superfluous host interfaces

Harry Schmalzbauer freebsd at
Mon Feb 26 12:13:20 UTC 2018

Bezüglich Ruben's Nachricht vom 26.02.2018 11:34 (localtime):
> On 26/02/2018 10:56, Harry Schmalzbauer wrote:
>> Another, personally very significant, reason is that you'll get a
>> superfluous host interface for each if_bridge(4), which makes the output
>> of plain ifconfig(8) kind of unreadable.
> By superflous host interfaces, do you mean the tap interfaces configured
> for each vm together with the bridge interfaces they are "bundled" in?

Additionally to the if_tap(4) ethernet host interfaces, you also get
if_bridge(4) ethernet interfaces, named bridgeX if I remember correctly.
The if_bridge(4) host interface is for control purposes only on a VM-SDN
host – at least with my setups.  I never needed to make use of IP
numbered bridges.  And I don't need to utilize any if_bridge(4) features
like STP, so I consider the bridgeX host interfaces as superfluous in
the VM-SDN use case.

I'd call the if_tap(4) host interfaces likewise superfluous – you would
only need the corresponding character devices – but that's been
implemented long before the need for SDN setups, so it is like it is.
And using ng_bridge(4) instead of if_bridge(4) doesn't change the need
for if_tap(4).  Only with vale(4) switches, bhyve(8) was able to provide
virtio-net connection wihtout "spamming" the host's ethernet interface
list (no tapX, no bridgeX).

> Overall I'm very happy with my bhyve setups atm. If there are any
> speed-/administrative-advantages that come with bridge_ng however, I'm
> very interested in switching to such a setup (or at least play with it).
> I'm running my vm's without any helper project so I'm flexible enough to
> do some fiddling :P
> Do you know of any documentation on using bridge_ng together with bhyve?
> My search-engines don't turn up much Im affraid and I haven't stumbled
> on it before.

Unfortunately it's not too easy to get started with netgraph.
Besides numerous man pages for the different nodes (ng_bridge(4) e.g.),
I only know the following source for a good overview:

One convenience disadvantage with ng_bridge(4) is that you have to
assign MACs manually, while if_bridge(4) does that itself (adjustable by
And you need to script all setups yourself.  Almost all of my setups
seem to be awkward enough that I always had to do some local scripting,
so that wasn't really a disadvantage for me.

If you're happy with your setup, I don't think you gain anything from
switching to ng_bridge(4), besides learning to control netgraph(4)
(which is very desirable imho).
I haven't had time left to do useful benchmarking regarding ng_bridge(4)
vs. if_bridge(4). I even don't know if netgraph nodes are still limited
to single threads.  But rough load comparings on a IvyBride machine
showed similar resource usage for both bridges, both easy capable of
1GbE saturation with small frames (while I remember one run with
ng_bridge(4) and if_vmnet(4), which couldn't deliver 1GbE speed, and I
wanted to falsify for vmnet/tap difference... just ran out of time :-( ).


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