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[Contrail] Fourth fragment of UDP packet is dropped by vRouter, first three fragments passed



Article ID: KB35535 KB Last Updated: 11 Mar 2020Version: 1.0

There is a day-1 product limitation of Contrail vRouter agent such that if the first packet has more than three fragments, the first three fragments pass while the fourth fragment is dropped. This causes incomplete packets to be received. This fragment drop issue only occurs before the traffic flow has been created on vRouter. Therefore, the following fragmented packets should pass without problems. If the packet type is TCP, a TCP re-transmit mitigates this limitation. However, for some user UDP/SIP application where the first jumbo packet carries import session information, it may cause a call session fail to set up. This article describes this product limitation.


SIP invites sourced from VM 2001:1890:1001:22c4::7:3 are sent to VRF VM 2001:1890:1001:22c4::3:20. The SIP invite messages are dropped. VM's MTU is set to 1500 which is fixed. These SIP invites are large packet sizes >4500 so they are fragmented. Captured traces on the source host tap interface show that all four fragments of SIP invite are being passed to the vRouter. However, the fourth fragmentation of SIP invite is not seen on bond0 underlay interface. Packet sniffers at the same time on source tap/bond0, destination tap/bond0.

In the Source TAP capture, note that all 4 fragments are highlighted:

But in source bond0 capture, note that the 4th fragment is missing: