netdev - Re: [PATCH net-next 1/2] bridge: Redirect to backup port when port is administratively down

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Message-ID: <3e84e2ea-2b11-4e3d-99e9-4c6ecabe3bc4@blackwall.org>
Date: Tue, 12 Aug 2025 11:27:50 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Ido Schimmel <idosch@...dia.com>, netdev@...r.kernel.org,
 bridge@...ts.linux-foundation.org
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
 edumazet@...gle.com, petrm@...dia.com, horms@...nel.org
Subject: Re: [PATCH net-next 1/2] bridge: Redirect to backup port when port is
 administratively down

On 8/12/25 11:02, Ido Schimmel wrote:
> If a backup port is configured for a bridge port, the bridge will
> redirect known unicast traffic towards the backup port when the primary
> port is administratively up but without a carrier. This is useful, for
> example, in MLAG configurations where a system is connected to two
> switches and there is a peer link between both switches. The peer link
> serves as the backup port in case one of the switches loses its
> connection to the multi-homed system.
> 
> In order to avoid flooding when the primary port loses its carrier, the
> bridge does not flush dynamic FDB entries pointing to the port upon STP
> disablement, if the port has a backup port.
> 
> The above means that known unicast traffic destined to the primary port
> will be blackholed when the port is put administratively down, until the
> FDB entries pointing to it are aged-out.
> 
> Given that the current behavior is quite weird and unlikely to be
> depended on by anyone, amend the bridge to redirect to the backup port
> also when the primary port is administratively down and not only when it
> does not have a carrier.
> 

hehe I did ask that question long time ago while adding support for backup ports,
at the time wasn't needed for the MLAG case :-)

> The change is motivated by a report from a user who expected traffic to
> be redirected to the backup port when the primary port was put
> administratively down while debugging a network issue.
> 
> Reviewed-by: Petr Machata <petrm@...dia.com>
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> ---
>   net/bridge/br_forward.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
> index 29097e984b4f..870bdf2e082c 100644
> --- a/net/bridge/br_forward.c
> +++ b/net/bridge/br_forward.c
> @@ -148,7 +148,8 @@ void br_forward(const struct net_bridge_port *to,
>   		goto out;
>   
>   	/* redirect to backup link if the destination port is down */
> -	if (rcu_access_pointer(to->backup_port) && !netif_carrier_ok(to->dev)) {
> +	if (rcu_access_pointer(to->backup_port) &&
> +	    (!netif_carrier_ok(to->dev) || !netif_running(to->dev))) {
>   		struct net_bridge_port *backup_port;
>   
>   		backup_port = rcu_dereference(to->backup_port);

Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>