Having a small issue getting Jumbo frames to work here in SLES10 with
Xen and bridged networking. I set up the Ethernet interface and the
Bridge for jumbo frames (MTU='9000' in
/etc/sysconfig/network-scripts/ifcfg-*). I quickly figured out that the
Linux bridge auto-negotiates to the lowest MTU set by one of its
members. There doesn't seem to be a built-in way to force the MTU, so I
had to modify the /etc/xen/scripts/xen-network-common.sh script to grab
the MTU from the /sys/class/net/<BRIDGE>/mtu file and set it before
adding a new interface to the bridge. This works wonderfully for PV VMs
- the vif<x>.<y> interfaces get set with the 9000 MTU setting. However,
with HVM domUs this does not work, primarily because the HVM domUs seem
to generate both a vif interface AND a tap interface. I have no clue
why - the tap interface seems totally worthless. More to the point,
though, there's a limitation in the Linux kernel pre-2.6.24 that does
not allow tap interfaces to have MTU over 1500. This has two negative
consequences: 1) bridge MTU gets set to 1500, and 2) because bridge
MTU=1500, future vif MTUs also get set to 1500, resulting in no
communication on that bridge until I go manually run "ifconfig
vif<x>.<y> mtu 9000" at which point they start working, again.

So, first, has anyone had any success getting SLES (10 or 11 - I'm
happy to look at how 11 does it, too) set up with Xen AND Jumbo Frames?

Second, are there any negative consequences to not having the
tap<x>.<y> interfaces get added to the bridges? A quick test removing
it shows no bad side-effects. If there are, what are they? If not, how
do I modify the xen network scripts to prevent the tap interfaces from
getting added to bridges?

Thanks - Nick


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