Showing results for 
Search instead for 
Did you mean: 

Migrating SRST to ESRST for ISR 4k: Steps necessary?

Rising star
Rising star

Hi everyone,


Assuming you have SIP SRST in place as a backup to your CUCM 11.x cluster, what are the steps necessary to migrate from SRST to ESRST?


From what I can tell in configuration guides, it's just a matter of changing the mode to esrst under "voice register global".  However, I'm skeptical it's that easy.


1) What of configurations on the SIP trunks to the SRST, or within CUCM itself?

2) How would the admin select which directory numbers are being automatically synced between CUCM and the ESRST? Is it automatically infered based on SRST reference within the device pool?

3) Is the SRST licensing on the ISR 4k enough, or some other licensing/module necessary?

4) Are there any step by step guides or videos which show how to configure ESRST across the board?



1 Accepted Solution

Accepted Solutions



Yes, the points which you have mentioned is correct.


- Rohan


View solution in original post

9 Replies 9

VIP Alumni
VIP Alumni

Below guide will help


You might need additional hardware for UMG Gateway.


*** Please rate helpful post; Mark "Accept as a Solution" if applicable




The datasheet doesn't answer any of the questions I'm afraid. I'm aware of the benefits of E-SRST, that's the very motivation to migrate.

1. No additional configurations required on SIP trunk or on CUCM.
2. The selection of the phones is based on the Device Pool they are associated with.
4. The below link might help you,

- Rohan

Thanks for the reply,


I've read over the Admin Guide, but it was lacking some information. 


1) As far as I can tell the previous demand for a UMG hardware module is no longer supported, so can I assume all configurations between CUCM cluster and SRSTs require only the existing SIP trunks?


2) I haven't seen a single document that explains which phones are being synchronized over the SIP trunk. If it's as you say and it's by Device Pool, that means that I need to add each SIP Trunk to the same Device Pool as the phones connected to the SRST? This means I'd need to size the ISR 4k according to the total amount of phones in the same Device Pool, and that I would not be able to communicate with phones outside the same Device Pool unless I do so via the SIP trunk. 


3) Any idea if an IM&P server is required to enable BLF? The "presence enable" command seems a bit ambiguous, since BLF's to the same CUCM cluster never required an IM&P server. 


Thanks alot!



1. configurations between CUCM cluster and SRSTs require only the existing SIP trunks - Yes, this is correct.
2. The SRST reference is made only through the Device Pool configuration. The phones receive the SRST information from CUCM and they are not synchronized over SIP trunk.
You need to do the sizing of the CUBE based on the phones present at that particular site.
3. Can you please share more details about the requirement.

- Rohan



So basically:


1) Add "mode esrst" to the SRST CLI

2) All device pools with the SRST reference will automatically sync with that SRST so that DN's are mainained 

3) No need to touch anything with the existing SIP trunk to SRST

4) Remove any static registration pools within the SRST CLI since this is done automatically now via ESRST

5) Remove any cor lists which aren't used by dial-peers since CSS/PT configuration from CUCM is now being enforced by ESRST

6) Move any other registration pool level commands to the global level, such as codec classes


Would this be accurate?


The BLF question is just due to their being confusion on whether the "presence" CLI command requires a presence server as part of the CUCM cluster, mostly due to the synchronous nature of ESRST. I'd like all phones in the same device pools under ESRST to see whether or not a line is in use or not.



Yes, the points which you have mentioned is correct.


- Rohan


Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: