System Basics Introduction
Page 206 7210 SAS M, T, X, R6 Basic System Configuration Guide
Redundancy
The redundancy features enable the duplication of data elements to maintain service continuation
in case of outages or component failure.
Software Redundancy
Software outages are challenging even when baseline hardware redundancy is in place. There
should be a balance to provide high availability routing otherwise router problems typically
propagate not only throughout the service provider network, but also externally to other connected
networks possibly belonging to other service providers. This could affect customers on a broad
scale. Presently, there are several software availability features that contribute to the percentage of
time that a router is available to process and forward traffic.
To fully appreciate high availability you should realize that all routing protocols specify minimum
time intervals in which the peer device must receive an acknowledgement before it disconnects the
session.
• OSPF default session timeout is approximately 40 seconds. The timeout intervals are
configurable.
• BGP default session timeout is approximately 120 seconds. The timeout intervals are
configurable.
Therefore, router software has to recover faster than the specified time interval to maintain up
time.
Configuration Redundancy
Features configured on the active device CFM/CPM are saved on the standby CFM/CPM as well.
When the active device CFM/CPM fails, these features are brought up on the standby device
CFM/CPMCFM/CPM that takes over the mastership.
Even with modern modular and stable software, the failure of route processor hardware or
software can cause the router to reboot or cause other service impacting events. In the best
circumstances, failure leads to the initialization of a redundant route processor, which hosts the
standby software configuration, to become the active processor. The following options are
available.
• 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.
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