Cisco Cisco Packet Data Gateway (PDG)
Interchassis Session Recovery
▀ Overview
▄ ASR 5500 System Administration Guide, StarOS Release 17
302
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.
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.
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 loopback
interface.
Both chassis must have identical hardware.
Three contexts:
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.
AAA RADIUS server
Border Gateway Protocol (BGP) – ICSR uses the route modifier to determine the chassis priority.
Important:
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.
Chassis Session Recovery”. If the chassis is not licensed, please contact your Cisco account representative.
The following figure shows an ICSR network.