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From Sean Lair <sl...@ippathways.com>
Subject Traffic being tagged with VLAN 0 w/ KVM
Date Fri, 19 Jan 2018 06:36:44 GMT
Hi all,

We are seeing some strange behavior with our KVM guests.  When the guest VM is on the same
KVM host as the vRouter, traffic to the guest VM is being tagged with VLAN 0 (it should just
be untagged traffic).  This breaks connectivity for some operating systems that aren't expecting
packets to be tagged with a VLAN.  Technically, the guest VLAN is 306, and we see Cloudstack
creating the 306 VLAN sub-interface on the host and the corresponding bridge interface - that
all looks good.

This only occurs when the guest VM is on the same host as the vRouter.  However, it is likely
that the physical switch connecting the KVM hosts is being smart and stripping the VLAN 0
tag... and that is why we don't have the problem with the traffic flows between hosts.

Cloudstack 4.9.3 w/ Advanced Networking
CentOS 7.4 KVM host (fully up-to-date)
Bridge networking within the host

Here is a tcpdump on the guest VM, showing the dot1q "VLAN 0" tag on the response packets
only.  The guest doesn't support VLAN tagging, and just says request timed out to the ping.

01:18:22.723029 02:00:0c:a3:00:05 > 02:00:56:46:00:04, ethertype IPv4 (0x0800), length
98: 10.1.1.246 > 8.8.8.8: ICMP echo request, id 1375, seq 150, length 64
01:18:22.738067 02:00:56:46:00:04 > 02:00:0c:a3:00:05, ethertype 802.1Q (0x8100), length
102: vlan 0, p 0, ethertype IPv4, 8.8.8.8 > 10.1.1.246: ICMP echo reply, id 1375, seq 150,
length 64
01:18:23.724363 02:00:0c:a3:00:05 > 02:00:56:46:00:04, ethertype IPv4 (0x0800), length
98: 10.1.1.246 > 8.8.8.8: ICMP echo request, id 1375, seq 151, length 64
01:18:23.739301 02:00:56:46:00:04 > 02:00:0c:a3:00:05, ethertype 802.1Q (0x8100), length
102: vlan 0, p 0, ethertype IPv4, 8.8.8.8 > 10.1.1.246: ICMP echo reply, id 1375, seq 151,
length 64
01:18:24.725480 02:00:0c:a3:00:05 > 02:00:56:46:00:04, ethertype IPv4 (0x0800), length
98: 10.1.1.246 > 8.8.8.8: ICMP echo request, id 1375, seq 152, length 64
01:18:24.740498 02:00:56:46:00:04 > 02:00:0c:a3:00:05, ethertype 802.1Q (0x8100), length
102: vlan 0, p 0, ethertype IPv4, 8.8.8.8 > 10.1.1.246: ICMP echo reply, id 1375, seq 152,
length 64
01:18:25.726752 02:00:0c:a3:00:05 > 02:00:56:46:00:04, ethertype IPv4 (0x0800), length
98: 10.1.1.246 > 8.8.8.8: ICMP echo request, id 1375, seq 153, length 64
01:18:25.741749 02:00:56:46:00:04 > 02:00:0c:a3:00:05, ethertype 802.1Q (0x8100), length
102: vlan 0, p 0, ethertype IPv4, 8.8.8.8 > 10.1.1.246: ICMP echo reply, id 1375, seq 153,
length 64

Any idea how to stop KVM from adding that VLAN 0 dot1q tag?  KVM is a new add to our environment.

Thanks
Sean

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