Advertisements flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these advertisements are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per-interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected.
Action taken in state Circumstances Backup All other states _______________________________________________________________ Advertisement has No acknowledgment No acknowledgment been flooded back sent. sent. out receiving in- terface (see Sec- tion 13, step 5b). _______________________________________________________________ Advertisement is Delayed acknowledg- Delayed ack- more recent than ment sent if adver- nowledgment sent. database copy, but tisement received was not flooded from Designated back out receiving Router, otherwise interface do nothing _______________________________________________________________ Advertisement is a Delayed acknowledg- No acknowledgment duplicate, and was ment sent if adver- sent. treated as an im- tisement received plied acknowledg- from Designated ment (see Section Router, otherwise 13, step 7a). do nothing _______________________________________________________________ Advertisement is a Direct acknowledg- Direct acknowledg- duplicate, and was ment sent. ment sent. not treated as an implied ack- nowledgment. _______________________________________________________________ Advertisement's LS Direct acknowledg- Direct acknowledg- age is equal to ment sent. ment sent. MaxAge, and there is no current instance of the advertisement in the link state database (see Section 13, step 4). Table 19: Sending link state acknowledgements.
Several retransmitted advertisements may fit into a single Link State Update packet. When advertisements are to be retransmitted, only the number fitting in a single Link State Update packet should be transmitted. Another packet of retransmissions can be sent when some of the advertisements are acknowledged, or on the next firing of the retransmission timer.
Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing Link State Update packet (until the LS age field reaches its maximum value of MaxAge).
If the adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.