Router stops advertising IPv6 default route after enabling RA + DHCPv6 coexistence

Environment

Home router (consumer Linux-based firmware) running firmware v2.7.3, LAN uses SLAAC for IPv6 addresses, DHCPv6 server enabled to assign DNS-only (no-address) on /64, client devices: Ubuntu 22.04 desktop, macOS 13 laptop, several Android phones.

Symptoms

After enabling DHCPv6 to provide DNS while retaining SLAAC for addresses, clients receive IPv6 addresses but cannot reach IPv6-only websites. Some clients intermittently show no default IPv6 route (no ::/0) despite having a link-local gateway; issue appears 5–20 minutes after change and persists until router reboot.

Observations

ip -6 route on Ubuntu shows only fe80::/64 via link-local and per-interface routes; curl -6 to ipv6.google.com times out; router syslog contains repeated 'RA suppressed for interface br-lan' messages after DHCPv6 process starts; router GUI reports 'IPv6: enabled (SLAAC + DHCPv6)'. Captured RA packets with tcpdump show Router Advertisement messages missing the M flag and missing default route (route lifetime set to 0).

Already attempted

Rebooted router and clients; disabled DHCPv6 (clients regained default route immediately); toggled SLAAC-only and DHCPv6-only modes; upgraded firmware to v2.7.4 beta (no change); restored config from backup (same behavior); added static default route on clients as temporary workaround.

Question

What causes the router to stop advertising the IPv6 default route when DHCPv6 and SLAAC are both enabled, and how can I restore stable IPv6 default routing without disabling DHCPv6 DNS delivery?

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AI DIAGNOSTICS

Incident analysis

Updated 1w ago• Trouble ID

Most likely 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%

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.

Suggested Fixes

Diagnostic step 1

Do not factory reset again during active troubleshooting; preserve current config and export a config backup.

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.

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.