Bgp Interaction; Requirements - Cisco ASR 5500 Administration Manual

Interchassis Session Recovery

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.
Important
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.
If communication on the SRP link is lost, and both chassis in the redundant pair are claiming to be Active,
the previously Active chassis is still preferred since it is advertising a smaller AS-path into the BGP routing
domain. The route modifier is incremented as switchover events occur. A threshold determines when the route
modifier should be reset to its initial value to avoid rollover.

Requirements

ICSR configurations require the following:
• Two chassis configured for the same service types. The services must be bound on an SRP-activated
• Both chassis must have identical hardware.
• Three contexts:
• AAA RADIUS server
• Border Gateway Protocol (BGP) – ICSR uses the route modifier to determine the chassis priority.
Important
For ICSR you must configure busyout ip pool commands in the same order on Active and Standby chassis
to avoid SRP validation failures.
loopback interface.
• Redundancy – to configure the primary and backup chassis redundancy.
• Source – AAA configuration of the specified nas-ip-address must be the IP address of an interface
bound to an HA, or any core network service configured within the same context.
• Destination – to configure monitoring and routing to the PDN.
ICSR is a licensed Cisco feature. Verify that each chassis has the appropriate license before using these
procedures. To do this, log in to both chassis and execute a show license information command. Look
for "Inter-Chassis Session Recovery". If the chassis is not licensed, please contact your Cisco account
representative.
ASR 5500 System Administration Guide, StarOS Release 19
BGP Interaction
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