Device Availability Technologies

Most network designs have single points of failure, and the overall availability of the network might be dependent on the availability of a single device. A prime example of this is the access layer of a campus network. Most endpoint devices connect to the access switch through a single network interface card (NIC); this is referred to as being single-homed; therefore, access switches represent a single point of failure for all attached single-homed devices, including CTS codecs.

Note

Beginning with CTS 1.5 software, Cisco TelePresence Multipoint Switches can utilize a NIC teaming feature that can enable these to be multihomed devices, that is, devices that connect to multiple access switches. Multihoming eliminates the access switch from being a single-point of failure and thus improves overall availability.

Ensuring the availability of the network services is often dependent on the resiliency of the individual devices. Device resiliency, as with network resiliency, is achieved through a combination of the appropriate level of physical redundancy, device hardening, and supporting software features. Studies indicate that most common failures in campus networks are associated with Layer 1 failures, from components such as power supplies, fans, and fiber links. The use of diverse fiber paths with redundant links and linecards, combined with fully redundant power supplies and power circuits, are the most critical aspects of device resiliency. The use of redundant power supplies becomes even more critical in access switches with the introduction of Power over Ethernet (PoE) devices such as IP phones. Multiple devices are now dependent on the availability of the access switch and its capability to maintain the necessary level of power for all the attached end devices. After physical failures, the most common cause of device outage is often related to the failure of supervisor hardware or software. The network outages due to the loss or reset of a device due to supervisor failure can be addressed through the use of supervisor redundancy. Cisco Catalyst switches provides two mechanisms to achieve this additional level of redundancy:

• Cisco StackWise/StackWise-Plus
• Cisco Nonstop Forwarding (NSF) with Stateful Switchover (SSO)

Both of these mechanisms, discussed in the following sections, provide for a hot active backup for the switching fabric and control plane, thus ensuring that data forwarding and the network control plane seamlessly recover (with subsecond traffic loss, if any) during any form of software or supervisor hardware crash.