Networking Tips Tricks

  The OSPF DN (Down) bit is a loop prevention flag in the LSA Options field used in MPLS VPNs and SD-WANs, set by Provider Edge (PE) routers on Type 3, 5, and 7 LSAs to signal routes coming from a Customer Edge (CE) router, preventing the PE from re-advertising them back, thus avoiding routing loops when redistributing BGP routes into OSPF . When a PE receives an LSA with the DN bit set, it ignores it for route calculation, stopping the loop. How it works Setting the bit : When a PE router redistributes routes (like BGP routes) into OSPF, it sets the DN bit in the LSA header for Type 3 (Summary), Type 5 (External), and Type 7 (NSSA External) LSAs before sending them to the CE. Ignoring the bit : When the PE router receives an LSA from a CE with the DN bit set, it understands this is a route originating from the VPN backbone and should not be used in its own OSPF process. Preventing loops : Without the DN bit, a PE could send a route to a CE, the CE would send it back (possibly modi...