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| Title | Size | Download |
|---|---|---|
| 01-High availability overview | 43.03 KB |
High availability overview
Because communication interruptions can seriously affect widely-deployed value-added services such as IPTV and video conference, basic network infrastructures must be able to provide high availability.
The following are the effective ways to improve availability:
· Increasing fault tolerance.
· Speeding up fault recovery.
· Reducing impact of faults on services.
Availability requirements
Table 1 describes a typical availability model that divides availability requirements into different levels.
Table 1 Availability requirements
|
Level |
Requirement |
Solution |
|
1 |
Decrease system software and hardware faults |
· Hardware—Simplified circuit design, enhanced production techniques, and reliability tests. · Software—Reliability design and test. |
|
2 |
Protect system functions from being affected if faults occur |
Device and link redundancy and switchover. |
|
3 |
Enable the system to recover as fast as possible |
Performing fault detection, diagnosis, isolation, and recovery technologies. |
Consider level 1 availability requirements during the design and production processes of network devices.
Consider level 2 availability requirements during network design.
Consider level 3 availability requirements during network deployment, according to the network infrastructure and service characteristics.
Availability evaluation
Mean Time Between Failures (MTBF) and Mean Time to Repair (MTTR) are used to evaluate the availability of a network.
MTBF
MTBF is the predicted elapsed time between inherent failures of a system during operation. It is typically in the unit of hours. A higher MTBF means a high availability.
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 means a smaller MTTR and a higher availability.
High availability technologies
Increasing MTBF or decreasing MTTR can enhance the availability of a network. The high availability technologies described in this section meet the level 2 and level 3 high availability requirements in the aspect of decreasing MTTR.
High availability technologies can be classified as fault detection technologies or protection switchover technologies.
Fault detection technologies
Fault detection technologies enable detection and diagnosis of network faults:
· BFD is a generic fault detection technology that can be used at any layer.
· Track works along with other high availability technologies to detect faults through a collaboration mechanism.
Table 2 Fault detection technologies
|
Technology |
Introduction |
Reference |
|
BFD |
BFD provides a single mechanism to quickly detect and monitor the connectivity of links or IP forwarding in networks. To improve network performance, devices must quickly detect communication failures to restore communication through backup paths as soon as possible. |
"Configuring BFD" |
|
Track |
The Track module implements collaboration between different modules. The collaboration involves three sets of modules: application, Track, and detection. These modules collaborate with one another through collaboration entries. The detection modules trigger the application modules to perform certain operations through the Track module. The detection modules probe such items as link status and network performance, and inform the application modules of the detection result through the Track module. Once notified of network status changes, the application modules use the changes to avoid communication interruption and network performance degradation. |
"Configuring Track" |
Protection switchover technologies
Protection switchover technologies aim at recovering network faults. They back up hardware, link, routing, and service information for switchover in case of network faults to ensure continuity of network services.
Table 3 Protection switchover technologies
|
Technology |
Introduction |
Reference |
|
Ethernet link aggregation |
Ethernet link aggregation, or link aggregation, aggregates multiple physical Ethernet links into one logical link to increase link bandwidth beyond the limits of any one single link. This logical link is an aggregate link. It allows for link redundancy because the member physical links can dynamically back up one another. |
Layer 2—LAN Switching Configuration Guide |
|
MSTP |
As a Layer 2 management protocol, MSTP eliminates Layer 2 loops by selectively blocking redundant links in a network, and in the meantime, allows for link redundancy. |
Layer 2—LAN Switching Configuration Guide |
|
GR |
GR prevents forwarding discontinuity caused by a protocol restart (including a BGP, IS-IS, OSPF, or LDP) or an active/standby switchover. This feature requires the peer devices to implement routing information backup and recovery. |
Related chapters in Layer 3—IP Routing Configuration Guide and MPLS Configuration Guide |
|
Interface backup |
Interface backup enables you to configure multiple backup interfaces for a Layer 3 interface to increase link availability. When the primary interface fails or is overloaded, its backup interfaces can take over or participate in traffic forwarding. |
|
|
VRRP |
VRRP is an error-tolerant protocol, which provides highly reliable default links on multicast and broadcast LANs such as Ethernet, avoiding network interruption due to failure of a single link. |
"Configuring VRRP" |
