One Way audio problem

Unanswered Question
Jan 14th, 2010
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Hello Everyone.

First I would like to thank you all for taking the time to read this and possible answer.

Situation is as follows:

Main (HUB) site currently has a Cisco ASA5510 firewall running 8.0.5 code

Cisco Express 500 12 port gig switch

2 x Cisco Express 500 24 port POE switches with a 2 gig etherchannel to the CE500 12 port gig switch

UC500 - 48 user model connnected to CE500 12 port gig switch, PRI line (11 lines active) Running latest IOS

We only use the UC500 for voice so it is only on the voice vlan.

We currently have 8 staff members that currently work from home.

At there home we have 851w or 871w ISR routers

They have 7975 IP phones at home that connect to UC500 at the main site.

Home routers are single vlan with an easy vpn to the ASA firewall.


For the last two months / three months we have been experiencing one way audio problems on internal to internal calls. It happends when someone from the Main hub site calls a person at home as well as when someone at home calls into the Main hub site.  Problem does not seam to happen for outgoing or incoming calls on the PRI - PSTN to home offices or local Hub calls.

The work around is when the user experiences this problem if they put the caller on HOLD and then pick up the call again then

They get two way audio.  This has been happening for some time now and I have been un able to resolve.  The problem has been a bit sporatic so it has been hard to replicate when I have cisco tac on the phone.  Some people say it is only for their first call of the day, others say it happends randomly throught the day. I was wondering if anyone else is experiencing this type of behaviour?  I was running 8.0.4 code on the ASA at the main office but we where have VPN problems where for some reason we would have duplicate SAs and the VPN tunnel would go down.  Cisco tac said there is a bug in 8.0.4 code and to move to 8.0.5.  I am not sure if this was relavent information but I thought I would include it.

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Jeff Dorn Thu, 08/18/2011 - 09:12
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Had the same issue and it turned out that the EZVPN was not creating a Security Association to the network that the voice gateway was located.  At the beginning of the day, or after some inactivity, the sa would time out to our voice network.  On the 871, you can check your sa's by issuing the following command:

show crypto ipsec sa

It should list an sa for each network it has an association with.  If the network your gateway is sitting on is not listed, than traffic is not flowing to the remote phone.  Or, with an internal to internal call, the traffic needs to flow directly between those two networks.  With EZVPN, Security Associations are initiated from the user side.

To resolve the issue, we implemented IP SLA's on the 871 to send an icmp packet to the gateway, always keeping a sa with the voice network:

ip sla 10

icmp-echo source-interface Vlan1

ip sla schedule 10 life forever start-time now

Add multiple ip sla's if needed.  The source interface needs to be the interface your phone is on.  This will send a ping from the 871 to the voice gateway once every minute, ensuring that the sa stays up.  We've been using this method for a few months and haven't had this issue since.


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