Best Practice for SRST deployment at a remote site

Answered Question

What is the best practice for a SRST deployment at a remote site? Should a separate router such as a 3800 series be deployed for telephony in addition to another router to be deployed for Data? Is there a need for 2 different devices?

Correct Answer by Rob Huffman about 7 years 10 months ago

Hi Brian,


I can't think of any! Our SRST deployments only use one 2800 series ISR.


Cheers!

Rob

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Rob Huffman Tue, 08/04/2009 - 08:02
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Hi Brian,


This is typically done all on one ISR Router at the remote site :)There are two flavors of SRST. Here is the feature comparison;



SRST Fallback


This feature enables routers to provide call-handling support for Cisco Unified IP phones if they lose connection to remote primary, secondary, or tertiary Cisco Unified Communications Manager installations or if the WAN connection is down. When Cisco Unified SRST functionality is provided by Cisco Unified CME, provisioning of phones is automatic and most Cisco Unified CME features are available to the phones during periods of fallback, including hunt-groups, call park and access to Cisco Unity voice messaging services using SCCP protocol. The benefit is that Cisco Unified Communications Manager users will gain access to more features during fallback ****without any additional licensing costs.



Comparison of Cisco Unified SRST and

Cisco Unified CME in SRST Fallback Mode



Cisco Unified CME in SRST Fallback Mode


• First supported with Cisco Unified CME 4.0: Cisco IOS Software 12.4(9)T

• IP phones re-home to Cisco Unified CME if Cisco Unified Communications Manager fails. CME in SRST allows IP phones to access some advanced Cisco Unified CME telephony features not supported in traditional SRST

• Support for up to 240 phones

• No support for Cisco VG248 48-Port Analog Phone Gateway registration during fallback

• Lack of support for alias command

• Support for Cisco Unity® unified messaging at remote sites (Distributed Exchange or Domino)

• Support for features such as Pickup Groups, Hunt Groups, Basic Automatic Call Distributor (BACD), Call Park, softkey templates, and paging

• Support for Cisco IP Communicator 2.0 with Cisco Unified Video Advantage 2.0 on same computer

• No support for secure voice in SRST mode

• More complex configuration required

• Support for digital signal processor (DSP)-based hardware conferencing

• E-911 support with per-phone emergency response location (ERL) assignment for IP phones (Cisco Unified CME 4.1 only)



Cisco Unified SRST


• Supported since Cisco Unified SRST 2.0 with Cisco IOS Software 12.2(8)T5

• IP phones re-home to SRST router if Cisco Unified Communications Manager fails. SRST allows IP phones to have basic telephony features

• Support for up to 720 phones

• Support for Cisco VG248 registration during fallback

• Support for alias command

• Lack of support for features such as Pickup Groups, Hunt Groups, Call Park, and BACD

• No support for Cisco IP Communicator 2.0 with Cisco Unified Video Advantage 2.0

• Support for secure voice during SRST fallback

• Simple, one-time configuration for SRST fallback service

• No per-phone emergency response location (ERL) assignment for SCCP Phones (E911 is a new feature supported in SRST 4.1)



http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/prod_qas0900aecd8028d113.html




These SRST hardware based restrictions are very similar to the number of supported phones with CME. Here is the actual breakdown;



Cisco 880 SRST Series Integrated Services Router

Up to 4 phones



Cisco 1861 Integrated Services Router

Up to 8 phones



Cisco 2801 Integrated Services Router

Up to 25 phones



Cisco 2811 Integrated Services Router

Up to 35 phones



Cisco 2821 Integrated Services Router

Up to 50 phones



Cisco 2851 Integrated Services Router

Up to 100 phones



Cisco 3825 Integrated Services Router

Up to 350 phones



Cisco Catalyst® 6500 Series Communications Media Module (CMM)

Up to 480 phones



Cisco 3845 Integrated Services Router

Up to 730 phones




*The number of phones supported by SRST have been changed to multiples of 5 starting with Cisco IOS Software Release 12.4(15)T3.


From this excellent doc;


http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/vcallcon/ps2169/data_sheet_c78-485221.html



Hope this helps!

Rob


Correct Answer
Rob Huffman Tue, 08/04/2009 - 11:52
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Hi Brian,


I can't think of any! Our SRST deployments only use one 2800 series ISR.


Cheers!

Rob

stephen.himebau... Wed, 09/02/2009 - 12:06
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TO: Brian

FR: Steve


We have separate devices for networks & voice. I go so far as to not even use the term 'router' for an ISR voice gateway. There are 'routers' and there are 'voice gateways'.


We don't want network functions to interfere with voice functions.


Imagine a situation where your IOS has stable voice functions, but there's a bug in the networks functions. If you upgrade the IOS, you may affect voice functions.


There is also the situation where the network traffic/load can affect voice call processing. High volume of network traffic could affect call processing. And vice versa.


The trade-off is cost. The all-in-one setup is less expensive.

Tim Walker Tue, 08/04/2009 - 20:31
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CME as SRST also supports fallback for Unity Connection as well as Unity Enterprise.

Aaron Harrison Wed, 08/05/2009 - 04:14
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Hi All


The reason to use two routers is for fault tolerance.


Imagine you have a single router - the power supply unit dies, and the router shuts off.


You have lost both your link to the WAN for Callmanager call control, and you hve lost your SRST router. So you have no data connectivity, no CallManager telephony control, and no SRST router.


If you have a seperate SRST/telephony and WAN/Data router, with your local telephony circuits connected to the SRST router, you have proper fault tolerance:


- If you lose the WAN link, you have SRST fallback


- If you lose the Data router to a hardware fault, you have SRST available with local telephony circuits so you can still use the phones.


- If you lose the SRST router to a hardware fault, you lose your local telephony circuits, but you still have Callmanager call control via the WAN/Data router. You can therefore still use the phones, and potentially can send/receive calls over the WAN via an alternate gateway.


Hope this helps


Aaron

Rob Huffman Wed, 08/05/2009 - 04:43
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Hey Aaron,


Excellent points my friend! I have seen references to CME router redundancy but had never really thought of it in a SRST context. +5 points sir!


Cheers!

Rob



PS: here is the reference I was referring to :)


Here is a clip from the CME SRND Guide;


Redundant Cisco Unified CME Router


A second Cisco Unified CME router can be configured to provide call-control services if the primary Cisco Unified CME router fails. The secondary Cisco Unified CME router takes over and provides services seamlessly until the primary router becomes operational again.


When a phone registers to the primary router, it receives a configuration file from the primary router. Along with other information, the configuration file contains the IP addresses of the primary and secondary Cisco Unified CME routers. The phone uses these addresses to initiate a keepalive (KA) message to each router. The phone sends a KA message after every KA interval (30 seconds by default) to the router with which it is registered and after every two KA intervals (60 seconds by default) to the other router. The KA interval can be adjusted with the keepalive command.


If the primary router fails, a phone will not receive an acknowledgment (ACK) to its KA message to the primary router. If the phone does not get an ACK from the primary router for three consecutive KAs, it registers with the secondary Cisco Unified CME router.


During the time that the phone is registered to the secondary router, it keeps sending a KA probe to the primary router to see if it has come back up, now every 60 seconds by default or two times the normal KA interval. After the primary Cisco Unified CME router is operating normally, the phone starts receiving ACKs for its probes. After the phone receives ACKs from the primary router for three consecutive probes, it switches back to the primary router and reregisters with it. The reregistration of phones with the primary router is also called rehoming.


The physical setup for redundant Cisco Unified CME routers is as follows. The FXO line from the PSTN is split using a splitter. From the splitter, one line goes to the primary Cisco Unified CME router and the other goes to the secondary Cisco Unified CME router. When a call comes in on the FXO line, it is presented to both the primary and secondary Cisco Unified CME routers. The primary router is configured by default to answer the call immediately. The secondary Cisco Unified CME router is configured to answer the call after three rings using the voice-port ring number 3 command. If the primary router is operational, it answers the call immediately and changes the call state so that the secondary router does not try to answer it. If the primary router is unavailable and does not answer the call, the secondary router sees the new call coming in and answers after three rings.


The secondary Cisco Unified CME router should be connected in some way on the LAN, either through the same switch or through another switch that may or may not be connected to the primary Cisco Unified CME router directly. As long as both routers and the phones are connected on the LAN with the appropriate configurations in place, the phones can register to whichever router is active.


Configure primary and secondary Cisco Unified CME routers identically, with the exception that the FXO voice port from the PSTN on the secondary router should be configured to answer after more rings than the primary router, as previously explained. The ip source-address command is used on both routers to specify the IP addresses of the primary and secondary routers.


http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmesystm.html#wp1024559



Keep Rockin' Aaron!

Rob

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