OSPF - LSA checksum

When deploying OSPF, each LSA that is received by an OSPF router contains a checksum. This checksum can be seen as part of the OSPF database. For example, the following output shows various LSAs and each one has a checksum displayed:

Router Link States (Area 0)

  Link ID         ADV Router      Age  Seq#       Checksum  Link count
  1.1.1.1         1.1.1.1         1200 0x80000005 0x0027A3  3
  2.2.2.2         2.2.2.2         850  0x80000007 0x0045B1  4
  3.3.3.3         3.3.3.3         1300 0x80000003 0x0031A2  2

Network Link States (Area 0)

  Link ID         ADV Router      Age  Seq#       Checksum  
  192.168.1.1     1.1.1.1         600  0x80000002 0x001BC3  
  10.1.1.1        2.2.2.2         800  0x80000003 0x002FD1  

Summary Net Link States (Area 0)

  Link ID         ADV Router      Age  Seq#       Checksum  Metric
  192.168.2.0     1.1.1.1         1200 0x80000001 0x001A12  20
  10.2.2.0        3.3.3.3         950  0x80000004 0x001C45  10

ASBR Summary Link States (Area 0)

  Link ID         ADV Router      Age  Seq#       Checksum  Metric
  4.4.4.4         3.3.3.3         950  0x80000002 0x001D88  30

Type-5 AS External Link States

  Link ID         ADV Router      Age  Seq#       Checksum  Route Tag
  172.16.10.0     4.4.4.4         1200 0x80000001 0x0045CD  0
  192.168.50.0    4.4.4.4         1100 0x80000002 0x0029F1  0

The 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.

Links:

https://forum.networklessons.com/t/ospf-multi-area-configuration/1698/58?u=lagapidis

https://networklessons.com/ospf/how-to-read-the-ospf-database