Next step in the flooding procedure

When a new (and more recent) LSA has been received, it must be flooded out some set of the router’s interfaces. This section describes the second part of flooding procedure (the first part being the processing that occurred in Section 13), namely, selecting the outgoing interfaces and adding the LSA to the appropriate neighbors’ Link state retransmission lists. Also included in this part of the flooding procedure is the

maintenance of the neighbors’ Link state request lists.

This section is equally applicable to the flooding of an LSA that the router itself has just originated (see Section 12.4).

For these LSAs, this section provides the entirety of the flooding procedure (i.e., the processing of Section 13 is not performed, since, for example, the LSA has not been received from a neighbor and therefore does not need to be acknowledged).

Depending upon the LSA’s LS type, the LSA can be flooded out only certain interfaces. These interfaces, defined by the following, are called the eligible interfaces:

AS-external-LSAs (LS Type = 5)

AS-external-LSAs are flooded throughout the entire AS, with the exception of stub areas (see Section 3.6). The eligible interfaces are all the router’s interfaces, excluding

virtual links and those interfaces attaching to stub areas.

All other LS types

All other types are specific to a single area (Area A). The

eligible interfaces are all those interfaces attaching to the Area A. If Area A is the backbone, this includes all the virtual links.

Link state databases must remain synchronized over all

adjacencies associated with the above eligible interfaces. This is accomplished by executing the following steps on each

eligible interface. It should be noted that this procedure may decide not to flood an LSA out a particular interface, if there is a high probability that the attached neighbors have already received the LSA. However, in these cases the flooding

procedure must be absolutely sure that the neighbors eventually do receive the LSA, so the LSA is still added to each

adjacency’s Link state retransmission list. For each eligible interface:

(1) Each of the neighbors attached to this interface are examined, to determine whether they must receive the new LSA. The following steps are executed for each neighbor:

(a) If the neighbor is in a lesser state than Exchange, it does not participate in flooding, and the next neighbor should be examined.

(b) Else, if the adjacency is not yet full (neighbor state is Exchange or Loading), examine the Link state request list associated with this adjacency. If there is an instance of the new LSA on the list, it indicates that the neighboring router has an instance of the LSA already. Compare the new LSA to the neighbor’s copy:

o If the new LSA is less recent, then examine the next neighbor.

o If the two copies are the same instance, then delete the LSA from the Link state request list, and

examine the next neighbor.[20]

o Else, the new LSA is more recent. Delete the LSA from the Link state request list.

(c) If the new LSA was received from this neighbor, examine the next neighbor.

(d) At this point we are not positive that the neighbor has an up-to-date instance of this new LSA. Add the new LSA to the Link state retransmission list for the adjacency.

This ensures that the flooding procedure is reliable;

the LSA will be retransmitted at intervals until an acknowledgment is seen from the neighbor.

(2) The router must now decide whether to flood the new LSA out this interface. If in the previous step, the LSA was NOT added to any of the Link state retransmission lists, there is no need to flood the LSA out the interface and the next interface should be examined.

(3) If the new LSA was received on this interface, and it was received from either the Designated Router or the Backup Designated Router, chances are that all the neighbors have received the LSA already. Therefore, examine the next interface.

(4) If the new LSA was received on this interface, and the interface state is Backup (i.e., the router itself is the Backup Designated Router), examine the next interface. The Designated Router will do the flooding on this interface.

However, if the Designated Router fails the router (i.e., the Backup Designated Router) will end up retransmitting the updates.

(5) If this step is reached, the LSA must be flooded out the interface. Send a Link State Update packet (including the new LSA as contents) out the interface. The LSA’s LS age must be incremented by InfTransDelay (which must be > 0) when it is copied into the outgoing Link State Update packet (until the LS age field reaches the maximum value of

MaxAge).

On broadcast networks, the Link State Update packets are multicast. The destination IP address specified for the Link State Update Packet depends on the state of the interface. If the interface state is DR or Backup, the

address AllSPFRouters should be used. Otherwise, the address AllDRouters should be used.

On non-broadcast networks, separate Link State Update

packets must be sent, as unicasts, to each adjacent neighbor (i.e., those in state Exchange or greater). The destination IP addresses for these packets are the neighbors’ IP

addresses.