CUCM pub/sub and SRST

[frsisqo14344]

Hi Everyone,

Is placing the CUCM pub or sub and SRST in the same network recommended? Are there any best practices for this configuration? basically, I want to place the publisher and the srst on the same network and location and the subscriber on another location. ANy help will be very much appreciated

[Chris Deren]

When you say on the same network, do you mean at the same location?

There is no need to have SRST at the head end location where the CUCM servers are.

HTH,

Chris

[Vipul Jindal]

SRST are always use for the site which dont have call manager server at there physical location!!

thanks,

Vipul Jindal

[David Hailey]

The reason that you don't need a CUCM and SRST at the same location is because the phones and the gateway at that location would always register to the local CUCM server (assuming it is running the CallManager service and is online).  SRST is generally used to provide a survivability option for locations where there is no local CUCM server.  If connectivity to the CUCM cluster is lost for whatever reason, then the local gateway will failover to SRST mode and the phones can register to it for basic telephony services during the outage.  When connectivity to CUCM is restored, the gateway and phones will re-register to the CUCM cluster.

Hailey

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[Aaron Harrison]

Hi

If you have only a single CUCM server, you may have an SRST router as a backup at the same site to provide some service during upgrades/failures.

If you have more than one CUCM in the cluster (even if these are on different sites) you would typically not deploy SRST as you are unlikely to lose both CUCM servers, or the local CUCM server and the WAN at the site time.

Regards

Aaron Harrison

Principal Engineer at Logicalis UK

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[David Hailey]

Good clarification point on that.

Hailey

[frsisqo14344]

Hi the reason I ask is because we had an issue where we both lost our WAN links connecting to the pub/sub. When this happened, it took awhile for the ip phone to re-register to the SRST. Another issue is losing the WAN links in split seconds wherein the phones were registering to SRST but since the pub/sub are up they started reregistering to pub. As we use the phones as a customer service line, during the outage we lost calls.

@ Chris, same location and on the same subnet . Are there any security issues if both srst and pub or sub on the same network?

[Aaron Harrison · Accepted solution]

Hi

The point is that if you have a sub at your remote location, then you are unlikely to benefit from SRST. If the WAN fails, you have a local sub, and if the local sub fails, you more than likely have a WAN connecting you to the publisher.

There are no additional security issues caused by having SRST and any type of CUCM on the same network (leaving aside any security concerns you might have with placing any type of servers at your remote sites).

The phones do not always register back/forth as quickly as you might like - they will retry each CUCM in their list, typically 30 seconds each before trying the SRST router.

Similarly they fail back quite slowly... have a look at this document:

http://www.cisco.com/en/US/docs/voice_ip_comm/uc_system/UC5.0.2/ipt_system_arch/UC05_FFR.html#wp1099291

The bottom line is that network stability is very important; even if you have a local subscriber, you will still experience issues if the network is unstable as the callmanager servers will see each other as 'out of service' when the WAN is flapping.

Regards

Aaron Harrison

Principal Engineer at Logicalis UK

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[frsisqo14344]

Thanks Aaron.

"The bottom line is that network stability is very important; even if you  have a local subscriber, you will still experience issues if the  network is unstable as the callmanager servers will see each other as  'out of service' when the WAN is flapping"

This can only be true if some IP phones are registered on the callmanager servers that is across the WAN links. However if the phones are registered on the local callmanager server, the only issue will be losing connectivity to the phones registered on the callmanager located on another location. Is this correct?

Oh and thanks for the link

[Aaron Harrison]

Hi

Yes, that's it basically - the servers will log 'SDLLinkOOS' errors and effectively you lose connection between them.

The phones registered locally will appear stable at least! Unless they are on a call accross the WAN at the time...

Aaron

[frsisqo14344]

"The phones registered locally will appear stable at least! Unless they are on a call accross the WAN at the time..."

"Inter office calls" as we call it, is of low priority as it can always be rectified by dialing out of the srst once WAN connections are lost . Of course there will be cost involve e.g. national calls.

Thanks for the inputs