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Public bug reported:
Hi,
It was non-obvious to me (and I suspect it will be non-obvious to a lot
of others) that it really is absolutely necessary to increase the MTU
when using VXLAN encapsulation (and presumably GRE and other tunnelling
as well). The principal difficulty in diagnosis is that resulting VM
behaviour is just so strange: depending on the guest and hypervisor
kernel used and whether GSO is disabled on each using ethtool, network
traffic may almost not flow at all, or be almost normal.
Very belatedly I spotted the doc note referenced in openstack-manuals
commit c3ba506e97d59969ee2ad9476e8d1dfa55001641 , and this is a good
start.
However, I think more is needed. This RFE is for neutron to sanity
check for obvious MTU mismatches at startup (it may not be possible to
catch all of them, but surely the obvious ones would be better than
nothing) and print out a big warning.
For example, I'd say if VXLAN is configured + the MTU on the default
gateway is 1500 + the MTU on the VXLAN network is <= 1554 (or whatever),
then print a warning. Ditto for GRE, etc., etc.
It would be good if the warning said something like "MTU settings *may*
be wrong: consult <OpenStack documentation link> for more details."
** Affects: neutron
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1375815
Title:
RFE: detect obvious MTU mismatches for tunnelling and print a warning
Status in OpenStack Neutron (virtual network service):
New
Bug description:
Hi,
It was non-obvious to me (and I suspect it will be non-obvious to a
lot of others) that it really is absolutely necessary to increase the
MTU when using VXLAN encapsulation (and presumably GRE and other
tunnelling as well). The principal difficulty in diagnosis is that
resulting VM behaviour is just so strange: depending on the guest and
hypervisor kernel used and whether GSO is disabled on each using
ethtool, network traffic may almost not flow at all, or be almost
normal.
Very belatedly I spotted the doc note referenced in openstack-manuals
commit c3ba506e97d59969ee2ad9476e8d1dfa55001641 , and this is a good
start.
However, I think more is needed. This RFE is for neutron to sanity
check for obvious MTU mismatches at startup (it may not be possible to
catch all of them, but surely the obvious ones would be better than
nothing) and print out a big warning.
For example, I'd say if VXLAN is configured + the MTU on the default
gateway is 1500 + the MTU on the VXLAN network is <= 1554 (or
whatever), then print a warning. Ditto for GRE, etc., etc.
It would be good if the warning said something like "MTU settings
*may* be wrong: consult <OpenStack documentation link> for more
details."
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