5.5 Cisco Unified Communications Manager Express in SRST Mode

This topic describes how Cisco Unified Communications Manager Express in SRST mode works.

Cisco Unified Communications Manager Express in SRST mode is used only as a backup for Cisco Unified Communications Manager. In a WAN failure, the following occurs:

  • Cisco Unified Communications Manager Express provides Cisco CallManager service to IP phones at the remote site.
  • Cisco Unified Communications Manager Express controls voice gateway functions of the Cisco IOS router at the remote site.

SRST fallback support using Cisco Unified Communications Manager Express is a feature that enables routers to provide call-processing 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 Communications Manager Express, provisioning of phones is automatic and most Cisco Unified Communications Manager Express features are available to the phones during periods of fallback. The benefit is that Cisco Unified Communications Manager users will gain access to more features during fallback without any additional licensing costs.

Advantages of Cisco Unified Communications Manager Express in SRST Mode

This section describes the advantages of using Cisco Unified Communications Manager Express in SRST mode when compared with standard SRST.

  • Provides many more features than standard SRST. All features of Cisco Unified Communications Manager Express can be used.
  • Allows a mix of preconfigured phones and directory numbers, and phones and directory numbers learned by SNAP.
    1. Only those phones and directory numbers that require individual configuration have to be preconfigured.
    2. Preconfiguration can be based on MAC address or on directory number.
    3. Phones that do not require individual settings do not have to be preconfigured.
    4. Additional settings can be applied to phone and directory number configuration learned by SNAP, using ephone and ephone-dn templates.

The main advantages are as follows:

  • Cisco Unified Communications Manager Express provides more features than standard SRST (in general).
  • Cisco Unified Communications Manager Express in SRST mode allows a mix of preconfigured phones and directory numbers for those phones, and directory numbers that require individual settings and phones that are not configured are learned by SNAP.
  • You can combine the advantages of Cisco Unified Communications Manager Express (more features) and standard SRST (dynamic provisioning of phone configuration during fallback, using SNAP) by preconfiguring the required individual settings in Cisco Unified Communications Manager Express.
  • Ephone configuration is based on MAC addresses; ephone-dn configuration is based on the directory number.
  • All phones or directory numbers that collectively require identical configuration that is not provided by SNAP do not have to be preconfigured, but the additional configuration can be applied by templates.

These advantages allow flexible configuration of any Cisco Unified Communications Manager Express features in a scalable way, because only those phones and directory numbers that require additional features (or individual settings) have to be manually preconfigured.

Phone Provisioning Options

The table compares the phone provisioning options of Cisco Unified Communications Manager Express in SRST mode.

Ephone and Ephone-dn Ephone and Automatic Assignment Ephone-dn and SRST Provisioning Complete SRST Provisioning
Cisco IOS Configuration ephone 1 mac-address… type 7960 button 1:6 ephone-dn 6 number 3001 ephone 1 mac-address… type 7960 (no button…) telephony-service auto assign… ephone-dn 6 number 3001 telephony-service srst mode auto-provision… telephony-service srst mode auto-provision…
Resulting Directory Number Configuration Existing Unified CME* configuration Unified CME* ephone-dn configuration referenced by automatic assignment Unified CME* ephone-dn configuration of matching phone directory number Phone directory number configuration plus Unified CME* SRST ephone-dn template (if used)
Ephone and Ephone-dn Ephone and Automatic Assignment Ephone-dn and SRST Provisioning Complete SRST Provisioning
Resulting Phone Configuration Existing Unified CME* configuration Existing Unified CME* configuration Unified CME* ephone template (if used) Unified CME* ephone template (if used)
Note

Unified CME = Cisco Unified Communications Manager Express

Unlike the standalone Cisco Unified Communications Manager Express, when you are configuring Cisco Unified Communications Manager Express in SRST mode, no phones have to be configured, because they can be learned by SNAP.

However, Cisco Unified Communications Manager Express in SRST mode allows any combination of these configurations:

  • Manually configured ephone with associated ephone-dn: In this case, the phone is completely configured. Both the ephone and an ephone-dn, which is associated with the ephone, exist. This configuration is used for phones that require additional configuration settings that cannot be learned from the phone via SNAP. These settings should be applied only to this phone (or to a few phones). An ephone template or ephone-dn template cannot be used, because these templates apply to all learned phones or directory numbers.
  • Manually configured ephone with no associated ephone-dn: This configuration is useful if specific phone configuration parameters are required (parameters that cannot be assigned from a template) but no specific directory number is required. If an ephone is preconfigured in Cisco Unified Communications Manager Express and it is not associated with a directory number, the directory number is not learned via SNAP. Therefore, the phone will not have a directory number unless you enable automatic assignment (which is equivalent to autoregistration in Cisco Unified Communications Manager).
    Note

    This combination is not very common because it combines the need for specific phone configuration parameters with the dynamic assignment of directory numbers.

  • Manually configured ephone-dn: An ephone-dn is not associated with an ephone. The reason to configure the ephone-dn but not the ephone is that only an individual ephone-dn configuration is required. Default settings or a single template can be used for the ephone (which is added once the phone is registered).
  • No manual configuration: In this case, the ephone-dn and the ephone are learned via SNAP. By configuring the appropriate templates, you can apply configuration settings that are not supported by SNAP to such newly added phones and directory numbers.

Phone Registration Process

This section describes the phone registration process for Cisco Unified Communications Manager Express in SRST mode.

When an IP phone registers to Cisco Unified Communications Manager Express in SRST mode during fallback, the following occurs:

  • The router searches for an existing ephone with the MAC address of the phone.
    1. If found, the ephone must be configured with a directory number (button command), or automatic assignment must be enabled.
    2. If not found, the router searches for an existing ephone-dn that matches the IP phone directory number (learned by SNAP).
      • If found, an ephone is added that refers to the matched ephone-dn.
      • If not found, an ephone is added that refers to a newly created ephone-dn autoconfigured by SNAP.
  • Whenever an ephone is added by Cisco Unified SRST, the SRST ephone template is applied.
  • Whenever an ephone-dn is added by Cisco Unified SRST, the SRST ephone-dn template is applied.

When a phone loses connectivity to the Cisco Unified Communications Manager, it registers to its configured SRST reference.

If that SRST reference is Cisco Unified Communications Manager Express in SRST mode, the Cisco Unified Communications Manager Express router first searches for an existing (preconfigured) ephone with the MAC address of the registering phone. If the router finds an ephone, the stored ephone configuration is used. No phone configuration settings that are provided by SNAP are applied, and no ephone template is applied. If the configured ephone is configured with one or more ephone-dns, the stored configuration is used for the ephone-dn or ephone-dns of the phone. Neither the information that is provided by SNAP nor the ephone template that is configured under the telephony-service configuration is applied. If the configured ephone is not configured with an ephone-dn, automatic assignment has to be enabled for the phone to become associated with an ephone-dn. SNAP is not an option in this case.

If no ephone is found for the MAC address of the registering phone, Cisco Unified Communications Manager Express adds the ephone (and applies the ephone template, if configured) by using SNAP. If the directory number exists, it is bound to the added phone; otherwise, the directory number is learned using SNAP. If configured, the ephone-dn template is applied.

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