In HPE Aruba Networking (AOS-CX and AOS-Switch) OSPF implementation, the routing behavior for external routes (Type 5 LSAs) distinguishes between two types of external advertisements:
E1 (Type-1 external) — The total path cost is calculated as the sum of the internal cost to reach the ASBR (Autonomous System Boundary Router) plus the external cost as advertised in the LSA.
E2 (Type-2 external) — The external cost is considered independent of the internal OSPF path cost to reach the ASBR. Thus, the metric used is only the external cost from the LSA.
When both an E1 and an E2 route exist to the same external destination, OSPF gives preference to the E1 route, regardless of metric values, because the E1 route represents a more accurate total cost to the destination (including internal OSPF cost).
Extract (as per HPE Aruba OSPF Technical Overview and AOS-CX Routing Guide):
“When both Type-1 (E1) and Type-2 (E2) external LSAs for the same destination are present, the router always prefers the Type-1 route. Type-1 routes include both internal and external costs in the total metric, while Type-2 routes use only the external cost. The E1 path is therefore considered more precise and is selected as the preferred route.”
This is consistent across Aruba’s OSPF implementation and follows standard OSPF behavior as defined by the protocol (RFC 2328).
Therefore, when both E1 and E2 routes are available and have the same overall cost, the router will always prefer the E1 path.
[References:• HPE Aruba Networking AOS-CX Routing Configuration Guide — OSPF External Route Preference (Section: OSPF External LSAs).• HPE Aruba Certified Switching Professional (ACSP) Study Guide — OSPF Route Selection and External Type Behavior.• HPE ArubaOS-Switch Management and Configuration Guide — OSPF External Route Types (E1 vs E2)., ]
