What is a fundamental difference between Type 1 and Type 2 external routes in OSPF?

The fundamental difference between Type 1 and Type 2 external routes in OSPF lies in how they calculate their cost, particularly concerning the internal costs associated with the Autonomous System (AS). Type 1 external routes, also known as E1 routes, take into account both the external cost to reach the destination and the internal OSPF cost. This means that the total cost is a combination of both the OSPF metric and the external route metric, making Type 1 routes suitable when internal OSPF path metrics are significant for routing decisions.

In contrast, Type 2 external routes, or E2 routes, do not consider the internal OSPF costs. They only use the cost of the external route itself when determining the overall cost for routing decisions. Given this distinction, it's clear that the way these two types of external routes handle the cost influences how OSPF selects routes, particularly when choosing between multiple paths to the same destination.

This understanding is essential for network design and optimization in environments where OSPF is employed, as it informs decisions on how routes are preferred and managed.