Friday, November 30, 2012

CUCM SRST Redundancy

A little while back I was thinking about how you could add redundancy for your SRST gateways.  CUCM allows you to configure a primary, secondary, and tertiary CM along with a single SRST reference.     However, I thought somebody would appreciate having redundancy for their local SRST gateways.

The first idea for redundancy was to configure the SRST IP address reference with a virtual IP address.  You would then configure two routers with HSRP/GLBP/VRRP on the same LAN, setting the IP address to CME/SRST to the virtual IP address.  I did not test this method, but did a little Google searching on the matter.  Some people say it works, however there an open enhancement bug CSCtz96320 labeled ‘SRST failback not working as desired when HSRP is enabled.’  I am not aware of any documentation that says HRSP with SRST is a supported application.

The next idea would was to configure the phone with a a secondary IP address in the SRST reference.  When you look at the Unified CM Configuration on a Cisco IP phone you will notice that there is room for 5 Unified CMs.  If CUCM supports 3 CUCMs in a Call Manager group with one SRST reference, what is the use of the 5th field?

It seems like the phone was capable of using another SRST or Unified CM reference.  I dug into the Cisco Unified Communications Data Dictionary  and looked at the SRST table.  Ah ha! There was field for ipaddr2, ipaddr3, sipipaddr2, and sipipaddr3.

The remarks were nicely labeled ‘IP address for second/third device in the SRST config list.”
I already knew that the CM Administration webpage would only let you set one IP address so I would need to make changes straight to the database to test the functionality out.

Before I begin, use this configuration at your own risk, Cisco TAC probably won’t support this configuration.

I created an SRST reference SRST_SC:




To test the functionality of a secondary SRST reference, I configured the IP address to an unreachable IP address (1.1.1.1).

I then checked the configuration directly in the ccm database using the SQL query run sql select name,ipaddr1,port1,ipaddr2,port2 from srst where name like ‘SRST_SC’


 
Now the test; let’s try to populate the field ipaddr2 with a working SRST IP address.  I used the command ‘run sql update srst set ipaddr2=’177.1.254.3’ where name like ‘SRST_SC’.



Let’s check the ccm database again to see if the change took.



Now I reset my phone so that the changes took effect.  After the reset, I verified on the IP phone that I had two SRST references!



Now I turned down the WAN link to simulate SRST.  I suspected that the first SRST reference would be checked (1.1.1.1), it would be unreachable, and then the phone would finally register to my secondary SRST reference (172.1.254.3).
 
It worked! Here is an output from the router with the phone registered to the secondary SRST reference.


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