OSPF features, as shown in Figure 1, include:
- Classless - It is classless by design; therefore, it supports VLSM and CIDR.
- Efficient - Routing changes trigger routing updates (no periodic updates). It uses the SPF algorithm to choose the best path.
- Fast convergence - It quickly propagates network changes.
- Scalable - It works well in small and large network sizes. Routers can be grouped into areas to support a hierarchical system.
- Secure - It supports Message Digest 5 (MD5) authentication. When enabled, OSPF routers only accept encrypted routing updates from peers with the same pre-shared password.
Administrative distance (AD) is the trustworthiness (or preference) of the route source. OSPF has a default administrative distance of 110. As shown in Figure 2, OSPF is preferred over IS-IS and RIP.