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We're running Mitaka using OVS and I have been troubleshooting some MTU
issues when I stumbled across this curiosity.

The VM OS in question has a interface MTU of 1500. When I do a tcpdump on
the tap interface on the compute node that correlates to that VM, I see
frames with a length of up to 10,000 bytes... Mostly I see frames that are
up to 7300 bytes though. How is that possible per the VM that that
interface is directly attached to has a MTU of 1500?

Here's a snippit of a tcpdump from that VM's tap interface:

[(openstack-mitaka) root at prv-0-18-compute user]# tcpdump -nni
tap3bbe0f9d-6b src x.y.120.23 and dst x.y.224.45
tcpdump: WARNING: tap3bbe0f9d-6b: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap3bbe0f9d-6b, link-type EN10MB (Ethernet), capture size
65535 bytes
15:22:26.560455 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [S], seq
3200123597, win 65535, options [mss 1460,nop,wscale 2,nop,nop,sackOK],
length 0
15:22:26.562741 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], ack
3169246826, win 53248, length 0
15:22:26.562853 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [P.], seq
0:36, ack 1, win 53248, length 36
15:22:26.563175 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
36:4416, ack 1, win 53248, length 4380
15:22:26.875289 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [P.], seq
0:1460, ack 1, win 53248, length 1460
15:22:26.876591 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
4416:10256, ack 1, win 53248, length 5840
15:22:27.781465 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
4416:5876, ack 1, win 53248, length 1460
15:22:27.782627 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
10256:17556, ack 1, win 53248, length 7300
15:22:30.484624 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
10256:11716, ack 1, win 53248, length 1460
15:22:30.486111 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
17556:24856, ack 1, win 53248, length 7300
15:22:35.203339 IP x.y.120.23.53540 > x.y.224.45.5001: Flags [.], seq
3585180775:3585182235, ack 1746533046, win 53248, length 1460
15:22:35.204632 IP x.y.120.23.53540 > x.y.224.45.5001: Flags [.], seq
7300:14600, ack 1, win 53248, length 7300
15:22:38.594013 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
17556:19016, ack 1, win 53248, length 1460
15:22:38.595570 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
24856:32156, ack 1, win 53248, length 7300
15:23:02.906420 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
24856:26316, ack 1, win 53248, length 1460
15:23:02.908021 IP x.y.120.23.53541 > x.y.224.45.5001: Flags [.], seq
32156:39456, ack 1, win 53248, length 7300

Any help on this would be appreciated!

Steve
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