Tag: segment-routing

What is Fast Reroute (FRR) in an MPLS network?

In an MPLS network with Segment Routing (SR), ensuring rapid recovery from link or node failures is crucial for maintaining high network availability. Cisco’s IOS-XR supports various Fast Reroute (FRR) mechanisms within MPLS Segment Routing environments, including Link Protection, Node Protection, and Topology Independent Loop-Free Alternate (TI-LFA). This article will provide a configuration guide and explanations for these MPLS-SR FRR mechanisms in an MPLS SR-enabled network running OSPF as the Interior Gateway Protocol (IGP).

What is TI-LFA in an MPLS network?

TI-LFA (Topology-Independent Loop-Free Alternate) is a fast reroute (FRR) mechanism in MPLS Segment Routing (MPLS-SR) designed to provide sub-50ms recovery from link or node failures in IP/MPLS networks. Unlike traditional LFA, which relies on specific topological conditions and may not cover all failure scenarios, TI-LFA is “topology-independent,” meaning it ensures protection for all traffic flows regardless of network topology. TI-LFA works by precomputing backup paths based on Segment Routing (SR) policies, allowing traffic to be quickly rerouted through alternate paths without complex recalculations. In the event of a failure, traffic is redirected via pre-established repair paths that adhere to the shortest path routing principles (SPF), minimizing packet loss and maintaining high availability across the network.

MPLS-SR Lab Setup (Baseline)

Using Cisco’s Modeling Labs (CML) I build the following MPLS-SR lab using OSPF as the IGP.

• 2 x PE router ( Left ) (PE5, PE6) running XRv with IOS-XR.
• 4 x P router ( Center )  (P1, P2, P3, P4) running XRv with IOS-XR.
• 2 x PE router ( Right ) (PE7, PE8) running IOSv with IOS.

Logical View:

Interfaces:

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What is MPLS Segment Routing (MPLS-SR)?

In short MPLS Segment Routing (MPLS-SR) is a modern approach to routing in MPLS (Multiprotocol Label Switching) networks. It allows for the efficient steering of traffic through predefined network segments. These segments are advertised by link-state routing protocols (IS-IS, OSPFv2, and OSPFv3) within IGP topologies. With MPLS-SR, path control and traffic engineering can be achieved without the need for protocols like LDP or RSVP-TE, which are typically used to set up traffic-engineered paths in traditional MPLS networks. These segments are stacked as labels in packet headers, allowing routers to follow predefined paths for traffic without this additional state tracking. This simplifies traffic engineering, enhances scalability, and enables efficient routing.

In this lab I will demonstrate the process of migrating from a traditional MPLS setup (using MPLS, OSPF, and LDP) to an MPLS-SR configuration on IOS-XR and IOS-XE. In this guide, you will see the configuration steps required and differences between both operating systems.
Following this, I will configure a Segment Routing Mapping Server (SRMS) to map the prefixes of IOSv routers since they don’t support MPLS-SR.

MPLS Lab Setup (Baseline)

Using Cisco’s Modeling Labs (CML) I build the following MPLS lab using OSPF and LDP neighbor relationships. 

• 2 x PE router ( Left ) (PE5, PE6) running CSR1000v with IOS-XE.
• 4 x P router ( Center )  (P1, P2, P3, P4) running XRv with IOS-XR.
• 2 x PE router ( Right ) (PE7, PE8) running IOSv with IOS.

Logical View:

Interfaces:

Firmware:

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MPLS – Segment Routing (MPLS-SR)

Multiprotocol Label Switching (MPLS) is a data-forwarding technique that uses labels to route packets along predefined paths, rather than traditional IP routing, which relies on layer-3 addresses. By attaching labels to packets, MPLS allows for high-speed data transfers and efficient traffic engineering, making it a go-to for large-scale carrier and enterprise networks.

Segment Routing (SR), is an extension for link-state IGPs (OSPF and IS-IS). Traditional MPLS forms Label Switched Paths (LSPs) through label distribution protocols such as LDP or RSVP-TE. SR simplifies this by eliminating these protocols and allowing source-based routing. In SR, the source node attaches a list of segments (labels) to the packet, guiding it through the desired path without the need for intermediate nodes to compute routing decisions. This shift enables networks to be more adaptable and scalable.

• RFC 8660 – Segment Routing with the MPLS Data Plane
• RFC 8665 – OSPF Extensions for Segment Routing
• RFC 8667 – IS-IS Extensions for Segment Routing

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