While investigating and Observing OSPF behavior with SD-WAN and why the DN bit is used as loop prevention mechanism when redistributing between OMP and OSPF. It seems that with Cisco SD-WAN, the WAN Edge which is natively a CE device that connects to MPLS Provider BEHAVES also like a PE Device in SD-WAN Fabric.
And the SD-WAN Fabric behaves like MPLS Superbackbone.
This implies that when running OSPF on service VPN (LAN), putting an ABR in the internal network in the middle between two WAN Edges can cause somes routes ignored by the WAN Edges if they are learned through a non-backbone area.
Also Knowing that originally the DN bit is used in MPLS L3 VPN between PE routers to prevent loops, I think this is why the DN bit is also used as a loop prevention between WAN Edges when redistributing between OMP and OSPF.
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