LEARN. PRACTICE. GROW.Concept and mechanism
Route installation and packet forwarding are distinct decisions. For sources of the same prefix, administrative distance helps choose the preferred source; metrics belong to their respective protocols. Then, among installed routes matching a destination, the longest prefix wins. An old /24 can continue attracting traffic despite a new /16. A floating static route uses lower preference than the primary for the same prefix. This does not automatically detect every remote failure: if the primary stays installed, the alternative may not take over.
Guided application
In OSPFv2, interpret states with network type and router roles. Two DROTHER routers on a broadcast network can remain 2-Way with each other and Full with DR/BDR. A higher-priority candidate does not automatically preempt a stable DR. Incompatible areas on a link should be corrected according to design without introducing duplicate router IDs. HSRP protects the first-hop reference, but upstream health depends on suitable routing and tracking. Finally, an IPv6 link-local next hop needs link context. Record these conditions when handing diagnosis to another team.
For 10.60.4.17, an installed /24 beats /16 and default. Before withdrawing the old route, confirm VRF, approved change, and return path.
Common pitfalls
Comparing AD across different lengths for forwarding; treating every 2-Way as failure; confusing a reachable gateway with a recovered service.
Related topics: DHCP, NAT, logs, and quality of service · Policy, identity, and secure management
Validate the decision for the specific destination, including return path and path dependencies.
Reference: Administrative distance and route selection · 200-301 CCNA v1.1