Remote Site Telephony and Branch Redundancy Options
Date: Mar 5, 2016
Sometimes it is not feasible or cost-effective to have a full-blown Cisco Unified Communications Manager (CUCM) solution for remote sites or smaller office locations. To integrate these remote sites, small to medium-size businesses (SMBs), or small office/home office (SOHO) with enterprise networks, Cisco IOS Unified Communications (UC) gateways offers a wide variety of functionality, ranging from call control features, survivability features, and redundancy options for remote site infrastructure.
This chapter covers the fundamentals of Cisco remote site telephony options and survivable remote site telephony (SRST) and its variations.
Upon completing this chapter, you will be able to meet these objectives:
- Describe remote site telephony options
- Describe Cisco SRST
- Describe Cisco E-SRST
- Describe Cisco SIP SRST
- Describe MGCP fallback
- Describe CFUR
Cisco Unified Communications Manager Express
Cisco Unified Communications Manager Express (Cisco Unified CME) is the express version of Cisco’s enterprise call control solution. A Cisco Unified CME-based solution is ideal for small businesses or remote sites in an enterprise environment. Figure 5-1 illustrates a Cisco Unified CME-based telephony solution connecting two SOHO/SMB sites.
Figure 5-1 Cisco Unified CME-Based Telephony Solution
A Cisco Unified CME is capable of the following:
- Supporting Skinny Client Control Protocol (SCCP) and Session Initiation Protocol (SIP) phones
- Terminating public switched telephone network (PSTN) (traditional E1/T1) or SIP trunks
- Tight integration with CUCM and Unity Connection
- Integrating with Cisco Unity Express
- Offering features like Call-Park, Ad-Hoc, or Meet-Me conferences, and Transcoding
- Offering an interactive self-service solution: Basic Automatic Call Distribution (B-ACD)
Hence, Cisco Unified CME can successfully service a site ranging from 25 to 1500 users (or even more depending on the model of router and licenses chosen). As discussed later in this chapter, Cisco Unified CME supports both SIP and SCCP endpoints and enterprise-grade features during regular and survivability mode, including video endpoints, conferencing, music on hold (MOH), paging, call-park, and so on.
Cisco Business Edition
Cisco offers another solution for SMB and remote offices in the form of Cisco Business Edition. Cisco Business Edition (5000/6000/7000) is a feature-rich, enterprise-class collaboration solution aimed at midsize businesses, where the full-blown capability of CUCM and relevant enterprise applications would be much more than required. Cisco Business Edition can scale from a few hundred to about 1000 users and also offers contact center ability for the agents. Cisco Business Edition comes with preloaded images of core applications, such as the following:
- CUCM
- Instant messaging/presence with Jabber
- Cisco Unity Connection
- Unified Contact Center Express
- Unified Attendant Console
The applications and licenses depend on the model of Cisco Business Edition, and the size of users/agents it can support varies as per the underlying platform (for example, Cisco UCS server [B-Series/C-Series]). Figure 5-2 depicts the Cisco Business Edition Collaboration setup on a Cisco B-Series UCS server.
Figure 5-2 Cisco Business Edition: Collaboration Setup for a Remote Site
Survivable Remote Site Telephony
All Cisco endpoints register with CUCM by default unless there is another call control that can support the same or if an endpoint has specific call control requirements (for example, video endpoints that register with VCS). In the unlikely event of a WAN failure, the remote site endpoints need a backup call control. In such case, Cisco Unified survivable remote site telephony (SRST) can provide necessary call control features based on the Cisco IOS router, such as a Cisco Integrated Services Router (ISR) or Aggregation Services Router (ASR). Cisco Unified SRST (hereafter referred to as SRST) is an IOS gateway-based (license) feature that enables administrators to configure redundant call control for sites that do not have a local CUCM server. Cisco Unified SRST function can be co-resident with many other IOS features, including Cisco Unified Border Element (CUBE).
Cisco IOS gateways support two types of SRST:
- SRST (call manager fallback, traditional)
- Cisco Unified CME-based SRST or enhanced SRST (E-SRST)
The major difference between these is that traditional SRST offers basic telephony features and E-SRST provides an enhanced feature set for the endpoints. E-SRST builds on top of SRST by automatically synchronizing user, phone, hunt group, partitions, and so on from CUCM to Cisco Unified CME. Any change on CUCM is propagated to the branch sites without admin intervention. So when the WAN is down, the phone resembles the last changes performed on CUCM. When in E-SRST mode, Cisco Unified CME syncs automatically with CUCM to get updates as and when they happen on CUCM. Figure 5-3 depicts Cisco Unified SRST solution.
Figure 5-3 Cisco Unified SRST solution
The Figure 5-4 illustrates Cisco Unified CME-based SRST.
Figure 5-4 Cisco Unified CME SRST Solution
When the endpoints lose connectivity with CUCM (WAN failure or remote server is unavailable), SRST kicks in and allows the endpoints to register with the configured SRST reference—the IOS voice gateway (as defined in CUCM). In the case of traditional SRST, the router builds on the configured application service. The SRST gateway detects newly registered IP phones and queries these IP phones for their configuration, and then autoconfigures itself using the Simple Network Auto Provisioning (SNAP) technology. Only one instance of SRST is allowed per device pool in CUCM.
Cisco Unified SRST is mostly leveraged with Media Gateway Control Protocol (MGCP) fallback, as explained later in this chapter. Cisco Unified SRST supports the following features:
- Simple one-time configuration of basic SRST functions
- Customizable programmable line keys and button layout control
- Support for Forced Authorization Code (FAC)
- Normalized +E.164 support
In case the SRST reference is Cisco Unified CME at the remote site/branch office, the IOS router first searches for an existing configured ephone with the MAC address of the phone that tries to register. If an ephone is found, the stored configuration is used. The phone configuration settings provided by SNAP are not applied any ephone template. If no ephone is found for the MAC address of the registration phone, the router adds the ephone and applies the ephone template (similar to configuration using SNAP). Following are some E-SRST features that distinguish it from traditional SRST:
- Automatic provisioning of remote branch sites.
- E-SRST router is in sync with CUCM that pushes the updates to the branch routers. Automatic sync for moves, adds, and deletions from CUCM to router.
- GUI interface for provisioning, monitoring, reporting, and troubleshooting.
- On-demand information sync with CUCM.
Cisco Unified CME supports two modes in which it can be configured using E-SRST: Skinny Client Control Protocol (SCCP) and Session Initiation Protocol (SIP). This allows support for analog, SIP, and SCCP phones while in SRST mode.
SRST and E-SRST Configuration
As explained in the previous section, SRST and E-SRST are the two possible ways to provide call control redundancy for the remote sites. This section details the configuration for the these two scenarios.
Example 5-1 outlines SRST configuration.
Example 5-1 SRST Configuration
SRST-Router(config)# call-manager-fallback SRST-Router(config-cm-fallback)# ip source-address 10.76.108.78 port 2000 SRST-Router(config-cm-fallback)# max-ephones 10 SRST-Router(config-cm-fallback)# max-dn 100 SRST-Router(config-cm-fallback)# max-conferences 4 gain -6 SRST-Router(config-cm-fallback)# transfer-system full-consult SRST-Router(config-cm-fallback)# secondary-dialtone 9 SRST-Router(config-cm-fallback)# moh music-on-hold.au SRST-Router(config-cm-fallback)# time-format 24 SRST-Router(config-cm-fallback)# date-format dd-mm-yy SRST-Router(config-cm-fallback)# system message SRST Mode
Example 5-1 defines IOS commands that help configure SRST on Cisco IOS router. The key commands are as follows:
- ip source-address defines the address of a physical or virtual (loopback) interface on the router with which the phones should register while using the SRST process. This is defined along with a port (2000 for SCCP).
- max-ephones defines the total number of ephones, and max-dn defines the maximum number of directory numbers under call-manager-fallback for the phones at the remote site.
- secondary-dialtone gives the same dialing experience for plain switched telephone network (PSTN) calls to end users (as they would experience when phones are registered with CUCM). In Example 5-1, pressing 9 gives an outside dialtone (it can be set to 0 for Europe and other regions as required).
- moh (music on hold) command defines the filename that is used for music on hold. This in turn allows playing MOH when remote site is in SRST mode.
- time and date format set the right format for the phones.
- system message helps display a message (configurable by administrator) that can help make users aware that the endpoints are running in SRST mode.
Example 5-2 explains Cisco Unified CME based SCCP based E-SRST.
Example 5-2 SCCP E-SRST Configuration
CUCME-Router(config)# telephony-service CUCME-Router(config-telephony)# srst mode auto-provision none CUCME-Router(config-telephony)# srst dn line-mode dual CUCME-Router(config-telephony)# srst ephone template 1 CUCME-Router(config-telephony)# srst dn template 1 CUCME-Router(config-telephony)# max-ephone 20 CUCME-Router(config-telephony)# max-dn 40 CUCME-Router(config-telephony)# ip source-address 10.76.108.76 port 2000 CUCME-Router(config-telephony)# moh music-on-hold.au CUCME-Router(config-telephony)# max-conferences 4 gain -6 CUCME-Router(config-telephony)# secondary-dialtone 9 CUCME-Router(config-telephony)# system message SRST Mode ! CUCME-Router(config)# ephone-template 1 CUCME-Router(config-ephone-template)# keep-conference local-only ! CUCME-Router(config)# ephone-dn-template 1 CUCME-Router(config-ephone-template)# hold-alert 25 idle
Example 5-2 has a number of IOS commands that define the following tasks:
- telephony-service initiates Cisco Unified CME configuration.
- srst command before usual Cisco Unified CME commands defines Cisco Unified CME mode for E-SRST provisioning.
- The mode for ephone-dns is set to dual-line.
- ephone-template applies common ephone configuration for any newly configured phone on Cisco Unified CME in SRST mode.
- dn template refers to the template used for defining common properties for a DN for a new ephone configured.
Other options are similar to traditional SRST configuration, as discussed earlier in Example 5-1.
Example 5-3 describes the configuration for Cisco Unified CME based SIP SRST.
Example 5-3 SIP E-SRST Configuration
CUCME-Router(config)# voice register global CUCME-Router(config-register-global)# mode srst CUCME-Router(config-register-global)# source-address 10.76.108.76 port 5060 CUCME-Router(config-register-global)# max-dn 40 CUCME-Router(config-register-global)# max-pool 10 CUCME-Router(config- register-global)# system message SRST Mode
As shown in Example 5-3, the following commands help define various tasks:
- SIP configuration starts with change of mode from cme to srst.
- max-dn defines the maximum directory numbers (DNs) to be supported during SRST.
- max-pool defines the maximum number of SIP voice register pools.
SRST IOS Dial Plan
When the endpoints enter SRST/E-SRST, a dial plan is required such that in absence of CUCM the router can route calls to/from PSTN/ITSP (SIP provider). Example 5-4 shows a basic dial North America Numbering Plan (NANP) for the SRST gateway to accept incoming calls and enable users to dial emergency, local, national, and international numbers. The configuration also includes routing for incoming calls.
Example 5-4 SRST Router Dial Plan Configuration
SRST-Router(config)# dial-peer voice 100 pots SRST-Router(config-dial-peer)# description all incoming calls SRST-Router(config-dial-peer)# incoming called-number . SRST-Router(config-dial-peer)# direct-inward-dial SRST-Router(config-dial-peer)# port 0/1:23 SRST-Router(config-dial-peer)# forward-digits all ! SRST-Router(config)# dial-peer voice 10 pots SRST-Router(config-dial-peer)# description Local outgoing calls SRST-Router(config-dial-peer)# destination-pattern 9[2-9]...... SRST-Router(config-dial-peer)# port 0/1:23 SRST-Router(config-dial-peer)# forward-digits all ! SRST-Router(config)# dial-peer voice 20 pots SRST-Router(config-dial-peer)# description Long Distance outgoing calls SRST-Router(config-dial-peer)# destination-pattern 91[2-9]..[2-9]...... SRST-Router(config-dial-peer)# port 0/1:23 SRST-Router(config-dial-peer)# forward-digits all ! SRST-Router(config)# dial-peer voice 30 pots SRST-Router(config-dial-peer)# description International outgoing calls SRST-Router(config-dial-peer)# destination-pattern 9011T SRST-Router(config-dial-peer)# port 0/1:23 SRST-Router(config-dial-peer)# forward-digits all ! SRST-Router(config)# dial-peer voice 911 pots SRST-Router(config-dial-peer)# description Emergency calls to 911 SRST-Router(config-dial-peer)# destination-pattern 911 SRST-Router(config-dial-peer)# port 0/1:23 SRST-Router(config-dial-peer)# forward-digits all
The next section describes the CUCM side of configuration to support SRST.
CUCM SRST Configuration
CUCM SRST configuration is similar for both Cisco Unified SRST or Cisco Unified CME-based SRST. The IOS router must be defined as an SRST reference in CUCM. Then this SRST reference consecutively needs to be added to the appropriate device pool to enable the phones at remote sites to leverage SRST. To configure an SRST reference and add it to a device pool, follow these steps:
Step 1. Go to CUCM Administration GUI > System > SRST, click Add New, and enter the required details about the remote site gateway, as shown in Figure 5-5. Ensure that the SRST gateway’s hostname/IP address is defined and the correct protocol (SCCP/SIP) port is chosen. If the gateway is referenced by hostname, be sure that CUCM can resolve the name and has DNS servers configured.
Figure 5-5 Cisco SRST Reference Configuration
- Step 2. Browse to System > Device Pool and select a device pool for remote site phones. Let’s assume that you have created a device pool per remote site. Assign the appropriate SRST reference per device pool. Configure the SRST reference in the device pool and save the configuration.
At this time, the CUCM part of SRST reference configuration is complete and the remote site gateway or Cisco Unified CME needs to be configured (described in earlier sections of this chapter) for SRST or E-SRST, respectively.
Multicast Music on Hold in SRST
While in SRST, the IOS router can play an audio file from its flash as a multicast MOH to users in the remote sites (similar to multicast MOH explained in Chapter 6, “Cisco Collaboration Solution Bandwidth Management”). This feature has the following limitations:
- A single MOH source should be used for all phones at remote sites. Multicast MOH can only support one file.
Cisco Unified SRST multicast MOH supports only G.711, and MOH from an SRST router cannot be unicast. Multicast MOH can work in one of the following two ways:
- Nonfallback mode: This occurs when the WAN link is up and the phones are controlled by CUCM. It allows the phones to consult a local MOH file instead of reaching out to CUCM across the WAN. This option is covered in detail in Chapter 6.
- Fallback mode: This occurs when SRST is active; i.e., the remote site has lost connectivity to the central-site CUCM. In this case, the branch router can continue to provide multicast MOH.
For configuring multicast MOH, CUCM and voice gateway at remote sites must be configured to support multicast. See multicast configuration for supporting MOH in Chapter 6. Assuming that multicast routing in the router is enabled and PIM dense mode is configured for identified interfaces, both CUCM and the IOS router need to be configured to support multicast MOH. To configure CUCM for supporting multicast MOH, follow these steps:
Step 1. Go to CUCM Administration GUI > Media Resources > Music on Hold Audio Source and select the audio source you are enabling for multicast. Ensure that the Allow Multi-casting check box is checked, as shown in Figure 5-6.
Figure 5-6 Multicast MOH Audio Source
- Step 2. Go to Media Resources > Music on Hold Server Configuration. Enable multicast support and select the option between port number or IP address. Ensure that the MOH server has a G.711 only enabled device pool assigned to it.
Step 3. Go to Media Resources > Media Resource Group (MRG) and click Add New. Add a new MRG and ensure that this MRG has the multicast-enabled MOH server assigned to it and is multicast enabled, as shown in Figure 5-7.
Figure 5-7 Multicast-Enabled MRG
- Step 4. Assign the MRG to an MRGL by going to Media Resources > Media Resource Group List and assign the MRGL to a device pool.
Example 5-5 outlines the configuration of an IOS SRST router.
Example 5-5 SRST Router Multicast Configuration
SRST-Router(config)# ccm-manager music-on-hold ! SRST-Router(config)# interface loopback 1 SRST-Router(config-if)# ip address 10.86.108.82 255.255.255.255 ! SRST-Router(config)# call-manager-fallback SRST-Router(config-cm-fallback)# ip source-address 10.76.108.78 port 2000 SRST-Router(config-cm-fallback)# moh music-on-hold.au SRST-Router(config-cm-fallback)# multicast moh 239.1.1.1 port 16384 route 10.86.108.82 10.76.108.78
MGCP Fallback
MGCP fallback provides call control functionality using application alternate H.323 operation when a gateway is configured for MGCP in CUCM. An MGCP gateway registers with CUCM like SCCP/SIP phones. MGCP fallback enables a gateway to act as local call control when the CUCM server to which the remote site phones and gateway register is offline or WAN connectivity is lost (in which case, Cisco Unified SRST kicks in and offers call control functionality). This is specifically useful when a centralized call control using MGCP is desired for all voice gateways.
Figure 5-8 shows MGCP fallback (that is, MGCP application to default application H.323).
Figure 5-8 MGCP Fallback Operation
To configure MGCP fallback and enable default H.323 application-based SRST, perform the following steps:
- Step 1. Configure SRST reference in CUCM and assign it to the appropriate CUCM device pool (refer to the CUCM SRST Configuration section).
- Step 2. Configure the Call Forward Unregistered (CFUR) internal and external destinations on line of the remote site phones to an E.164 number (for example, a shared line on the main site or voicemail number). This is accomplished by going to Device > Phone, selecting a phone, selecting a line, and setting CFUR.
- Step 3. Configure the MGCP fallback and Cisco Unified SRST on the remote site gateways and implement the SRST dial plan on the remote site gateways. MGCP fallback configuration is shown in Example 5-6.
Example 5-6 MGCP Fallback Configuration
MGCP-Router(config)# ccm-manager fallback-mgcp MGCP-Router(config)# application MGCP-Router(config-app)# global MGCP-Router(config-app-global)# service alternate default
At this time, the IOS router is ready for fallback to H.323 application (defined as service alternate default in Example 5-6). Upon loss of connectivity (server/CallManager service is unavailable or WAN connection is down) to the CUCM server, the phones register with the gateway and continue to operate as normal (in SRST mode) until the link or CUCM service is restored.
Cisco Call Forward Unregistered
One of the problems with SRST is that the CUCM is unable to identify if a whole site has gone offline and that it may possibly be in SRST mode. In such a case, a user trying to call a phone at remote site gets either a busy signal or rolls to voicemail (or the call handling behavior depending on the configuration). How do you reach the remote site from the HQ or main site if the remote site is in SRST mode? The problem is solved by the Call Forwarding Unregistered (CFUR) feature of CUCM.
On each device line directory number (DN), CFUR is a call forwarding option that is engaged when a device is not registered. This is true in the following instances:
- The phone is unregistered, which implies the phone was registered earlier
- The phone is unknown, which implies the phone was never registered
- A user device profile is not logged in, which implies the user profile configured with CFUR for a physical phone or softphone that is not logged into extension mobility (and this is a segue to a later discussion of another potential use of CFUR).
When an active DN is unregistered, a call to this line is redirected based on the CFUR configuration. Figure 5-9 explains the CFUR process in conjunction with SRST scenario.
Figure 5-9 CFUR-Based Call Routing
The following steps explain the CFUR operation in conjunction with SRST:
- Step 1. The phone with DN 2003 goes off-hook and dials 1003 at the remote site. The CUCM dial plan comes to action associated with the digits dialed (digit analysis).
- Step 2. Because the remote site is in SRST mode, all phones at that site have the status of unregistered, which is one of the preliminary conditions for CFUR to begin.
- Step 3. The CUCM call processing node handling DN 2003’s call setup request passes the call decision process to the ForwardManager subprocess for a call routing decision. In turn, the ForwardManager subprocess determines that DN 1003 has a CFUR setting of 914087771003 (a valid E.164 number routable on the PSTN).
- Step 4. The call is routed over PSTN to the remote site, where the call is handled as a direct inward dialing (DID) call (assuming that the calling search space [CSS] configured for CFUR has valid public switched telephone network [PSTN] access). After digit manipulation by the remote site’s SRST/E-SRST router, a call setup is sent to DN 1003 that is registered with the SRST/E-SRST router as ephone, and the audio channel is established thereafter (following the normal call setup process).
Another potential use case for CFUR apart from use of CFUR in conjunction with SRST for availability is that CFUR can also be implemented when a user is leveraging a soft client. For example, if a mobile user has a laptop with a softphone (for instance, Cisco IP Communicator or Cisco Jabber) and the user shuts down the laptop or places it into standby/sleep mode, CFUR can forward calls destined for the unregistered softphone to the user’s cell phone. The user does not have to set up Call Forward All (CFA) manually before closing the softphone application. If the softphone is not registered, calls are forwarded to the user’s cell phone. This is another application of the CFUR feature that improves availability in CUCM deployments.
Summary
The following key points were discussed in this chapter:
- Cisco offers a host of solutions for remote sites/smaller sites so that they can connect to and leverage the services from the enterprise network.
- Keeping redundancy in mind, Cisco IOS gateways host the endpoints at the remote site and enable the remote site users to continue using enterprise-grade telephony and collaboration features.
- SRST (and E-SRST) allow the SCCP (and SIP) endpoints to register to local IOS gateways and enable calling within the remote site as well as outside using PSTN or SIP trunks.
References
For additional information, refer to these resources:
Cisco Systems, Inc. Cisco Collaboration Systems 10.x Solution Reference Network Designs (SRND), May 2014. http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab10/collab10.html
Cisco Systems, Inc. Cisco Collaboration Systems 11.x Solution Reference Network Designs (SRND), July 2015. http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab11/collab11.html
Review Questions
Use these questions to review what you have learned in this chapter. The answers appear in Appendix A, “Answers Appendix.”
Which of the following best describes Cisco Collaboration Solution’s extension for remote site telephony?
- Cisco Unified Communications Manager Express (Cisco Unified CME)-based call control
- Cisco Business Edition
- Cisco Unified Attendant Console
- a and b
- b and c
Which of the following are two most commonly used remote site survivability solutions? (Choose two.)
- Cisco Unified SRST
- Cisco Unified CME-based SRST
- Cisco Business Edition
- CUBE
What is the default fallback application in Cisco IOS routers?
- SIP
- H.323
- ISDN
- SCCP
True or false: Multiple multicast MOH sources can be used in SRST mode.
- True
- False
Cisco Unified CME can support which three type of endpoints? (Choose three.)
- Analog
- Digital
- SCCP
- SIP
- MGCP
How many instances of SRST can be defined in a device pool on CUCM?
- 1
- 2
- 3
- As many as configured in CUCM
Which two commands are used for SRST configuration at the Cisco IOS router?
- telephony-server
- ccm-manager fallback
- service alternate default
- call-manager-fallback
Which of the following five commands are configured in telephony service E-SRST configuration mode? (Choose five.)
- create cnf-files
- ephone
- srst ephone-dn
- max-ephones
- max-dn
- ip source-address
- moh
- number