Network & Infrastructure / IPv6, TCP & Transport Engineering
IPv6 default-router lifetime versus prefix lifetime
Hosts keep a valid global IPv6 address after losing their default route. Address validity and router availability are governed by different advertised lifetimes and should not be treated as one state.
Environment and inputs: Dual-stack routed networks and transport endpoints; validate the complete path and the active operating-system version. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Capture router advertisements, default-router entries, prefix information and host route expiry. Compare the router lifetime with preferred and valid prefix lifetimes and any duplicate gateway sources.
Explain the result
A prefix can remain valid after a router stops being usable as a default. Conversely, a router advertisement can supply routing information without granting every advertised prefix autonomous address configuration.
Plan the fix
Correct the intended RA policy and redundant-router behavior, taking host implementation differences into account. Do not shorten address lifetimes indiscriminately to compensate for a missing default advertisement.
Check the result and rollback
Observe host routing across gateway failure and return for at least the configured lifetime window. Restore prior settings if addresses churn or clients lose a valid backup router.