Skip to main content

4.5. Flooding

Most of the flooding algorithm remains unchanged from the IPv4 flooding mechanisms described in Section 13 of [OSPFV2]. However, the addition of flooding scope and unknown LSA type handling has caused some changes in the OSPF flooding algorithm.

The encoding of flooding scope in the LS type and the need to process unknown LS types cause modifications to the processing of received Link State Update packets. For IPv6, Steps 2 and 3 are modified as follows:

(2) Examine the LSA's LS type. Discard the LSA and get the next one from the Link State Update packet if the interface area has been configured as a stub or NSSA area and the LS type indicates "AS flooding scope".

(3) Else if the flooding scope in the LSA's LS type is set to "reserved", discard the LSA and get the next one from the Link State Update packet.

For IPv6, the eligible interfaces are selected based on the following factors:

  • The LSA's flooding scope
  • For LSAs with area or link-local flooding scope, the particular area or interface with which the LSA is associated
  • Whether the LSA has a recognized LS type
  • The setting of the U-bit in the LS type

Choosing the set of eligible interfaces breaks into three cases:

Case 1: The LSA's LS type is recognized. Set eligible interfaces depending on the flooding scope encoded in the LS type.

Case 2: The LS type is unrecognized and the U-bit is set to 0. There is a single eligible interface, namely, the interface on which the LSA was received.

Case 3: The LS type is unrecognized and the U-bit is set to 1. Select the eligible interfaces based on the encoded flooding scope as in Case 1.

4.5.3. Installing LSAs in the Database​

There are three separate places to store LSAs, depending on their flooding scope:

  • LSAs with AS flooding scope are stored in the global OSPF data structure
  • LSAs with area flooding scope are stored in the appropriate area data structure
  • LSAs with link-local flooding scope are stored in the appropriate interface data structure

When storing the LSA into the link-state database, a check must be made to see whether the LSA's contents have changed. When an LSA's contents have been changed, parts of the routing table must be recalculated based on the LSA's LS type.

Last updated on