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Phones behind CISCO PIX no incoming calls

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Has anyone had any experience of setting up phones behind a CISCO PIX.


We have 2 x SNOM phones at a customer location, behind a CISCO PIX firewall, registering to a hosted PBX.


The SNOM phones are registering successfully, however in the phone log the phones register every 1800 seconds instead of the usual 15 secs on working phones.


[2]9/4/2008 09:56:46: Registered at registrar as 3551@pbx.domain.co.uk (Expires: 1800 secs)


This is typical where the phone is behind a device with a SIP ALG, and usually disabling the SIP ALG solves this.


We have tried the following command on the PIX


no ip nat service sip


but continue to have problems with incoming calls.


So if anyone has any experience/suggestions they would be gratefully recieved!




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That is probably because the firewall is SIP-aware. Sometimes the firewall wants to help, but just makes things worse. You can bypass the firewall using TLS (well, if that's okay with your security guidelines). Or just turn that part of the firewall off (actually, in this case the help is not needed).

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Yes the CISCO does seem to be altering the SIP REGISTER packets, there seems to be SIP fixup enabled in the CISCO config


fixup protocol sip 5060

fixup protocol sip udp 5060


Disabling these has not made a difference though, however we have been able to get incoming calls working by reducing the "Maximum Registration Time" to 1 minute.


Whether this is advisable or not as a long term solution though I'm not sure!

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