libvirt users - Apr 2010 - STP default behavior for bridged (off) and NAT (on) networking in libvirt

Hello,

when talking about networking in hypervisors, it's easy to distinguish
the two classic configurations, NAT and bridged, and the official
libvirt wiki does exactly in this way here [1]. NAT networking is
already configured by libvirt creating a bridge called libvirt0, while
bridged networking have to be configured manually by the user.

The libvirt0 bridge for NAT networking is default configured in this way:
STP=on
MAXWAIT=0

While suggested br0 bridge for bridged networking is configured in this way:
STP=off
MAXWAIT=5

Please, can you explain me why STP is on for NAT and should be off for
bridged networking? It seems much easier to me to create loops when
using bridged networking than NAT. Moreover, reading this old debian
bug about complaints of MAXWAIT != 0 when STP off, I'm wondering if
the suggested configurations aren't actually switched.

Thanks,
Francesco

[1] http://wiki.libvirt.org/page/Networking
[2] http://www.mail-archive.com/debian-bugs-dist at
lists.debian.org/msg700924.html

On Thu, Apr 15, 2010 at 09:23:46PM +0200, Francesco Pretto
wrote:
> Hello,
> 
> when talking about networking in hypervisors, it's easy to distinguish
> the two classic configurations, NAT and bridged, and the official
> libvirt wiki does exactly in this way here [1]. NAT networking is
> already configured by libvirt creating a bridge called libvirt0, while
> bridged networking have to be configured manually by the user.
> 
> The libvirt0 bridge for NAT networking is default configured in this way:
> STP=on
> MAXWAIT=0
> 
> While suggested br0 bridge for bridged networking is configured in this
way:
> STP=off
> MAXWAIT=5
I don't believe we're recommending STP=off for bridging. Our recommended
config is

# cat > ifcfg-br0 <<EOF
DEVICE=br0
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes
DELAY=0
NM_CONTROLLED=no
EOF

And IIUC, STP defaults to on if not specified.

I agree STP should always be on for bridging to avoid netloops. The main
important param DELAY=0 is there to ensure that you get instant network
connectivity when doing live migration - without it you'll get a 30 second
network blackout

Daniel
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