In OSPF, when an Autonomous System Boundary Router (ASBR) redistributes routes into the OSPF domain, it assigns them a " Type. " Understanding the difference between Type 1 (E1) and Type 2 (E2) is fundamental for controlling traffic flow out of the network.
Type 2 (E2) Routes (Default): The cost of an E2 route is only the external cost (seed metric) assigned by the ASBR. OSPF does not add the internal path cost to reach the ASBR when comparing two E2 routes.
Tie-Breaking: If two ASBRs advertise the same prefix with the same E2 metric, OSPF then uses the internal cost to the ASBR as a tie-breaker. It will choose the path to the ASBR that is mathematically " closest " in terms of OSPF cost.
Closest Exit Point Strategy (Option D): This is a very common design in Aruba campus networks with multiple Internet or Data Center exits. By configuring all ASBRs to use the same E2 seed metric (the " appropriate " metric), you ensure that every switch in the network will automatically choose its own closest exit point based on the local topology.
Type 1 (E1) Routes: These routes calculate the total cost as External Cost + Internal Path Cost. Use E1 when you want the internal network ' s performance/hops to always influence the path choice, even if external costs differ.
Why other options are incorrect:
Option B: This is the definition of an E1 route, not E2.
Option C: While E2 with the same metric can result in ECMP (Equal Cost Multi-Path), this only happens if the internal costs to both ASBRs are also identical. The more fundamental architectural goal for E2 is favoring the closest exit.
Option A: You can achieve primary/backup with either E1 or E2; it is not a requirement to use E1.
