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...
OSPF Interview Questions The purpose of this post is to get you equipped with the best set of questions asked for OSPF in interview: Q1: Why are loopbacks advertised as /32 host routes in OSPF? A1: Loopbacks are advertised as /32 host routes in OSPF to ensure uniqueness and stability, enabling precise reachability to individual loopback interfaces. Q2: How do I change the reference bandwidth in OSPF? A2: You can change the OSPF reference bandwidth using the "auto-cost reference-bandwidth" command followed by the desired bandwidth value. Q3: How does OSPF calculate its metric or cost? A3: OSPF calculates its metric/cost based on the bandwidth of the link. Cost = Reference bandwidth / Interface bandwidth. Q4: What algorithm is used by OSPF if equal cost routes exist? A4: OSPF uses the Shortest Path First (SPF) algorithm and load-balances over equal cost routes. Q5: Are OSPF routing protocol exchanges authenticated? A5: Yes, OSPF exchanges can be authenticated using me...