LEARN. PRACTICE. GROW.Concept and mechanism
A path may accept small requests and fail with larger messages. Tunnels add overhead and may reduce useful available size. In IPv6, Path MTU Discovery uses Packet Too Big feedback so the source can adapt size; routers do not fragment in-transit packets as a remedy. Blocking that feedback may create a black hole where a connection appears to work until larger traffic appears. Confirm sizes, the point emitting the message, and whether it reaches the source. Correction should permit necessary feedback under policy without indiscriminately disabling the firewall. Test varied sizes after the change.
Guided application
Congestion requires a different analysis. Shaping can smooth a rate through queues, adding delay; it creates no physical capacity. Measure backup traffic, interactive latency, and policy effects before promising an outcome. Diagnostic tools also have limits: a silent traceroute hop may still forward while simply not responding to probes. If the endpoint and subsequent hops reply, do not declare complete failure at that hop. Finally, validate the service that matters to the business. Ping does not prove TLS, authorization, complete transfer, or reconciliation. Use the agreed test transaction, confirm monitoring, record change and outcome, and give APS escalation and rollback criteria.
After failover, login works but a large report stalls: include representative payloads and sizes in acceptance.
Common pitfalls
Testing only ping; treating all ICMP as dispensable; shaping as new bandwidth; longer timeout as a corrected cause.
Related topics: Layers, subnets, and addressing plans · Protocols, cloud, and shared dependencies
Demonstrated recovery requires representative traffic and clear criteria as well as infrastructure indicators.
Reference: Path MTU Discovery for IPv6 · N10-009 V9