I believe I've narrowed this down to a GNS3 issue, as none of the other devices in my network experience this behavior - it is only traffic passing through the GNS3 CloudEthernet interface which are being reassembled, as far as I can tell. Observe packets exceeding 1500 byte MTU.Run traffic through GNS3 and the virtual router. Create a project with a virtual router (IOU) and two GNS3 Clouds to VM's Ethernet interfaces in different portgroups/vSwitches.This leads to tremendously poor performance, as a majority of packets are discarded between the end host (behind the virtual router) and the Internet (northbound of the virtual router, through Cloud-to-interface, through vSwitch, out physical ESXi host NIC). These packets are then discarded by the virtual router as "giants" which do not conform to the MTU. However, when I packet capture via GNS3, I am seeing packets much larger than 1500 bytes. If I packet capture inbound packets on my ESXi host, or the southbound side of my ESXi vSwitch, none exceed the 1500 bytes (1518 bytes at layer 2) MTU. This host VM interface is in an ESXi port group on a virtual switch, which has one physical port uplink to the rest of my network. I am linking the interface on the virtual router to a GNS3 Cloud, which is itself linked to an ethernet interface on the host VM. I am seeing strange behavior where GNS3/GNS3VM is reassembling packets that had been fragmented to not exceed MTU, and these packets are being discarded by my virtual router inside GNS3.
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