OSPF Acknowledgments

OSPF Acknowledgments

To ensure that LSAs are transmitted successfully, OSPF requires each LSA to be acknowledged. LSAs are acknowledged by one of the following acknowledgment types:

·         Implicit acknowledgment— Occurs when the sending router receives a duplicate LSA from the neighbor. By seeing the neighbor report the LSA, the router knows "implicitly" that it received the LSA. A neighbor can implicitly acknowledge the receipt of an LSA by including a duplicate of the LSA in an update back to the originator. Implicit acknowledgments are more efficient than explicit acknowledgments in some situations, for instance, when the neighbor was intending to send an update to the originator anyway.

·         Explicit acknowledgment— Requires the receiving router(s) to send a specific link-state acknowledgement packet in response to the LSA. A neighbor explicitly acknowledges the receipt of an LSA by sending a Link State Acknowledgment packet. A single Link State Acknowledgment packet is capable of acknowledging multiple LSAs. The packet carries only LSA headers—enough to completely identify the LSA—not the complete LSA. When a router first sends an LSA, a copy of the LSA is entered into the Link State Retransmission list of every neighbor to which it was sent. The LSA is retransmitted every RxmtInterval until it is acknowledged or until the adjacency is broken. The Link State Update packets containing retransmissions are always unicast, regardless of the network type.

To ensure that LSAs are current and valid, each LSA has a sequence number, a checksum, and a MaxAge value. The sequence number and checksum verify that the LSA is valid, whereas the age parameter ensures that the LSA is the most current LSA. MaxAge is used to verify how old the LSA was. MaxAge is set for 3600 seconds, or 1 hour. When a router originates an LSA, it sets the MaxAge at 0. Each time the LSA is flooded by another router, its MaxAge is incremented by another timer called InfTransDelay, which has a default value of 1. When the LSA reaches the MaxAge value, the LSA is reflooded throughout the network. Routers also use the MaxAge parameter when comparing two LSAs to determine which one is more current. This type of flooding can be excessive on large stable networks. In Cisco IOS 12.1, Cisco introduced a concept called LSA flooding reduction.