Segment Routing (SR) represents a revolutionary paradigm shift in network architecture, combining the simplicity of source routing with the power of MPLS forwarding. This comprehensive guide explores how SR and Topology Independent Loop-Free Alternate (TI-LFA) work together to create resilient, scalable service provider networks.
Understanding Segment Routing Fundamentals
Segment Routing simplifies network operations by encoding forwarding instructions as an ordered list of segments in the packet header. Unlike traditional MPLS that requires per-flow state at every hop, SR maintains state only at the ingress node, dramatically reducing protocol complexity.
Key Components of Segment Routing
The SR architecture consists of several critical elements:
- Prefix Segments (Prefix-SID): Global identifiers associated with IGP prefixes, enabling shortest-path forwarding
- Adjacency Segments (Adj-SID): Local identifiers representing specific links, allowing strict forwarding paths
- Segment List: An ordered sequence of segments that defines the complete forwarding path
- SR Global Block (SRGB): A range of labels reserved for segment routing, ensuring consistent label allocation
Topology Independent Loop-Free Alternate (TI-LFA)
TI-LFA leverages SR capabilities to compute backup paths that guarantee loop-free forwarding even during network failures. Traditional LFA mechanisms often fail to find protection in complex topologies, but TI-LFA solves this limitation elegantly.
Practical Implementation
Configuring SR-MPLS with TI-LFA
Here’s a practical implementation on Cisco IOS-XR:
! Enable Segment Routing globally
segment-routing
global-block 16000 23999
!
!
router isis CORE
is-type level-2-only
net 49.0001.1921.6800.1001.00
address-family ipv4 unicast
metric-style wide
segment-routing mpls
!
interface Loopback0
passive
address-family ipv4 unicast
prefix-sid absolute 16001
!
!
interface GigabitEthernet0/0/0/0
point-to-point
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
!
!This configuration enables:
- Segment Routing with SRGB range 16000-23999
- IS-IS as the IGP with SR extensions
- Prefix-SID allocation for the loopback interface
- TI-LFA protection on transit interfaces
TI-LFA Path Computation
The TI-LFA algorithm computes backup paths using a two-stage approach:
- Post-Convergence Path Analysis: Determines the shortest path after link/node failure
- Segment List Construction: Builds an SR segment stack to steer traffic around the failure
The backup path typically consists of:
- A node segment to reach the post-convergence next-hop
- Optional adjacency segments for specific link traversal
- The destination prefix segment
Laboratory Exercise
Verification Commands
Verify TI-LFA operation with these commands:
# Check SR-MPLS forwarding table
show mpls forwarding
# Verify segment routing labels
show segment-routing mpls lb
# Examine TI-LFA backup paths
show isis fast-reroute detail
# Display segment routing topology
show isis segment-routing label tableExpected output shows primary and TI-LFA backup paths with their respective segment lists.
Benefits of SR with TI-LFA
| Feature | Traditional MPLS + LDP | Segment Routing + TI-LFA |
|---|---|---|
| Convergence Time | 50-500ms | <50ms (sub-50ms) |
| Protection Coverage | 60-80% | 100% |
| Protocol Complexity | High (LDP, RSVP) | Low (IGP only) |
| State Requirements | Per-LSP state | Source-only state |
Advanced Implementation Scenarios
Anycast SID for Disaster Recovery
Deploy anycast prefix-SIDs across multiple data centers for automatic failover:
! DC1 Edge Router
interface Loopback100
description Anycast SID for DC Services
ipv4 address 192.168.100.1 255.255.255.255
!
router isis CORE
interface Loopback100
address-family ipv4 unicast
prefix-sid absolute 16100
!Traffic Engineering with SR Policies
SR policies enable explicit path control for traffic optimization:
- Low-latency paths: Segment lists avoiding congested links
- Disjoint paths: Primary and backup paths with zero shared risk
- Bandwidth optimization: Load balancing across multiple SR paths
Conclusion
Segment Routing with TI-LFA represents the future of service provider networking, offering unprecedented simplicity, scalability, and resilience. By eliminating LDP/RSVP complexity while achieving 100% fast-reroute coverage, SR-MPLS enables operators to build more reliable networks with lower operational overhead.
As networks continue to grow in scale and complexity, the combination of SR and TI-LFA provides the architectural foundation needed for 5G transport, cloud interconnect, and next-generation services.
Parveen Jindgar
Engineering Leader
I am an engineering leader with over 20 years of experience working on large-scale packet networks in Service Provider and latency-sensitive environments. My work has centered on end-to-end architectural ownership of backbone and edge infrastructures — spanning platform and vendor selection, routing and control-plane design, and the evolution of networks under real operational and economic constraints. PacketBytes is a personal space where I document the engineering thinking behind these systems — examining architectures, control planes, and design trade-offs as they behave under real traffic, real failures, and real operational pressure, beyond configuration and into intent.