Voice call fail over

[rohan_bhaktha]

Hi,

I have been having this question for a long time and have not got a proper answer for it from anywhere. Hoping to get it sorted out here :)

There is a standard site deployment with 2 CPE and dual last miles running as active backup with voice calls running on them.

What will happen if the primary link goes down, will all the active call seamlessly fail-over to the secondary link?

If the answer is YES, then please do answer the following:

what are the timers within which the switch over should happen.

Are these default setting or can be tweaked with

Where can I see or change these settings from.

Please let me know if any more clarification is required.

Thanks in advance.

[Ayodeji Okanlawon]

We need more details to answer these. What is the call processing platform you are using. You mentioned that voice calls is running on the CPE. Does that suggest you have CCME running on the routers?

[rohan_bhaktha]

The issue would be applicable for both, CCM or CME.

Scenario 1 using CCM.

Site A CPE is connected to centralized CCM over MPLS. Since there are 2 last miles, CPE has two paths to stay connected to the CCM and to remote phone at site B.

Once the call is connected RTP is flowing directly between site A & B. if the primary last mile of Site A fails will the active call be intact while it switches over to secondary link.

Scenario 2 using CME.

The same setup, 2 last miles connected to MPLS. Site A calls Site B signaling and RTP flows directly between 2 sites, primary link fails what happens?

@ Denis You are correct RTP doesnot know of any alternate paths but the communication/ reachability happens by the "contact" parameter in SDP if the IP is reachable from an different path RTP should be flowing seamlessly. 

This is my understanding because there are always issues with the service providers backbone and path switching keep happening, if calls would drop because of this then there would have been a lot of call drops.

There is also a factor of BFD which will switch the traffic from Pri to Sec within 5-10ms which is very fast so I feel the calls should not be drop in such scenarios. There should be some RTP timers which will hold the call before it decides to terminate the connection.

Thanks

Rohan

[Dennis Mink]

RTP has no mechanism to discover alternative paths to the remote end. Neither does any call signalling protocol once the call is already established.  so no, your call will not gracefully fail over and follow the redudant link.  The call will need to be re-established to get it flowing again.

cheers

[Tagir Temirgaliyev]

if 2 last mile configured as L2 etherchanel so if one link fail all rtp will go throw second. and all the active call will fail-over to the secondary link.

also possible L3 redundancy based on some routing protocol.

[rohan_bhaktha]

Thanks for the response Tagir but these are 2 independent LM also am very much interested in knowing what are the timelines that we are dealing with within which we have to make sure the failover should happen.

As mentioned below we can use BFD for WAN convergence which will happen within 5-10ms but still there is a mystery of will it work and what are the threshold.....

[Ayodeji Okanlawon]

RTP would normally failover without issues if routing convergence is fast.
Traffic including RTP would failover to second circuit. by default BGP takes longer to converge 3min sometimes. If BGP timers are tuned and BFD is enabled on the circuits it would be seamless and signaling and RTP will continue to flow.

[rohan_bhaktha]

Thanks Ayodeji but I am really looking forward to know the parameters and the thresholds on CME, CCM or SIP/SDP that control the failover mechanism.

[Ayodeji Okanlawon]

Rohan,

If the gateway is involved in media termination I dont see how RTP can fail over. The information I provided only assume that the gateway is used for transport purposes. Hence all the failover will be dependent on the WAN routing protocol

[Tagir Temirgaliyev]

 RTP can fail over if loopback interface is source

[Ayodeji Okanlawon]

Tagir,

Thanks for the comment however you need to educate us on how how that will work. Suppose that CUCM informs IP phone of where to send its media. If the media is the loop of gateway 1..now the wan link fails while call is ongoing..How will the media be redirected to gateway 2. How will the IP phone know the new destination to send its media to

[Tagir Temirgaliyev]

I mean CUBE and it is CE router

and if CUBE has several wan links to provider

It looks like without a clear scheme (drawing), we are talking about different things each