Cisco Cisco Packet Data Gateway (PDG) Guida Dell'Amministratore
Interchassis Session Recovery
Overview ▀
Cisco ASR 5500 System Administration Guide ▄
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Interchassis Communication
Chassis configured 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 an Hello message from the active chassis within the dead interval,
the standby chassis transitions to the active state. In situations where the SRP link goes out of service, a priority scheme
is used to determine which chassis processes the session. The following priority scheme is used:
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 an Hello message from the active chassis within the dead interval,
the standby chassis transitions to the active state. In situations where the SRP link goes out of service, a priority scheme
is used to determine which chassis processes the session. The following priority scheme is used:
route modifier
chassis priority
MIO/UMIO MAC address
Checkpoint Messages
Checkpoint messages are sent from the active chassis to the standby chassis. These messages are sent 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.
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.
AAA Monitor
AAA servers are monitored using the authentication probe mechanism. AAA servers are considered Up if the
authentication-probe receives a valid response. AAA servers are considered Down when the max-retries count
specified in the configuration of the AAA server has been reached. SRP initiates a switchover when none of the
configured AAA servers responds to an authentication probe. AAA probing is only performed on the active chassis.
authentication-probe receives a valid response. AAA servers are considered Down when the max-retries count
specified in the configuration of the AAA server has been reached. SRP initiates a switchover when none of the
configured AAA servers responds to an authentication probe. AAA probing is only performed on the active chassis.
Important:
A switchover event caused by an AAA monitoring failure is non-revertible.
If the newly active chassis fails to monitor the configured AAA servers, it remains as the active chassis until one of the
following occurs:
following occurs:
a manual switchover
another non-AAA failure event causes the system to switchover
a CLI command is used to clear the AAA failure flag and allow the chassis to switch to standby
BGP Interaction
The Service Redundancy Protocol implements revertible switchover behavior via a mechanism that adjusts the route
modifier value for the advertised loopback/IP Pool routes. The initial value of the route modifier value is determined by
the chassis’ configured role and is initialized to a value that is higher than a normal operational value. This ensures that
in the event of an SRP link failure and an SRP task failure, the correct chassis is still preferred in the routing domain.
modifier value for the advertised loopback/IP Pool routes. The initial value of the route modifier value is determined by
the chassis’ configured role and is initialized to a value that is higher than a normal operational value. This ensures that
in the event of an SRP link failure and an SRP task failure, the correct chassis is still preferred in the routing domain.
The Active and Standby chassis share current route modifier values. When BGP advertises the loopback and IP pool
routes, it converts the route modifier into an autonomous systems (AS) path prepend count. The Active chassis always
has a lower route modifier, and thus prepends less to the AS-path attribute. This causes the route to be preferred in the
routing domain.
routes, it converts the route modifier into an autonomous systems (AS) path prepend count. The Active chassis always
has a lower route modifier, and thus prepends less to the AS-path attribute. This causes the route to be preferred in the
routing domain.