On Mon, Apr 2, 2018 at 8:26 AM, Chas Williams <3ch...@gmail.com> wrote: > On Mon, Apr 2, 2018 at 11:08 AM, Roopa Prabhu <ro...@cumulusnetworks.com> > wrote: >>
[snip] >> they are popular...in-fact they are the default bridge mode on our >> network switches. >> And they have been around for some time now to ignore its users. >> Plus it is not right to change default mtu behavior for one mode of the >> bridge >> and not the others (bridge mtu handling from user-space is complex enough >> today >> due to dynamic mtu changes on port enslave/deslave). > > I don't see the issue with one mode of bridge behaving differently > from another mode. > The VLAN behavior between the two bridge modes is completely different so > having > a different MTU behavior doesn't seem that surprising. > > You are potentially mixing different sized VLAN on a same bridge. The only > sane > choice is to pick the largest MTU for the bridge. This lets you have > whatever MTU > is appropriate on the child VLAN interfaces of the bridge. If you > attempt to forward > from a port with a larger MTU to a smaller MTU, you get the expected behavior. you mean larger MTU on the vlan device on the bridge to a smaller MTU on the bridge port ?. this will result in dropping the packet. how is this supposed to be expected default behavior ?. > > Forcing the end user to configure all the ports to the maximum MTU of > all the VLANs > on the bridge is wrong IMHO. > You then risk attempting to forward > oversize packets > on a network that can't support that. I am a bit confused: Are you trying to solve the config problem by implicitly making it the default and there by creating the oversize packet drop issue by default ? > >> >>> >>> I don't think those drops are unexpected. If a user has misconfigured >>> the bridge >>> we can't be expected to fix that for them. It is the user's >>> responsbility to ensure >>> that the ports on the VLAN have a size consistent with the traffic >>> they expect to >>> pass. >>> >> >> By default they are not expected today. The problem is changing the bridge >> to max mtu changes 'all' the vlan devices on top of the vlan aware bridge to >> max mtu by default which makes drops at the bridge driver more common if the >> user had mixed mtu on its ports. > > That's not been my experience. The MTU on the vlan devices is only > limited by the > bridges's MTU. Setting the bridge MTU doesn't change the children > VLAN devices MTUs. It does not, but it now allows vlan devices on the bridge to have a larger MTU if they need to (some or all of them). This is consistent with vxlan driver as well: picks default mtu to be lower or equal to the default dst dev mtu and allows user to override it with a larger MTU.
