Link Continuity; Internal Bypass Mechanism - Cisco SCE8000 GBE Installation And Configuration Manual

Chapter 3 Cisco SCE8000 GBE Topology and Topology-Related Parameters
This fail-over solution preserves the Cisco SCE8000 GBE functionality and the network link:
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Link Continuity

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The internal bypass mechanism of the Cisco SCE8000 GBE allows traffic to continue to flow, if desired,
even if the device itself is not fully functioning. In addition, the Cisco SCE8000 GBE is designed with
the ability to control up to four external optical bypass devices (each optical bypass supports two links).
This is needed because the internal bypass mechanism cannot maintain traffic flow in all cases.
Note that when the Cisco SCE8000 GBE is connected to the network through an optical splitter, a failure
of the Cisco SCE8000 GBE does not affect the traffic flow, as the traffic continues to flow through the
optical splitter.

Internal Bypass Mechanism

The Cisco SCE8000 GBE includes a SPA Interface Processor module with a bypass mechanism that is
enabled upon Cisco SCE8000 GBE failure.
The SPA Interface Processor card supports the following three modes:
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The SPA Interface Processor card cannot preserve the link in the following circumstances:
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The two Cisco SCE8000 GBE s are simultaneously aware of the subscriber contexts, and subscriber
states are constantly exchanged between them, such that if the active Cisco SCE8000 GBE fails, the
standby can take over with minimum state loss.
When one Cisco SCE8000 GBE fails (depending on the type of failure) its link traffic is still
bypassed to the functioning Cisco SCE8000 GBE and processed there, so the traffic processing
continues for both the links.
The bypass of the traffic through the failed Cisco SCE8000 GBE is configurable, and the user may
choose to always cutoff the line that goes through the failed Cisco SCE8000 GBE . In this case
network redundancy protocols like HSRP are responsible for identifying the line cutoff and
switching all the traffic to go through the functioning Cisco SCE8000 GBE GBE.
In addition, optional external optical bypass devices provide link continuity in the event of any
failure of the Cisco SCE8000 GBE GBE. This ensures 100% link continuity at the expense of
service inaccuracy as a result of the traffic of the failing box not being processed.
Internal Bypass Mechanism, page 3-7
External Optical Bypass, page 3-8
Bypass — The bypass mechanism preserves the network link, but traffic is not processed for
monitoring or for control.
Forwarding — This is the normal operational mode, in which the Cisco SCE8000 GBE processes
the traffic for monitoring and control purposes.
Cutoff — There is no forwarding of traffic, and the physical link is forced down (cutoff
functionality at layer 1).
During platform reboot (SW reload), there is an 11-second period during which the link is forced
down (cutoff functionality). If any routing or spanning tree protocols are used in the network, this
delay may be extended.
During a power failure (The Cisco SCE8000 GBE has two power supplies. A power failure occurs
only when both of them fail).
Cisco SCE8000 GBE Installation and Configuration Guide
Link Continuity
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