[ovs-discuss] MTU / Fragmentation / VXLAN

Justin Pettit jpettit at ovn.org
Wed Aug 28 00:31:00 UTC 2019

> On Aug 25, 2019, at 9:38 PM, Heim, Dennis <Dennis.Heim at wwt.com> wrote:
> I have an issue with SIP phones registered to a Cisco Call Manager Express (CME).
> Network Layout:
> Phone->Physical Switch->OvS(A)----VXLAN Overlay-----OvS(B)—CME.
> When the packet reaches  OvS(A) the sip (invite) packet for the phone call is in tact (packet 51).
> <image006.jpg>
> However, when I look at traces on OvS(B), I do not see that SIP invite. Interesting enough, the SIP registration message arrives as the phone is registered with the CME.

Are you sure that Packet 51 is the one that requires fragmentation?  According to your screenshot, it looks like the packet is 114 bytes.  1500 bytes is the most common MTU and a VXLAN encapsulation overhead is probably around 40 bytes, so there should be plenty of room for it.

The ICMP "fragmentation needed" error message would normally contain the first 64 bytes or so of the packet that needs fragmenting.


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