High Availability
Page 216 7750 SR OS Basic System Configuration Guide
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.
However, in a centralized architecture, packet processing and forwarding is performed in a central
shared route processor and the individual IOMs (line cards) are relatively simple. The cards rely
solely on the route processor for routing and forwarding intelligence and, should the centralized
route processor fail, there is greater impact to the system overall, as all routing and packet
forwarding will stop.
In a distributed system, the packet forwarding functionality is situated on each IOM. Distributing
the forwarding engines off the central route processor and positioning one on each IOM lowers the
impact of route processor failure as the line cards can continue to forward traffic during an outage.
The distributed system is better suited to enable the convergence of business critical services such
as real-time voice (VoIP), Video, and VPN applications over IP networks with superior
performance and scalability. The centralized architecture can be prone to performance bottleneck
issues and limits service offerings through poor scalability which may lead to customer and
service SLA violations.
Service Redundancy
All service-related statistics are kept during a switchover. Services, SDPs, and SAPs can be
interrupted during a CPM switchover with the following exceptions:
• IES services and GRE services with no keepalives are not interrupted during a CPM
switchover.
• Services using MPLS tunnels or spanning tree can be impacted during a CPM switchover.
Accounting Configuration Redundancy
When there is a switchover and the standby CPM becomes active, the accounting servers will be
checked and if they are administratively up and capable of coming online (media present, etc.), the
standby will be brought online and new accounting files will be created at that point. Users must
manually copy the accounting records from the failed
CPM.
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