16.2 SAF and CCD Overview

This topic provides an overview of SAF and CCD.

Dial Plan Scalability Issues in Large Networks

  • Call-routing information between separate call-routing domains must be manually configured:
    1. Full-mesh configuration
      • Extremely complex; suitable only for small networks
    2. Hub-and-spoke configuration when centralized call-routing entities (SIP network services or H.323 gatekeepers) are used:
      • Scales better than full-mesh topologies
      • Requires redundant deployment of central services
  • Changes must be manually configured.
  • PSTN backup must be implemented independently at each call-routing domain.
  • ILS is one solution, but it does not support Cisco IOS products.

In large networks with several call agents—such as Cisco Unified Communications Manager Express, Cisco Unified Communications Manager, Cisco Unified Border Element, Cisco Unified SRST, and Cisco IOS gateways—the implementation and maintenance of dial plans can be complex.

The use of H.323 gatekeepers or SIP network services reduces the complexity. However, dial plan implementation still does not scale well in very large deployments.

The main scalability issues in large networks are caused by the fact that call-routing information must be configured separately at each call-routing domain.

Without centralized services (such as H.323 gatekeepers or SIP network services), a full-mesh configuration is required. In other words, each call control domain must be configured with call-routing information toward all other call-routing domains. This implementation model does not scale at all and therefore is suitable only for smaller deployments.

In a hub-and-spoke deployment model, call-routing information for each call-routing domain is configured only once at the centralized call-routing entity. This centralized call-routing entity can be a SIP network service or an H.323 gatekeeper. Such a solution scales better than full-mesh topologies; however, it introduces a single point of failure and therefore requires redundant deployment of the centralized service. In addition, the centralized call routing must still be manually configured. For example, if telephone number ranges or prefixes are changed at one of the call-routing domains, these changes must also be manually performed at the centralized call-routing service. Further, PSTN backup must be implemented independently at each call-routing domain.

In summary, there is no dynamic exchange of call-routing information between call-routing domains, and there is no automatic PSTN backup. ILS is one solution, but ILS works only between Cisco Unified Communications Manager clusters and does not support Cisco IOS Software products.

Scalable Dial Plan Solution for Large Networks

This section describes a scalable dial plan solution for large networks.

  • Solutions exist for dynamic exchange of routing information in Cisco IOS setting:
    1. Dynamic IP routing protocols:
      • Routers have local networks attached.
      • Routers advertise local networks to other routers.
      • All routers learn all available networks and how to get there.
  • Same concept can be used for call-routing information:
    1. Call-routing domains advertise telephone numbers or number ranges:
      • Internal numbers and IP address for VoIP
      • External numbers for PSTN backup
  • CCD was introduced with Cisco Unified Communications Manager Version 8.
    1. Call agents can advertise and learn call-routing information using the following:
      • Cisco Unified Communications Manager
      • Cisco Unified Communications Manager Express
      • Cisco Unified SRST
      • Cisco Unified Border Element
      • Cisco IOS gateway

The problem of dynamically distributing reachability information exists in areas other than call routing. In IP networks, for example, routing has changed from simple static routing to large, fully dynamic clouds, such as the Internet.

Dynamic routing protocols provide the solution for scalable IP routing. IP routers have local networks that are attached. IP routers advertise these locally known networks to other routers so that all routers can learn about all available networks and the path to get to those networks.

The same concept can be used to distribute call-routing information. Each call-routing domain advertises locally known telephone numbers or number ranges. Because local numbers are typically used by internal patterns (using VoIP) as well as via the PSTN, each call-routing domain advertises both the internally used numbers and the corresponding external PSTN numbers.

Cisco CCD, a feature that was introduced in Cisco Unified Communications Manager Version 8, provides exactly such a service. It allows Cisco Unified Border Element, Cisco Unified SRST, Cisco Unified Communications Manager, Cisco Unified Communications Manager Express, and Cisco IOS gateways to advertise and learn call-routing information in the form of internal directory numbers and PSTN numbers or prefixes.

CCD Overview

This section provides an overview of CCD.

  • CCD-enabled call agents advertise to and learn from “the network.”
  • SAF distributes information within the network.
  • SAF forwarders interact with CCD-enabled call agents (SAF clients):
    1. SAF forwarder learns information from SAF client.
    2. SAF forwarders exchange information with each other.
    3. SAF forwarder advertises all learned information to SAF client.

With CCD, each CCD-enabled call agent advertises locally found directory numbers or directory number ranges and their corresponding PSTN numbers or prefixes to the SAF-enabled network. In addition, each CCD-enabled call agent learns call-routing information from the network.

SAF is used to propagate information within the SAF-enabled network. SAF forwarders interact with CCD-enabled call agents (that is, SAF clients). A SAF forwarder learns information from a SAF client. SAF forwarders exchange learned call-routing information with each other so that the SAF-enabled network is aware of all learned call routes. SAF forwarders learn from SAF clients and then advertise all learned information to SAF clients. That way, all SAF clients are aware of all available call-routing information—internal directory numbers and their corresponding PSTN numbers.