknowledgment.

NETWORK LESSON