Disable SIP SDP modification

[mpla]

Cisco C8000V. Cisco IOS XE Software, Version 17.06.05.

Trying to implement static PAT to translate SIP traffic between private and public voice gateways.

<172.17.20.36:3002>    <Cisco NAT 1.2.3.4>      <5.6.7.8:5060>

ip nat inside source static udp 172.17.20.36 3002 1.2.3.4 3002 extendable

But seeing SIP payload being modified after message processed by CISCO.

Received

User Datagram Protocol, Src Port: 5060, Dst Port: 3002
Session Initiation Protocol (200)
Status-Line: SIP/2.0 200 OK
Message Header
Via: SIP/2.0/UDP 1.2.3.4:3002;branch=z9hG4bK00B2699857923461da3
From: <sip:1.2.3.4>;tag=gK007b1f05
To: <sip:5.6.7.8>;tag=SDjpkq999-611519991-1713538972837
Call-ID: 131850216_1293583290@1.2.3.4
[Generated Call-ID: 131850216_1293583290@1.2.3.4]
CSeq: 180544 OPTIONS
Content-Length: 0

Sent

User Datagram Protocol, Src Port: 5060, Dst Port: 3002
Session Initiation Protocol (200)
Status-Line: SIP/2.0 200 OK
Message Header
Via: SIP/2.0/UDP 172.17.20.36:3002;branch=z9hG4bK00B2699857923461da3
From: <sip:172.17.20.36>;tag=gK007b1f05
To: <sip:5.6.7.8>;tag=SDjpkq999-611519991-1713538972837
Call-ID: 131850216_1293583290@172.17.20.36
[Generated Call-ID: 131850216_1293583290@172.17.20.36]
CSeq: 180544 OPTIONS
Content-Length: 0

Looking for a way to disable SIP payload modifications. Tried following but without difference:

no ip nat service sip udp port 3002
then
no ip nat service sip udp port 5060
then
no ip nat service all-algs

[Roger Kallberg]

Using PAT or NAT would do this, so it is the expected result and also needed for call services to be operational as you cannot present the internal IP information to the external party as they wouldn’t know how to route the response traffic. You’ll also need to use a SIP profile to modify the required IP information in the SIP dialogue that isn’t modified by PAT/NAT.

[mpla]

Is there any mechanism to disable such modifications? Internal signaling gateway is capable to work behind NAT and doesn't expect changes in SIP payload.

[Roger Kallberg]

Not using PAT or NAT, other than that nothing that I’m aware of.