LEARN. PRACTICE. GROW.Concept and mechanism
Analyze the route table actually associated with the subnet sending traffic. The most specific matching destination route takes priority. A propagated /24 can beat a static /16; static preference applies to identical destinations in the described context. Do not automatically transfer VPC-table rules to every TGW routing variant. In a Transit Gateway, distinguish attachment association with the table used for forwarding from propagation of its prefixes into tables. Publishing a route in several places can unexpectedly increase reach. Segmentation policy should identify necessary connections and expressly forbidden connections, including the return direction.
Guided application
When integrating a VPC, prepare a short test matrix: production to shared services should pass; development to production should fail; authorized responses should return. Accepting peering does not create every required route. In Direct Connect, diagnose physical and logical prerequisites such as partner-agreed VLAN before changing BGP advertisements without evidence. An MTU change can also update the physical connection and affect other VIFs. Identify shared applications, owners, and the window. Retain previous configuration, define stop signals, and confirm behavior after the change. One configuration line can affect several applications; the change plan should reflect that dependency regardless of diff size.
10.80.12.9 matches /16 and /24. Select /24 before comparing static and propagated preference.
Common pitfalls
Wrong table; propagation treated as association; positive-only testing; MTU treated as isolated change.
Related topics: Resolution, endpoints, and automation · Evidence-led diagnosis
Follow the packet and also verify traffic that must not pass.
Reference: ANS-C01 domain 2 · ANS-C01