SRST Call Flow with respect to CME

[Rohit Khajuria]

Hi All

Thanks in advance !!!

I have a little confusion in a scenario where a Branch Site is connected with HQ Site with MPLS and then Branch Site is further connected with PBX.

So my confusion is, what if MPLS connectivity is lost between the HQ Site and Branch SIte, what would b the call flow then when SRST enabled ? How the callers from outside the cluster (PSTN) will connect to Branch Site routers with reference to both Incoming Calls and Outgoing Calls? will it go through CUCM or directly with CME site ?

Please find the pik attached for your reference.

[GaryB-UK]

Hi

If I understand your question and diagram correctly this is what you are asking.

Normally a call from the PSTN line (Branch site) will call a phone at the branch site that is configured to normally register with CUCM. You want to know what happens if the MPLS link between the 2 sites is dropped. This following is assuming that SRST is all setup and configured.

The phones are normally registered to CUCM and when the WAN link fails the phones will look for their SRST reference stated under their device pool configuration. This should be the IP address of the branch router. When the phones are unable to register to CUCM they "fallback" and attempt to register directly with the SRST router. Assuming they register with the SRST router then they will continue to have basic functionality for inbound and outbound calling.

The actual RTP streams should remain the same

PSTN - Gateway - Phone

The call signalling would alter

Normal operation

PSTN - Gateway - CUCM - Phone

Failure

PSTN - Gateway - Phone

Below is a document describing the SRST process in more detail.

http://www.cisco.com/c/en/us/products/collateral/unified-communications/unified-survivable-remote-site-telephony/data_sheet_c78-678873.html

Hope this helps

Gary

[Rohit Khajuria]

Thanks for your reply Gary.

I know the SRST Process but the only confusion is that, after the MPLS Connectivity loss, the phones get unregistered and then get registered with SRST. Now my point is what is the fun of CFUR configuration in this scenario if there is any ?

And will the call directly hit CME Router (SRST Enabled) and then will establish the call directly with the endpoint, without CUCM in the picture ?

[GaryB-UK · Accepted solution]

Hi

If you are talking about the CUCM CFUR you could put the external DDI of the branch phone into that field. So from a CUCM perspective the phone is unregistered (when the WAN is down) as it is not registered to the CUCM servers. So if an internal person at HQ, calls the extension of the unregistered phone the CUCM system will send the call out the HQ PSTN to the branch site and basically loop it back in again via the branch PSTN link.

As the phones are actually registered to the SRST router they will be able to answer and make PSTN calls without the need for CUCM as the router takes on basic call processing functionality during this outage period.

Gary

[Rohit Khajuria]

Ok.. so basically it means CFUR is needed only for Internal users and it has nothing to do with the PSTN users or external users who try to call the phones on Branch Site when WAN is down.

Thanks a lot for the information, it really helps me a lot.

Thanks again !!!