netdev - Re: Routing in case of GRE interface under a bridge

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Date: Thu, 29 Jun 2023 14:12:35 +0300
From: Ido Schimmel <idosch@...sch.org>
To: Nayan Gadre <beejoy.nayan@...il.com>
Cc: netdev@...r.kernel.org
Subject: Re: Routing in case of GRE interface under a bridge

On Wed, Jun 28, 2023 at 07:06:45PM +0530, Nayan Gadre wrote:
> I have a "l2gre0" and "eth0" interface under the bridge "br0".

I assume "l2gre0" is a gretap, not ipgre.

> If a packet comes to eth0 interface with a destination IP address say
> 10.10.10.1 which is not known on the Linux system, as there is no
> route for 10.10.10.1, will the l2gre0 interface encapsulate this
> packet and send it across the tunnel ?

The bridge doesn't care about IP addresses when forwarding unicast
packets. Forwarding happens based on DMAC. Packet will be transmitted
through "l2gre0" if the bridge has a matching FDB entry for the DMAC
with "l2gre0" as the destination bridge port or if there is no FDB entry
at all, in which case the packet will be flooded.

One of the attributes of the GRE device is the remote address, which is
the encapsulating destination IP. Linux needs to have a route telling it
how to reach this destination address or the packet will be dropped.

> The other endpoint is on a different Linux system with another l2gre0
> interface having IP address 10.10.10.1
> 
> Thanks
> N Gadre
>