SIP SRST Related Configuration

Survivable Remote Site Telephony (SRST) is a feature we can configure on Cisco ISR routers which will act as Call Managers when phones cannot reach CUCM. Mostly you will see this at WAN sites but for the sake of learning the technology, I am configuring it with in a LAN segment.

Following is a real world topology.

Following is the lab I am setting up..

Goal is to register the IP phones at Router when connection to CUCM is gone.
First let's look at the license capability of the Router. I am using a Cisco 2911.

Router#show license detail cme-srst

If the output looks like above, it's good to go.

(1) Create a SRST reference on CUCM

Go to CM Administration > System > SRST and Add New

IP address 192.168.137.100 is the IP address of the SRST router.

(2) Give the SRST Reference on Device Pool

Go to CM Administration > System > Device Pool and select SRST reference.

(3) Enable SIP Registrar Functionality on Router

Router(config)#voice service voip
Router(conf-voi-serv)#allow-connections sip to sip
Router(conf-voi-serv)#sip
Router(conf-serv-sip)#registrar server expires max 600 min 60
Router(conf-serv-sip)#bind all source-interface g0/0

(4) Globally Assign Characteristics to SIP Phones

Router(config)#voice register global
Router(config-register-global)#mode esrst
Router(config-register-global)#system message "SIP-SRST is Active"
Router(config-register-global)#max-dn 200
Router(config-register-global)#max-pool 20

"SIP-SRST is Active" will be displayed on screen of the IP Phones when it is registered at router.

(5) Configure SIP SRST Services for IP Phones

Router(config)#voice register pool 1
Router(config-register-pool)#id network 192.167.137.0 mask /24
Router(config-register-pool)#codec g711ulaw
Router(config-register-pool)#dtmf-relay rtp-nte cisco-rtp sip-notify

Above commands will be enough to register IP Phones at router when CUCM is down.

Router#show sip-ua status registrar will show the current registered phones.

If the router is configured correctly as an Gateway (with dial-peers etc) calls will be routed out.