SaMOG Inter-Chassis Session Recovery
This chapter describes the license-enabled inter-chassis session
recovery feature on the SaMOG gateway.
SaMOG is capable
of providing chassis-level and geographic-level redundancy and can
recover fully created sessions in the event of a chassis failure.
The Cisco ASR 5x00
and virtualized platforms provide industry leading carrier class redundancy.
The systems protects against all single points of failure (hardware
and software) and attempts to recover to an operat ional state when
multiple simultaneous failures occur.
The system provides
several levels of system redundancy:
- Under normal N+1
packet processing card hardware redundancy, if a catastrophic packet
processing card failure occurs all affected calls are migrated to
the standby packet processing card if possible. Calls which cannot
be migrated are gracefully terminated with proper call-termination
signaling and accounting records are generated with statistics accurate
to the last internal checkpoint.
- If the Session Recovery
feature is enabled, any total packet processing card failure will
cause a packet processing card switchover and all established sessions
for supported call-types are recovered without any loss of session.
Even though Cisco provides
excellent intra-chassis redundancy with these two schemes, certain
catastrophic failures which can cause total chassis outages, such
as IP routing failures, line-cuts, loss of power, or physical destruction
of the chassis, cannot be protected by this scheme. In such cases,
the SaMOG Inter-Chassis Session Recovery (ICSR) feature provides
geographic redundancy between sites. This has the benefit of not
only providing enhanced subscriber experience even during catastrophic
outages, but can also protect other systems such as the RAN from
subscriber reactivation storms.
ICSR allows for continuous
call processing without interrupting subscriber services. This is accomplished
through the use of redundant chassis. The chassis are configured
as primary and backup with one being active and one in recovery
mode. A checkpoint duration timer is used to control when subscriber
data is sent from the active chassis to the inactive chassis. If
the active chassis handling the call traffic goes out of service,
the inactive chassis transitions to the active state and continues
processing the call traffic without interrupting the subscriber
session. The chassis determines which is active through a propriety
TCP-based connection called a redundancy link. This link is used
to exchange Hello messages between the primary and backup chassis
and must be maintained for proper system operation.
How It Works
to support ICSR communicate using periodic Hello messages. These
messages are sent by each chassis to notify the peer of its current
state. The Hello message contains information about the chassis
such as its configuration and priority. A dead interval is used
to set a time limit for a Hello message to be received from the
chassis' peer. If the standby chassis does not receive a Hello message
from the active chassis within the dead interval, the standby chassis
transitions to the active state. In situations where the redundancy
link goes out of service, a priority scheme is used to determine
which chassis processes the session. The following priority scheme
- route modifier
- chassis priority
- SPIO MAC address
are sent from the active chassis to the inactive chassis at specific
intervals and contain all the information needed to recreate the
sessions on the standby chassis, if that chassis were to become
active. Once a session exceeds the checkpoint duration, checkpoint
data is collected on the session. The checkpoint parameter determines
the amount of time a session must be active before it is included
in the checkpoint message.
For more information
on inter-chassis session recovery support, refer to the Interchassis Session
Recovery chapter in the System
identifies limitations, restrictions, and dependencies for the SaMOG
- In this release, ICSR
support is available only for the following sessions types:
- PMIPv6 access-type
and GTPv2 network type sessions with DIAMETER-based authentication.
- EoGRE access-type and
GTPv2 network type sessions with DIAMETER-based authentication.
- ICSR support is not
available for recovering CDR information and SaMOG Local Breakout (LBO)
sessions (flow-based and LBO Basic).
- ICSR support is not
available for multiple SaMOG services configuration.