Content

| 7.6 |  | Network Design Issues for Voice |
| 7.6.5 |  | High availability |

The distributed architecture of a Cisco IP telephony solution provides the inherent availability that is a prerequisite for voice networking. Cisco IP telephony solutions are also inherently scalable, allowing seamless provisioning of additional capacity for infrastructure, services, and applications.

In the world of converged networking, in contrast to the world of the PBX, availability is designed into a distributed system rather than into a box. Redundancy is available in the individual hardware components for services such as power and supervisor modules. Network redundancy, however, is achieved with a combination of hardware, software, and intelligent network design practices.

Network redundancy is achieved at many levels. Physical connections exist from the edge devices where IP phones and computers are attached to two spatially diverse aggregation devices. In the event that an aggregation device fails, or connectivity is lost for any reason (such as a broken fiber or a power outage), failover of traffic to the other device is possible. By provisioning clusters of Cisco CallManagers to provide resilient call control, other servers can pick up the load if any device within the cluster fails.

Advanced Layer 3 protocols such as Hot Standby Router Protocol (HSRP) or fast converging routing protocols, such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP), can be used to provide optimum network layer convergence around failures.

Advanced tools are also available for the MAC layer (Layer 2). Tunable spanning tree parameters and the new 802.1s standards allow fast convergence. Value-added features such as uplink-fast and backbone-fast allow intelligently designed networks to further optimize network convergence.

High availability of the underlying network plays a major role in ensuring a successful deployment. This translates into redundancy, resiliency, and fast convergence.