3750 altering tcp ack number

[Phil Curwood]

Hi All,

We have a 3750 core switch in our DC. I'm having an issue whereby the tcp ack number is being changed for traffic going to/from a Cisco 887 router we're using.
I have narrowed this issue down by performed wireshark captures on the following interfaces on the core switch:

The interface connected to the router
The Port Channel interface connected to the HyperV host

I have then tried to establish rdp sessions in both directions (i.e. from a VM on the HyperV host to a VM in Azure, and vice versa). and made a note of the captures.

Has anyone every seen this behaviour before?

Robbie

core switch interfaces connecting a HyperV host and t  traffic coming through the router (via a VPN from Azure)

[Aaron O'Hare]

Normally only when NAT is in place, are you doing any NATing on the 887?

-AO

[Masoud Pourshabanian]

Hello,

As Aaron mentioned, NAT may cause changing the Sequence numbers and the following ack numbers, because router starts to communicate with outside as an agent for inside network so it picks a random sequence number for each flow resulting the sequence numbers of inside network would be different from the acks coming from the outside network. Router will set the correct acks when it translates back.

Masoud

[Phil Curwood]

Hi Guys,

Thanks for the quick reponse. I found the issue to be aysmetric routing which I've now resolved.

Thanks again.

Robbie.