MBIM Interface in Bridge Mode

Bjørn Mork bjorn at mork.no
Tue Sep 22 11:19:32 UTC 2020

Sassy Natan <sassyn at gmail.com> writes:

> In my setup I'm using VPP <https://wiki.fd.io/view/VPP> to create a high
> end linux router using DPDK .

VPP is very cool.  Never thought it would include LTE interfaces.  But
why not?

> *One thing to note here is that my requirements must set the LTE device
> inside a linux bridge. *

IMHO you should rethink that.

The drivers, and sometimes modem firmware, add some smoke and mirror
looking like ethernet headers.  But the real interface towards the
mobile network is an IP only point-to-point tunnel interface.  I believe
it's easier to stick as close to that as possible when doing low-level
stuff like VPP.

This means that none of the L2 stuff can be supported on an LTE port. It
should simply be considered a point-to-point IP tunnel regardless of how
the driver creates the associated netdev.

> To keep it simple for now, maybe someone can guide me how to set this in a
> bride mode?

This is not supported.  Every time I thought about implementing bridge
support I just ended up with completely pointless complexity, trying to
fake an L2 header based on more advanced L3 to L2 mapping than the
currently implemented scheme.

The LTE netdev created by qmi_wwan or cdc_mbim is an IP point-to-point
tunnel, similar to a PPP interface, and you should just treat it like
that.  It cannot be bridged.  Any L2 stuff you need will have to be
mapped to  L3.

Yes, sure, it is possible to pretend the netdev can be a bridge port,
just like we pretend it is an ethernet interface. You are just never
going to actually have any L2 there, so it all depends on smart L3 rules
making sure the packets end up on the correct port with the correct

IMHO, it is much easier to avoid the indirect L2 stuff, and just do the
smart L3 rules.


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