1
Diagnostic step 1
Do not factory reset again during active troubleshooting; preserve current config and export a config backup.
FULL AI ANALYSIS
Router stops advertising IPv6 default route after enabling RA + DHCPv6 coexistence
Generated Oct 3, 2026, 12:35:03 AM
Analysis summary
Symptoms and packet captures indicate router's RA advertisements are being sent without a usable default route (zero lifetime) shortly after the DHCPv6 service starts. This commonly points to a firmware bug or misconfiguration where the router's RA generator and DHCPv6 daemon conflict over which component owns the default route advertisement; some implementations suppress RA prefixes or default route when DHCPv6 obtains a delegated prefix or attempts to manage routing. Differences in M/O flags, RA lifetime values, or kernel sysctls that suppress RAs can produce the observed behavior. Client-side fixes (static route) worked briefly because they bypassed RA reliance.
Possible causes
Router firmware bug where DHCPv6 daemon disables or zeroes default-route options in RAs when DHCPv6 is active.
90%Race condition between RA and DHCPv6 processes that results in periodic RA updates with route lifetime=0.
80%Misconfigured IPv6 settings: router configured to expect delegated prefix (PD) but LAN uses single /64, causing RA generator to skip default-route advertisements.
40%Kernel or network stack sysctl (e.g., accept_ra or sysctl handling of RA when managed flag differs) suppressing RAs after DHCPv6 attaches.
15%Suggested fixes
2
Diagnostic step 2
Capture RA and DHCPv6 traffic for analysis: run tcpdump -i <lan-if> icmp6 -vv and capture DHCPv6 packets (UDP 547/546) for 60–120s immediately after enabling services.
3
Diagnostic step 3
Check router IPv6 config for PD vs SLAAC modes and ensure LAN has a proper /64 assigned; if a PD is configured, try switching to static /64 to test behavior.
4
Diagnostic step 4
Temporarily disable DHCPv6 (as you discovered) to confirm RA restores default route; if so, use DHCPv6 disabled + static DNS as interim solution.
5
Diagnostic step 5
Look for firmware release notes and known issues for RA/DHCPv6 interaction; if running a beta, revert to the latest stable firmware that previously worked or downgrade to last-known-good firmware and re-test thoroughly (use DDU-like clean config restore if vendor recommends). Do this during a maintenance window because routing will change for all devices. If downgraded firmware still exhibits the problem or packet captures show malformed RAs (route lifetime=0 coming from RA generator), collect: router syslog, full tcpdump pcap, current IPv6 config output and open a support ticket with the router vendor including those logs. Escalate to vendor with captures—this likely requires vendor patching the RA/DHCPv6 interaction.