14MTTRMTTR is the average time required to repair a failed system. MTTR in a broad sense also involvesspare parts management and customer services.MTTR = fault detection time + hardware replacement time + system initialization time + linkrecovery time + routing time + forwarding recovery time. A smaller value of each item means asmaller MTTR and a higher availability.High availability technologiesAs previously mentioned, increasing MTBF or decreasing MTTR can enhance the availability of anetwork. The high availability technologies described in this section meet the level 3 highavailability requirements in the aspect of decreasing MTTR.High availability technologies can be classified as fault detection technologies or protectionswitchover technologies.Fault detection technologiesFault detection technologies enable detection and diagnosis of network faults.• DLDP and Ethernet OAM are data link layer fault detection technologies.• BFD is a generic fault detection technology that can be used at any layer• NQA is used for diagnosis and evaluation of network quality• Monitor Link and Track work along with other high availability technologies to detect faultsthrough a collaboration mechanism.See Table 2 for the details of these technologies.Table 2 Fault detection technologiesTechnology Introduction ReferenceDLDPDLDP deals with unidirectional links that may occur in a network. Ondetecting a unidirectional link, DLDP, as configured, can shut down therelated port automatically or prompt users to take actions to avoidnetwork problems.High AvailabilityConfigurationGuide/DLDPConfigurationEthernet OAMAs a tool monitoring Layer 2 link status, Ethernet OAM is mainly usedto address common link-related issues on the “last mile”. You canmonitor the status of the point-to-point link between two directlyconnected devices by enabling Ethernet OAM on them.High AvailabilityConfigurationGuide/EthernetOAMConfigurationBFDBDF provides a single mechanism to quickly detect and monitor theconnectivity of links or IP forwarding in networks. To improve networkperformance, devices must quickly detect communication failures torestore communication through backup paths as soon as possible.High AvailabilityConfigurationGuide/BFDConfiguration
