Testing SRST

Hello Everyone,

Recently I have received many questions on how to test SRST. There are multiple options that are available, with varying consequences to the rest of the topology. We'll go over a few of them below:

If you have phones that are directly connected to the router performing SRST BR1 (R2) or at least connected on the same LAN segment, the process is easier, and simply requires blocking WAN connectivity to the CUCM cluster. You could do this by either shutting down the WAN interface, or by creating a null route for traffic to the cluster (ip route null 0) This route would send all traffic destined for the /24 subnet to Null or in other words nowhere.

Now, if you have phones that are connected through a VPN connection that goes through the PSTN router, the data path does not go through R2, and therefore shutting down the WAN interface will not force the phones to drop registration from CUCM and re-home to the SRST router.  You could then block traffic at HQ (R1), but all communication with Phones registered to CUCM, and Voicemail/Presence/CUCCX would then be broken. Instead, you can block packets through the PSTN router. Use the same ip route statement from above, and apply it to the PSTN router. Now the phones will register to BR1 (R2), and you can perform testing, including making calls back to the HQ site.

Don't forget to remove your IP route after performing testing! Otherwise, many other parts of your lab will not function! Perform a ping to the CUCM, to ensure you have removed the route, or brought up your relevant interfaces.

Hope this helps


  • Hi Josh,


    It seems to me that "ip route null 0" applying on the PSTN router blocks all traffic towards, including all traffic towards PUB and SUB.

    As I use only softphones for my labs, when I want to test BR1 SRST while applying this route on PSTN router, traffic from my HQ IP Phone is also blocked as it passes through PSTN router before to reach CUCM cluster...

    What would be the best solution in my case ? Please note that all my soft phones are running in virtual machines and are seen with the same IP address in CUCM...


    Best regards,


  • Hi SB,


    Due to the fact that your phones are all coming from the same IP, it will be difficult to perform testing with phones registered to both the CUCM and the router running SRST. You can at least test the phones functionality in SRST though.

    In addition, you can test to ensure general call routing through normal operation without the route for the CUCM phones. You can also test 4 digit dialing to the phones, by setting up a call forward unregistered destination under the DN settings, with the long distance prefix attached, and then shutting down one of the phones, and checking the isdn debugs on the routers to ensure the call is processed.


    Hope this helps

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