✅ OSPF – Bit level Understanding

Router Role Identification (Type 1 – Router LSA)
🔹B-bit (Border bit): Indicates the router is an Area Border Router (ABR), responsible for inter-area routing and summary LSAs.
🔹E-bit (External bit): Indicates the router is an Autonomous System Boundary Router (ASBR), injecting external routes into the OSPF domain
🔹V-bit (Virtual link bit): Indicates participation in an OSPF virtual link used to maintain backbone (Area 0) connectivity.
External Route Handling (Type 5 & Type 7 LSAs)
🔹E-bit (External metric type): Defines external route type:
o  E1: Internal OSPF cost + external cost
o  E2: External cost only (default)
🔹P-bit (Propagate bit – NSSA): Controls whether Type 7 LSAs are translated by the ABR into Type 5 LSAs for propagation outside the NSSA.
How It Works (Simple)
·  If P-bit = 1 → ABR can translate Type 7 → Type 5
·  If P-bit = 0 → Route stays inside NSSA only

‼️ Advanced OSPF Features
🔹Understanding of how ABRs use B-bit and E-bit information to generate Type 3 and Type 4 LSAs.
🔹Knowledge of Type 7 → Type 5 translation behavior in NSSA environments.

📝 Question: In a multi ABR non backbone area, how does a router know which ABR to use for communication outside its area?
Answer:
1️⃣ ABRs advertise inter-area routes via Type 3 LSAs
🔹Each ABR summarizes routes from other areas and floods them into its area.
2️⃣ Routers use SPF calculation
🔹Router checks all Type 3 LSAs in its LSDB.
🔹Each LSA has a metric (cost) to reach the destination.
🔹Router chooses the lowest cost path to reach the destination (which automatically chooses the “best ABR”).
3️⃣ Tie-Breakers (if costs are equal)
🔹Router uses router ID of ABR to break ties.
🔹The one with lower router ID is preferred.