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Hi

I was helping a friend of mine debug weird issue with
Xen networking the other day.  The setup involves a
dom0 with a single NIC (fxp(4)), vlan(4) interfaces
attached to the fxp in the dom0, and bridged to additional
xennet(4) interfaces in the domU.  Because the xvif/xennet pair
seems to have a hard 1500 byte limit, the tagged packets can't
bridged without a drop in MTU, which is undesirable.

Anyway, while from the dom0 the vlan(4) interfaces work
as expected, the connection of the domU's xennet to the
tagged frames on the copper is acting extremely weird.

The domU can ping6 ff02::1%xennetX both itself and the xvif
in the dom0. The MAC table in the dom0's bridge for this interface
shows the addresses I expect it should, sometimes entries even
appear in the NDP/ARP tables in remote machines.  But not so
much as a icmp ping response seems to get received by the domU.

So, outgoing from the domU seems to be working, but incoming
seems not to.

All interfaces are marked up, and whatnot.

I tested this on two 4.99.4 Xen3 dom0s,
and a 3.x Xen2 dom0 as well.  All the same.

Any ideas of where to look for clues as to what's causing this?

	Jonathan Kollasch

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