This topic describes dial plan scalability issues.
Dial plans are difficult to implement in large Cisco Unified Communications deployments:
- Static configuration for multiple sites or domains is complex because of any-to-any call-routing requirements.
- Centralized H.323 gatekeepers or SIP network services offer dial plan simplification.
- Less configuration because of any-to-one call-routing topology
- Static configuration nevertheless
- No dynamic recognition of routes
- No automatic PSTN rerouting
- No built-in redundancy
- No optimal solution for large deployments.
- GDPR and CCD allow dynamic learning of dial plans.
The main scalability issue of large deployments is that each call-routing domain (for example, a Cisco Unified Communications Manager cluster or a Cisco Unified Communications Manager Express router) must be aware of how to get to all other domains.
Such a dial plan can become very large and complex, especially when multiple paths (for example, a backup path for TEHO) must be made available. Because each call routing domain must be aware of the complete dial plan, a static configuration does not scale. For example, any changes in the dial plan must be applied individually at each call-routing domain.
Centralized H.323 gatekeepers or SIP network services can be used to simplify the implementation of such dial plans, because there is no need to implement the complete dial plan at each call-routing domain. Instead of an any-to-any dial plan configuration, only the centralized component must be aware of where to find each number. This approach, however, means that you rely on a centralized service. If the individual call-routing entities have no connectivity to the centralized call-routing intelligence, calls will fail. Also, the configuration is still static. Any changes at one call-routing domain (for example, new PSTN prefixes because of changing the PSTN provider) must also be implemented at the central call-routing component.
In addition, these centralized call-routing services do not have built-in redundancy. Redundancy can be provided, but requires additional hardware, additional configuration, and so on. Redundancy is not an integrated part of the solution.
The ideal solution for a large deployment would allow an automatic recognition of routes. Internal as well as external (for PSTN backup) numbers should be advertised and learned by call-routing entities. A dynamic routing protocol for call-routing targets would address scalability issues in large deployments.