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RVS4000 mangles RTP Packets

I've come across a strange issue with a Cisco RVS4000 router, firmware      V2.0.2.7

First of all, ALG is turned off in the router!  The RTP port range 10001-10999 is forwarded to the PBX internal IP.

A PBX sitting behind NAT with ports forwarded establishes a given  incoming call with a SIP trunk provider.  Packet sniffs have shown the  PBX and Trunk agree on given IP's and port numbers in Session  Description Protocol for the conversation.  The PBX behaves as you would  expect, sending the audio from the agreed ports: PBX  PrivateIP/Port(10202) --> Trunk PublicIP/Port(53544).

The strangeness sets in when the packets come out of the WAN side of  the router.  The call setup is identical however, the first 10 RTP  packets are changed to appear to come from a different port: PBX  PublicIP/Port(51062) --> Trunk PublicIP/Port(53544).

The remaining RTP packets after the first 10 come from the correct  port: PBX PublicIP/Port(10202) --> Trunk PublicIP/Port(53544).

The SIP trunk provider sends audio in the reverse direction: Trunk PublicIP/Port(53544) --> PBX  PublicIP/Port(51062)

However, the incoming audio works and arrives at the PBX on port 10202 - therefore the router is obviously applying NAT to those packets.

The result is that the SIP trunk provider ignore all RTP packets  after the first 10 (and thus one-way audio from the trunk to the PBX) because their system accepted the first 10 from the  "wrong" port and therefore ignores the remaining packets coming from the  "right" port.

To make things even stranger, the router behaves properly on  subsequent calls.  The issue usually only appears after a long period  without incoming calls (> 30 min.)

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