A linux bridge always adopts the smallest MTU of the enslaved devices. When no device are enslaved, it defaults to a MTU of 1500 and refuses to use a larger one. This is problematic when using bridges enslaving only virtual NICs (vnetX) like it's common with KVM guests.
Steps to reproduce the problem 1) sudo ip link add br-test0 type bridge # create an empty bridge 2) sudo ip link set br-test0 mtu 9000 # attempt to set MTU > 1500 3) ip link show dev br-test0 # confirm MTU Here, 2) returns "RTNETLINK answers: Invalid argument". One (cumbersome) way around this is: 4) sudo modprobe dummy 5) sudo ip link set dummy0 mtu 9000 master br-test0 Then the bridge's MTU can be changed from anywhere to 9000. This is especially annoying for the virtualization case because the KVM's tap driver will by default adopt the bridge's MTU on startup making it impossible (without the workaround) to use a large MTU on the guest VMs. https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1399064 Signed-off-by: Michael Braun <michael-...@fami-braun.de> Reported-by: Li RongQing <roy.qing...@gmail.com> -- If found https://patchwork.ozlabs.org/project/netdev/patch/1456133351-10292-1-git-send-email-roy.qing...@gmail.com/ but I am missing any follow up. So here comes a refresh patch that addresses the issue raised. --- net/bridge/br_if.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c index 4fe30b182ee7..f14e7d2329bd 100644 --- a/net/bridge/br_if.c +++ b/net/bridge/br_if.c @@ -496,7 +496,7 @@ static int br_mtu_min(const struct net_bridge *br) if (!ret_mtu || ret_mtu > p->dev->mtu) ret_mtu = p->dev->mtu; - return ret_mtu ? ret_mtu : ETH_DATA_LEN; + return ret_mtu ? ret_mtu : (64 * 1024); } void br_mtu_auto_adjust(struct net_bridge *br) -- 2.20.1
