This feature adds support for the extended LSA format in OSPFv3 as described in draft-ietf-ospf-ospfv3-lsa-extend, OSPFv3 LSA Extendibility.
Before this feature, the SR OS used the fixed-format LSA to carry the prefix and link information as described in RFC 5340, OSPF for IPv6. The fixed-format is not extensible, for this reason it needs to use the TLV format of the extended LSA.
With this feature, the default mode of operation for OSPFv3 is referred to as sparse mode, meaning that the router advertises the fixed-format for existing LSAs and adds the TLV-based extended LSA only when it needs to advertise new sub-TLVs. This mode of operation is very similar to the way OSPFv2 advertises the Segment Routing information. It sends the prefix in the original fixed-format prefix LSA and then follows with the extended prefix TLV which is sent in an extended prefix opaque LSA containing the prefix SID sub-TLV.
The extended-lsa only value enables the full extended LSA mode and this causes all existing and new LSAs to use the extended LSA format.
An OSPFv3 area inherits the instance level configuration but can also be configured independently to the sparse of full extended LSA mode.
The OSPFv3 instance must first be shut down before the user can change the mode of operation, because the protocol must flush all LSAs and re-establish all adjacencies.