SRST Reference explanation?

Answered Question
Jun 17th, 2010
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I have a simple question about the SRST reference and it's relationships - just for my education.


Why does the SRST reference in CCM reference an IP on the remote gateway?  If my primary link goes down, why does this reference need to know/what does it use the IP for?  CCM and the gateway don't have communcation at this point, so why is there a reference to an IP address that it cannot reach?  How does this IP address configuration in SRST Reference come into play in the whole scheme of things?


I'm just trying to understand the inner-workings of SRST and the relationships associated.


Thanks!

Correct Answer by Jaime Valencia about 7 years 1 week ago

The config file for all phones has the CUCM group, the SRST reference is included there.

When any of the 3 servers you can configure does not reply to the keepalives it then tries to register with the default TFTP and then with the SRST reference (or default GW if no reference is configured).

Other than telling the phone to try to register to that SRST GW, there is no other use for it. The rest of the process is actually the SRST magic.


Once a GW with SRST configured receives a registration attempt, SRST kicks in.


HTH


java


If this helps, please rate


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Correct Answer by Md. Mijanur Rahman about 7 years 1 week ago

Hi, first try to understand the technology behind of SRST reference.

After configuring SRST reference (the remote gateway IP) you have to set it under Device Pool and reset the device pool. As a result all of the phones get reset and the SRST IP sets as the lowest prefered CM under phone settings. You can see the SRST IP at IP Phone settings (Settings>Device Configuration>Unified CM Configuration>Unified CM X SRST)  where X = Number of Callmanager server in your cluster+1, so if you have 2 CM on your cluster if will be set at 3rd position.

Now think your WAN is down, so phones can't communicate with your CMs and consider the SRST CM as the prefered CM and it tries to get register with the remote gateway with H.323 or SIP as you have configured SRST on your remote gateway.


I think it will make you clear about the concept.


HTH.

Correct Answer by Jaime Valencia about 7 years 1 week ago

It's not for CUCM to use.

It's for the phones that cannot reach CUCM BUT can reach the SRST GW IP.


CUCM NEVER communicates with the SRST GW. The phones are the ones interested on communicating with SRST.

Once the phone goes thru it's CCM group servers and none of them reply, it starts sending keepalives to SRST and then SRST kicks in.


HTH


java


If this helps, please rate


www.cisco.com/go/pdihelpdesk

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Correct Answer
Jaime Valencia Thu, 06/17/2010 - 07:14
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It's not for CUCM to use.

It's for the phones that cannot reach CUCM BUT can reach the SRST GW IP.


CUCM NEVER communicates with the SRST GW. The phones are the ones interested on communicating with SRST.

Once the phone goes thru it's CCM group servers and none of them reply, it starts sending keepalives to SRST and then SRST kicks in.


HTH


java


If this helps, please rate


www.cisco.com/go/pdihelpdesk

ajrussell Thu, 06/17/2010 - 07:18
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So I think I understand, but to expand on that...  You're saying that this is a configuration that is sent to each phone from Call Manager & not actually used during SRST in any way?  Simply a config that is sent out to each phone in the DP to tell them what to do in the event of an outage/need for SRST activation?

Correct Answer
Jaime Valencia Thu, 06/17/2010 - 07:25
User Badges:
  • Cisco Employee,
  • Hall of Fame,

    2011

The config file for all phones has the CUCM group, the SRST reference is included there.

When any of the 3 servers you can configure does not reply to the keepalives it then tries to register with the default TFTP and then with the SRST reference (or default GW if no reference is configured).

Other than telling the phone to try to register to that SRST GW, there is no other use for it. The rest of the process is actually the SRST magic.


Once a GW with SRST configured receives a registration attempt, SRST kicks in.


HTH


java


If this helps, please rate


www.cisco.com/go/pdihelpdesk

Correct Answer
Md. Mijanur Rahman Thu, 06/17/2010 - 07:18
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  • Silver, 250 points or more

Hi, first try to understand the technology behind of SRST reference.

After configuring SRST reference (the remote gateway IP) you have to set it under Device Pool and reset the device pool. As a result all of the phones get reset and the SRST IP sets as the lowest prefered CM under phone settings. You can see the SRST IP at IP Phone settings (Settings>Device Configuration>Unified CM Configuration>Unified CM X SRST)  where X = Number of Callmanager server in your cluster+1, so if you have 2 CM on your cluster if will be set at 3rd position.

Now think your WAN is down, so phones can't communicate with your CMs and consider the SRST CM as the prefered CM and it tries to get register with the remote gateway with H.323 or SIP as you have configured SRST on your remote gateway.


I think it will make you clear about the concept.


HTH.

ajrussell Thu, 06/17/2010 - 07:54
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I got it and all your answers were very helpful.  I'm on the verge of completely grasping

the intricacies of Call Manger and am looking to "fill in the gaps".  Thanks for your responses!

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