Re: [PATCH net v1 1/2] ipv6: update route serial number on NETDEV_CHANGE

[Ido Schimmel <idosch@nvidia.com>]

On Mon, Apr 27, 2026 at 10:42:22PM +0000, Sagarika Sharma wrote:
> When using IPv6 ECMP routes, if a netdev listed as a nexthop experiences
> a carrier change event (e.g., a bond device generating a NETDEV_CHANGE
> event after its slaves go linkdown), established connections utilizing
> that nexthop fail to fail over to other available nexthops. Instead,
> these connections stall or drop.
> 
> This happens because the IPv6 FIB code does not invalidate the socket's
> cached destination when a NETDEV_CHANGE event occurs. While
> fib6_ifdown() correctly marks the nexthop with RTNH_F_LINKDOWN, it
> leaves the route's serial number unchanged. As a result, sockets with a
> previously cached dst do not realize the route is no longer viable and
> continue to try using the non-functional nexthop.
> 
> This behavior contrasts with IPv4, which actively flushes cached
> destinations on a NETDEV_CHANGE event (see fib_netdev_event() in
> net/ipv4/fib_frontend.c).
> 
> Fix this by updating the route serial number in fib6_ifdown() when
> setting RTNH_F_LINKDOWN. This invalidates stale cached destinations,
> forcing sockets to perform a new route lookup and fail over to a
> functioning nexthop.
> 
> Fixes: 51ebd3181572 ("ipv6: add support of equal cost multipath (ECMP)")
> Signed-off-by: Sagarika Sharma <sharmasagarika@google.com>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>