Survivable Remote Site Telephony (SRST)

        How do you design Unified Communications failover for your remote sites or your primary site with a single CUCM Publisher or CUCM Business Edition? Survivable Remote Site Telephony (SRST) is the answer! SRST is a Cisco IOS Software capability that acts as a backup call-processing agent in the event of a link (typically WAN) failure, providing users with a subset of capabilities. However, the choice is not that easy as there are multiple versions and configuration scenarios:

• SRST
• H.323
• MGCP
• SIP
• CME as SRST
• Enhanced SRST
• Dual SRST

        In order to decide on which one to select you have to decide on the foundation first. How many users are at each SRST site and what phone models are utilized? The number of users and phone types will help you decide which SRST to select, which router to utilize, and what code and licensing you'll need to deploy. 

Requirements:

• The SRST routers need sufficient PSTN paths (FXO, PRI, BRI, etc.) to support the remote site users
• Decide on a list of requirements for SRST
• Inbound and outbound PSTN connectivity to all phones
• Voicemail
• Call Forward No Answer or Busy
• Extension expansion to full 10-digit DIDs of other sites
• Music on Hold locally streamed from SRST router
• Conferencing

CUCM Configuration:

1. Go to the CM Administration page and select System > SRST
2. Click on Add New
3. Configure the Name as SiteCode_IPAddress (eg. NY_10.1.100.2)
4. Set the IP Address for both IP Address and SIP IP Address
5. Click Save
6. Go to System > Device Pool
7. Select the Remote Site Device Pool
8. Configure the corresponding SRST within the Device Pool
9. Click Save

 

VGW Configurations:

1. license accept end user agreement (if running IOS Version 15.x or higher)
2. license modify priority cme-srst high (if running IOS Version 15.x or higher)
3. clock timezone [zone] [hours-offset]
4. call-manager-fallback
5. ip source-address [ip address] [port]
6. max-dn [max-directory-numbers] [dual-line | octo-line] [preference] [number]
7. huntstop channel [1-8]
8. max-ephone [max-phones]
9. date-format [mm-dd-yy | dd-mm-yy | yy-dd-mm | yy-mm-dd]
10. time-format [12 | 24]
11. system message [primary | secondary] [custom message]
12. secondary-dialtone [digit-string]

Example

1. clock timezone PST -8
2. call-manager-fallback
3. ip source-address 10.1.100.2  2000
4. max-dn 300 octo-line
5. huntstop channel 6
6. max-ephone 100
7. date-format mm-dd-yy
8. time-format 12
9. system message primary CM Fallback
10. secondary-dialtone 9

Verifying that SRST is enabled:

1. Show running-config
2. show call-manager-fallback all
3. show ephone (while in fallback)
4. debug ephone [keepalive | register | state | detail | error | statistics | pak | raw]

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