Thanks for posting! The SIP proxy fail-over or "survivability" feature is triggered by the unit seeing three SIP Invites with no response. When no response is seen from the SIP server, the unit looks for locally configured trunks to which the call can be routed. For 911 calls, we sometimes use the following command to make sure that calls are routed by the local switchboard immediately, without being sent out to the SIP server.
sip proxy emergency-call-routing accept 911
This would be used when a local TDM trunk should be used for the call. You may also want to read about the SIP Proxy Monitor feature in the following linked guide, Configuring SIP Proxy in AOS. It works only in Stateful mode, but uses a SIP Options message to determine if the a primary SIP server is unreachable and a backup SIP server should be used.
Regarding your question about "high availability", this means different things to different people. What protocols are you hoping to implement? We would likely need to see a network diagram to see exactly what types of scenarios you are trying to mitigate. We do have a guide which goes over VRRP, Configuring IPv4 VRRPv2 in AOS,however this might not be the best solution in this case, since the switch and router are built together. Failover based on default router failure is likely to require switching phones to a new router/switch. This might be something best discussed over the phone, so feel free to call Technical Support at 888-423-8726 to open up a support ticket.
Thanks so much. That was very helpful. The instance I am using HA is
with 908e's now. But it's good to know the 6355 will do that too, same OS
anyway. We're really just using those as a gateway device and have been
replaced with the 908s. I'll take the 911 command below to our lab for
Sales Engineer IV
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