FTN Fallback Support for Flexible Algorithms
Flexible Algorithm (Flex-Algo) enables an IGP to compute paths based on specific constraints and topology attributes, providing distributed traffic-engineering capabilities. These algorithms allow users to define differentiated forwarding paths that meet various performance or resource objectives. Service routes that carry a color attribute can be steered through Flex-Algo–based transport paths using SR-ODN templates.
In previous software versions, services mapped to a particular Flex-Algo FTN had no protection if that FTN became unavailable. When the primary Flex-Algo transport path failed, the affected service FTNs were removed because no alternate path selection was supported, even if valid Native-SR or other Flex-Algo options existed in the network.
The Flex-Algo Fallback feature introduces a mechanism to maintain service continuity during Flex-Algo
transport failures. With this enhancement, colored services can automatically move to an alternate transport FTN when their primary Flex-Algo FTN goes down. The fallback can be directed either to a Native-SR FTN (ISIS-SR or OSPF-SR) or to a secondary Flex-Algo FTN defined by the user.
Only one fallback method is configured per color.
Feature Characteristics
| • | Fallback Mechanism: Provides a configurable fallback FTN if the primary FA-FTN is not available. |
| • | Service Continuity: Ensures that colored services remain mapped to a valid FTN without manual intervention. |
| • | Flexible Configuration: Allows fallback to either the native SR-FTN or another designated FTN. |
| • | Failure Handling: Automatically activates the fallback path during FA-FTN failure or unavailability. |
| • | User Control: Administrators can explicitly define the fallback preference. |
Benefits
This feature offers several key advantages:
| • | High Availability: Maintains uninterrupted forwarding even when the Flex-Algo FTN is down. |
| • | Operational Resilience: Reduces service disruption during control-plane or Flex-Algo-specific failures. |
| • | Simplified Management: Eliminates the need for manual reconfiguration during FTN failures. |
| • | Enhanced Flexibility: Provides fallback options to align with network design policies. |
| • | Improved Reliability: Ensures service-level agreements (SLAs) are met by avoiding traffic blackholing. |
Prerequisites
This feature requires the following:
| • | Ensure that SR-MPLS (ISIS-SR) is enabled and operational in the network. |
| • | Loopback interfaces are configured with prefix-SIDs for each Flex-Algo instance. |
| • | Flex-Algo instances (for example, 128 and 129) are defined with appropriate metric types. |
| • | MPLS TE is enabled with a valid SRGB range and router ID. |