Looking at the current mainline code, when you set the MTU is it clamped
by the bridge wide "lowest" mtu:
static int br_change_mtu(struct net_device *dev, int new_mtu)
{
struct net_bridge *br = netdev_priv(dev);
if (new_mtu < 68 || new_mtu > br_min_mtu(br))
return -EINVAL;
[...]
}
Which is calculated rather simplistically where there are no attached
devices:
int br_min_mtu(const struct net_bridge *br)
{
[...]
if (list_empty(&br->port_list))
mtu = ETH_DATA_LEN;
[...]
It is not entirely clear why we care to clamp the MTU artificially at
this level, as we will reevaluate the mtu and lower it when we do add a
new bridge port.
All of that said, if the MTU was set on the KVM tap before it was added
then the bridge MTU should follow that device, and that appears to be
the expected use model from the way the code is written.
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https://bugs.launchpad.net/bugs/1399064
Title:
bridges cannot have a mtu > 1500 by themselves
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