LEARN. PRACTICE. GROW.Concept and mechanism
In OSPF, failed DBD exchange can leave neighbors in Exstart or Exchange. If logs show incompatible MTU, align supported size and rehearse the path; mtu-ignore does not prove large-packet transport. Other causes exist too, so state alone is insufficient. Passive-interface suppresses neighbor formation on that interface while applicable network advertisement can remain. In the base BGP algorithm, path validity precedes preference comparison: an unreachable next hop needs resolution. Among valid paths, weight is compared before local preference. Additional mechanisms and policies can change context and should be explicit in diagnosis.
Guided application
EIGRP uses a strict condition for feasibility: reported distance below feasible distance. Equality does not satisfy it; do not confuse these values with any metric shown in another field. IP services add their own state. PAT created on one router can be absent on a return path through another without synchronization. In HSRP, a decrement produces intended preference only when it changes the comparison and other conditions exist, including preempt where needed. In a fictional example, 120 minus 30 leaves 90, below a peer with 100. For multicast (S,G), arrival on an interface different from that expected by RPF warrants analysis of the source path. Also document time and synchronization during investigations; deeper NTP/PTP, PBR, and multicast study remains necessary beyond these initial examples.
Weight 200/local-pref 100 beats weight 100/local-pref 300 under the supplied base algorithm.
Common pitfalls
Preference before validity; equality as feasibility; routing as NAT state; Full as complete transport.
Related topics: Architecture and surviving capacity · Virtualization, VRFs, and overlays · Switching and adjacency formation
Separate valid path, path selection, and session state requirements.
Reference: BGP best path selection · 350-401 ENCOR v1.2, effective 2026-03-19; core component of CCNP Enterprise