Just in case someone finds this hack useful, the following is the patch I used on 2.5.0. It will only do what is intended (hardcode advertised MTU to 1480) if "Use same settings as DHCPv6 server" is unchecked under the Router Advertisements configuration settings.

src/etc/inc/services.inc | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/etc/inc/services.inc b/src/etc/inc/services.inc index a3203aaaf7..1c63272ca1 100644 --- a/src/etc/inc/services.inc +++ b/src/etc/inc/services.inc @@ -130,7 +130,8 @@ function services_radvd_configure($blacklist = array()) { $radvdconf .= "\tAdvDefaultLifetime {$dhcpv6ifconf['raadvdefaultlifetime']};\n"; } - $mtu = get_interface_mtu($realif); + /*$mtu = get_interface_mtu($realif);*/ + $mtu = 1480; if (is_numeric($mtu)) { $radvdconf .= "\tAdvLinkMTU {$mtu};\n"; } else {