Re: DSA breaks clients' roaming between switch port and host interfaces

[Andrew Lunn <andrew@lunn.ch>]

On Sun, Apr 05, 2020 at 08:23:36PM +0800, DENG Qingfang wrote:
> Hello,
> I found a bug of DSA that breaks WiFi clients roaming.
> 
> I set up 2 WiFi routers as AP, both of them run kernel 5.4.30 and use DSA.
> 
>         +-------------------------+
> +-----------------------------+
>         |                         |                            |
>                       |
>         |                         |                            |
>                       |
>         |       AP1               |                            |
> AP2                   |
>         |                     LAN2+--------------------------->|LAN1
>                       |
>         |       10.0.0.1/24       |                            |
> 10.0.0.2/24           |
>         |                         |                            |
>                       |
>         |       MV88E6XXX DSA     |                            |
> MT7530 DSA            |
>         |                         |                            |
>                       |
>         |                         |                            |
>                       |
>         |                         |                            |
>                       |
>         +-------------------------+
> +-----------------------------+
>                      ^                                              ^
>                      |                                              |
>                      |                      Roams                   |
>                      |                     -------------------------+
>                      |
>                      +------------    +-------------------+
>                                       |     Wi-Fi         |
>                                       |     Client        |
>                                       |                   |
>                                       |     10.0.0.3/24   |
>                                       |                   |
>                                       |                   |
>                                       +-------------------+
> 
> When the client roams from AP1 to AP2, it cannot ping AP1 anymore for
> a few minutes, and vice versa.
> 
> With bridge fdb I found out the part that caused the problem.
> When the client is connected to AP1, bridge fdb on AP2 shows:
> 
> <client's mac> dev lan1 master br-lan
> <client's mac> dev lan1 vlan 1 self
> 
> It means AP2 should talk to the client via lan1, which is correct.
> 
> After the client roams to AP2, the problem comes:
> 
> <client's mac>  dev wlan0 master br-lan
> <client's mac>  dev lan1 vlan 1 self
> 
> >From iproute2 man page: "self" means the address is associated with
> the port drivers fdb. Usually hardware.
> 
> The lan1 is still there, which means the kernel has updated the
> forwarding table in br-lan, but forgot to delete the one in the switch
> hardware.
> 
> What happens when the client now tries to talk to AP1, such as ping
> 10.0.0.1? I debugged with tcpdump:
> 
> 1. The client sends ARP request: who-has 10.0.0.1?
> 2. The software part of the bridge of AP2 receives the ARP request,
> updates fdb, and sends it to the CPU port
> 3. The switch receives the client's ARP request from the CPU port, and
> floods it out of the LAN1 port. Although the source MAC address of the
> request is the client's, _auto learning of the CPU port is disabled in
> DSA_, so the switch does not update the MAC table.
> 4. AP1 receives the ARP request, then responds: 10.0.0.1 is-at <AP1's MAC>.
> 5. AP2's switch receives the response from LAN1, then looks it up in
> the MAC table, the egress port is the same as the ingress port (LAN1).
> To avoid loop, the ARP response is discarded.
> 
> If I manually delete the leftover fdb entry in the hardware via
> "bridge fdb del <client's MAC> dev lan1 vlan 1", the client can talk
> to AP1 immediately.
> And vice versa, the mv88e6xxx has the same bug, so I think it's with
> the general DSA part.
> 
> Does anyone know how to fix it?
> 
> Thanks.
> Qingfang

Hi Qingfang

I've had similar reports from somebody else.

Did you try playing with auto learning for the CPU port?

    Andrew