High Availability
242 Basic System Configuration Guide
• Warm standby — The router image and configuration is already loaded on the
standby route processor. However, the standby could still take a few minutes to
become effective since it must first re-initialize connections by bringing up Layer 2
connections and Layer 3 routing protocols and then rebuild routing tables.
• Hot standby — The router image, configuration, and network state is already loaded
on the standby and it receives continual updates from the active route processor and
the swapover is immediate. However, hot standby affects conventional router
performance as more frequent synchronization increases consumption of system
resources. Alcatel-Lucent’s newer generation service routers address this issue
because they already have extra processing built into the system.
Component Redundancy
Component redundancy is critical to reduce MTTR for the system and primarily consists of
the following router features:
• Dual route processor modules — For a highly available architecture, redundant
Control Processor Modules (CPM) are essential. The route processing functions of
the CPM calculate the most efficient route to an Internet destination and
communicate the best path information to peer routers. Rapid information
synchronization between the primary and secondary CPMs/CFMs is crucial to
minimize recovery time.
• Switch fabric (SFM) redundancy — Failure of a single switch fabric card with little
to no loss of traffic.
• Redundant line cards — LAG, ECMP and other techniques to spread traffic over
multiple line cards so that a failure of one line card does not impact the services being
delivered.
• Redundant power supply — A power module can be removed without impact on
traffic.
• Redundant fan — Failure of a fan module without impacting traffic.
• Hot swap — Components in a live system can be replaced or become active without
taking the system down or affecting traffic flow to/from other modules.
Router hardware architecture plays a key role in the availability of the system. The principle
router architecture styles are centralized and distributed. In these architectures, both active
and standby route processors, I/O modules (IOMs) (also called line cards), fans, and power
supplies maintain a low MTTR for the routing system.
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