LSA Acknowledgment and Validation

LSA Acknowledgment and Validation
LSA acknowledgments are sent from a router to the originating router to acknowledge the
receipt of a LSA. There are two different methods routers can use to acknowledge receipt
of LSAs:
Explicit acknowledgment The recipient sends a link-state acknowledgment packet to the originating
interface.
Implicit acknowledgment A duplicate of the flooded LSA is sent back to the originator.
Here is a packet decode of an explicit acknowledgment:
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 6
Type of Service: 0
Unused:
Total Length: 84
Identifier: 1285
Fragmentation Flags: 0
Fragment Offset: 0
Time To Live: 1
IP Type: 0x59 OSPF (Hex value for protocol number)
Header Checksum: 0x8dda
Source IP Address: 131.31.194.140
Dest. IP Address: 224.0.0.6
No Internet Datagram Options
OSPF - Open Shortest Path First Routing Protocol
Version: 2
Type: 5 Link State Acknowledgement
Packet Length: 64
Router IP Address: 142.42.193.1
Area ID: 1
Checksum: 0x6699
Authentication Type: 0 No Authentication
Authentication Data:
........ 00 00 00 00 00 00 00 00
Link State Advertisement Header
Age: 3600 seconds
Options: 100010
No AS External Link State Advertisements
Type: 3 Summary Link (IP Network)
154 Chapter 5  OSPF Operation in a Single Area
ID: 0x90fb6400
Advertising Router: 153.53.193.1
Sequence Number: 2147483708
Checksum: 0x3946
Link State Length: 28
Link State Advertisement Header
Age: 3600 seconds
Options: 100010
No AS External Link State Advertisements
Type: 3 Summary Link (IP Network)
ID: 0x90fb6400
Advertising Router: 131.31.193.1
Sequence Number: 2147483650
Checksum: 0x25c0
Link State Length: 28
Frame Check Sequence: 0x00000000
You can tell that this is a link-state acknowledgment packet based on the OSPF header information.
You will see that it is a type 5 OSPF packet, or a link-state acknowledgment packet.
There are two methods by which an implicit acknowledgment may be made:
Direct method The acknowledgment, either explicit or implicit, is sent immediately. The
following criteria must be met before the Direct method is used:
 A duplicate flooded LSA is received.
 LSA age equals MaxAge (one hour).
Delayed method The recipient waits to send the LSA acknowledgment with other LSA
acknowledgments that need to be sent.
Validation occurs through the use of the sequencing, checksum, and aging data contained
in the LSA update packet. This information is used to make sure that the router possesses the
most recent copy of the link-state database. One important item to know about LSAs is that
there are multiple types of LSAs. We discuss the different types of LSAs in more detail in the
next chapter.