1-2
MTTR
MTTR is the average time required to repair a failed system. MTTR in a broad sense also involves
spare parts management and customer services.
MTTR = fault detection time + hardware replacement time + system initialization time + link recovery
time + routing time + forwarding recovery time. A smaller value of each item, a smaller MTTR and a
higher availability.
High Availability Technologies
As previously mentioned, increasing MTBF or decreasing MTTR can enhance the availability of a
network. The high availability technologies described in this section meet the level 3 high availability
requirements in the aspect of decreasing MTTR.
High availability technologies can be classified into fault detection technologies and protection
switchover technologies.
Fault Detection Technologies
Fault detection technologies enable detection and diagnosis of network faults. CFD, DLDP and
Ethernet OAM are data link layer fault detection technologies; NQA is used for diagnosis and
evaluation of network quality; Monitor Link and Track work along with other high availability
technologies to detect faults through a collaboration mechanism. See
for the details of
these technologies.
Table 1-2
Fault detection technologies
Technology
Introduction
Reference
CFD
Connectivity Fault Detection (CFD), which conforms to IEEE
802.1ag Connectivity Fault Management (CFM) and ITU-T Y.1731,
is an end-to-end per-VLAN link layer Operations, Administration and
Maintenance (OAM) mechanism used for link connectivity detection,
fault verification, and fault location.
High Availability
Configuration
Guide/CFD
Configuration
DLDP
The Device link detection protocol (DLDP) deals with unidirectional
links that may occur in a network. On detecting a unidirectional link,
DLDP, as configured, can shut down the related port automatically
or prompt users to take actions to avoid network problems.
High Availability
Configuration
Guide/DLDP
Configuration
Ethernet OAM
As a tool monitoring Layer 2 link status, Ethernet OAM is mainly
used to address common link-related issues on the “last mile”. You
can monitor the status of the point-to-point link between two directly
connected devices by enabling Ethernet OAM on them.
High Availability
Configuration
Guide/Ethernet
OAM
Configuration
