SLES10 Xen Jumbo Frames

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/ script to grab
the MTU from the /sys/class/net//mtu file and set it before
adding a new interface to the bridge. This works wonderfully for PV VMs

  • the vif. 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. 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. 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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Hello. Same problem from here. I Can add the MTU 9000 on the Bridge, but
they don t works on the PV (on my case with XEN)…Don t know how can i
solve the MTU on the Xen server (SLES 11 SP1 with lastest patches)

Victor Galino Lopez, Promo-Soft, S.L.

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