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Date:   Thu, 16 Jul 2020 11:41:01 +0800
From:   Zhu Yanjun <zyjzyj2000@...il.com>
To:     "pbl@...tov.io" <pbl@...tov.io>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: Bonding driver unexpected behaviour

On Wed, Jul 15, 2020 at 8:49 PM pbl@...tov.io <pbl@...tov.io> wrote:
>
> I'm attempting to set up the bonding driver on two gretap interfaces, gretap15 and gretap16
> but I'm observing unexpected (to me) behaviour.
> The underlying interfaces for those two are respectively intra15 (ipv4: 10.88.15.100/24) and
> intra16 (ipv4: 10.88.16.100/24). These two are e1000 virtual network cards, connected through
> virtual cables. As such, I would exclude any hardware issues. As a peer, I have another Linux
> system configured similarly (ipv4s: 10.88.15.200 on intra15, 10.88.16.200 on intra16).
>
> The gretap tunnels work as expected. They have the following ipv4 addresses:
>           host           peer
> gretap15  10.188.15.100  10.188.15.200
> gretap16  10.188.16.100  10.188.16.200
>
> When not enslaved by the bond interface, I'm able to exchange packets in the tunnel using the
> internal ip addresses.
>
> I then set up the bonding driver as follows:
> # ip link add bond-15-16 type bond
> # ip link set bond-15-16 type bond mode active-backup
> # ip link set gretap15 down
> # ip link set gretap16 down
> # ip link set gretap15 master bond-15-16
> # ip link set gretap16 master bond-15-16
> # ip link set bond-15-16 mtu 1462
> # ip addr add 10.42.42.100/24 dev bond-15-16
> # ip link set bond-15-16 type bond arp_interval 100 arp_ip_target 10.42.42.200
> # ip link set bond-15-16 up
>
> I do the same on the peer system, inverting the interface and ARP target IP addresses.
>
> At this point, IP communication using the addresses on the bond interfaces works as expected.
> E.g.
> # ping 10.24.24.200
> gets responses from the other peer.
> Using tcpdump on the other peer shows the GRE packets coming into intra15, and identical ICMP
> packets coming through gretap15 and bond-15-16.
>
> If I then disconnect the (virtual) network cable of intra15, the bonding driver switches to
> intra16, as the GRE tunnel can no longer pass packets. However, despite having primary_reselect=0,
> when I reconnect the network cable of intra15, the driver doesn't switch back to gretap15. In fact,
> it doesn't even attempt sending any probes through it.
>
> Fiddling with the cables (e.g. reconnecting intra15 and then disconnecting intra16) and/or bringing
> the bond interface down and up usually results in the driver ping-ponging a bit between gretap15
> and gretap16, before usually settling on gretap16 (but never on gretap15, it seems). Or,
> sometimes, it results in the driver marking both slaves down and not doing anything ever again
> until manual intervention (e.g. manually selecting a new active_slave, or down -> up).
>
> Trying to ping the gretap15 address of the peer (10.188.15.200) from the host while gretap16 is the
> active slave results in ARP traffic being temporarily exchanged on gretap15. I'm not sure whether
> it originates from the bonding driver, as it seems like the generated requests are the cartesian
> product of all address couples on the network segments of gretap15 and bond-15-16 (e.g. who-has
> 10.188.15.100 tell 10.188.15.100, who-has 10.188.15.100 tell 10.188.15.200, ..., who-hash
> 10.42.42.200 tell 10.42.42.200).

Please check this
https://developers.redhat.com/blog/2019/05/17/an-introduction-to-linux-virtual-interfaces-tunnels/#gre

Perhaps gretap only forwards ip (with L2 header) packets.

Possibly "arp -s" could help to workaround this.

Zhu Yanjun
>
> uname -a:
> Linux fo-gw 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2+deb10u1 (2020-06-07) x86_64 GNU/Linux
> (same on peer system)
>
> Am I misunderstanding how the driver works? Have I made any mistakes in the configuration?
>
> Best regards,
> Riccardo P. Bestetti
>

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