Sip passing through nat but rtp is not - no audio

Unanswered Question
Jul 14th, 2010
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I'm looking at traffic leaving my router with a sniffer. I see SIP traffic but I do not see RTP traffic.  The phones ring on both sides but I do not get any audio.

interface f0/0.100

ip address

ip nat outside

ip nat pool VoIP prefix-length 24

ip nat inside source route-map VoIP pool VoIP overload
ip nat inside source static tcp 49201 49201 extendable

access-list 1 permit ip host any

route-map VoIP permit 10
match ip address 1
match interface  f0/0.100
set interface  f0/0.100

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Overall Rating: 5 (1 ratings)
patricianet Thu, 07/15/2010 - 06:24
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Do I need to enable anything voip related for this to work?

Like voice service voip ?

Nagaraja Thanthry Thu, 07/15/2010 - 06:46
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  • Cisco Employee,


You can enable "ip nat service sip" or "ip nat service h323" and "ip nat

service h225" commands. As per the documentation, they are enabled by

default. In the latest IOS there is a new feature added to Cisco IOS that

ensures that even RTP packets get translated to one of the allowed ports as

specified by the RFC. The command to enable the feature is "ip nat service



Hope this helps.



patricianet Mon, 07/19/2010 - 17:31
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Ok I configured the portmap command and I still do not get any audio.

This works with a linksys router but not with a cisco 2600 router

saif musa Tue, 03/31/2015 - 23:20
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Thanks Nagaraja Thanthry, that correct, I had the same issue and solved by using the instructions above ( ip nat service allow-sip-even-rtp-ports).




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