Network packet size over MTU in guests

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Hi,

We use KVM 75 on Gentoo hosts with kernel 2.6.27.8 with a variety of guests (Windows XP, 2000, 2003 & several Linux distributions) in a production setup. The hosts are Intel Xeon based. We use bridged networking with E1000 model inside of guests.

Recently, we began to have network problem (slowdowns, lags, timeouts) on several guests. Inside those guests, we saw with tcpdump that there were packets with a size well over MTU. On the host, tcpdump on the guest tap did show packets of 1500 bytes and that's all. For some of the guests (Debian based), changing the NIC model to rtl8139 instead of e1000 "fixed" the problem, but for others (Gentoo kernel 2.6.27.19) it didn't.

Would it be a glitch in the KVM code or a driver problem ? Does this ring a bell to anyone ?

Thanks.

FT
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