CM Fallback from time to time

Unanswered Question
Dec 4th, 2008

I have a customer whose remote site mentioned that the see the "CM Fallback" message on couple of phones at random. It appears that certain phones lose their keepalives while others are ok. This one remote site in particular has roughly 30 or so users. Any ideas why this is happening? Is it a QOS issue?

I have this problem too.
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morioka Thu, 12/11/2008 - 13:37

Not all of the phones go into SRST, only a few at random. The T1 link between the host and remote never goes down during this time. It may be because those particular phones are losing their keepalives.

allan.thomas Thu, 12/11/2008 - 14:03

Hi Michael, in this instance I would tend to agree. If not all phones are falling back to SRST then it is highly possible that the keepalives are being dropped or delay across the WAN.

Therefore I would suspect that QoS is the issue if there are no physical problems with the WAN or underlying LAN.

Firstly verify the switchports which these phones are connected to. Are there any reported errors on the ports? Duplex issues?

Also verify the integrity of the WAN interface. Is the reliability 255/255? are there CRCs, Input Errors.

Secondly is QoS provisioned end-to-end? It is possible that your signalling is not be trusted or being remarked. Do packets captures at either end to determine whether signalling packets still have the dscp value.

One other alternative if QoS is not an option would be to increase the keepalives, this would avoid issues with lossy low bandwidth WAN circuits. However this is clusterwide.

Hope this helps.



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