Segment Routing vs. Traditional MPLS: A Modern Approach to Traffic Engineering

 

1. Introduction

Modern networks strive for greater efficiency, simpler management, and enhanced flexibility. Segment Routing (SR) has emerged as a powerful alternative to traditional MPLS (Multiprotocol Label Switching), eliminating many of its limitations, such as reliance on Label Distribution Protocol (LDP) and RSVP-TE for signaling. This article compares SR-MPLS with classical MPLS and explores its key advantages, including a step-by-step migration process illustrated with network diagrams.

---

2. Traditional MPLS and Its Limitations

MPLS uses labels for efficient packet forwarding, relying on LDP for label distribution or RSVP-TE for traffic engineering. While MPLS has served networks well, it faces challenges in scalability and complexity.

Limitations of MPLS:

• LDP dependency: A separate protocol for label distribution, increasing overhead.

• No native ECMP support: RSVP-TE lacks equal-cost multipath (ECMP) forwarding.

• Complex control plane: Each router must maintain LDP sessions, increasing memory and processing requirements.

• Traffic engineering challenges: Requires additional mechanisms such as RSVP-TE or centralized SDN controllers.

---

3. Segment Routing: A Modern Alternative

Segment Routing (SR-MPLS) simplifies network design by encoding the forwarding path within the packet itself using Segment Identifiers (SIDs). Instead of relying on LDP, SR uses existing IGP (OSPF/IS-IS) extensions to distribute labels.

Advantages of SR:

• No need for LDP: Simplifies the control plane.

• Uses IGP for label distribution: Eliminates additional protocols.

• Stateless core: Reduces memory and processing overhead on routers.

• Better traffic engineering: Native support for SR-TE (Segment Routing Traffic Engineering).

• Built-in ECMP: Efficient utilization of available paths.

---

4. Migration from MPLS-LDP to Segment Routing

Migration to SR is typically done in phases to minimize service disruption. The following sections illustrate this transition with network diagrams.

4.1 Initial State: MPLS Network with LDP

In this stage, the network is fully MPLS-based, with LDP used for label distribution.

(Image: Traditional MPLS Network with LDP)

4.2 Hybrid State: Coexistence of LDP and SR

During migration, both LDP and SR run in parallel, allowing gradual migration of routers to SR.

(Image: Hybrid Network with MPLS-LDP and Segment Routing)

4.3 Fully Migrated State: Pure SR-MPLS Network

Once all routers support SR, LDP is removed, simplifying the network architecture.

(Image: Fully Migrated Segment Routing Network)

---

5. Configuration Examples

5.1 MPLS-LDP Configuration (Traditional Approach)

router isis 1
 net 49.0001.0000.0000.0001.00
 is-type level-2
 metric-style wide

mpls ip
mpls label protocol ldp
interface GigabitEthernet0/0/0
 mpls ip
 mpls ldp

router ldp
 mpls ldp router-id Loopback0 force

5.2 Segment Routing Configuration

router isis 1
 net 49.0001.0000.0000.0001.00
 is-type level-2
 metric-style wide
 segment-routing mpls

interface GigabitEthernet0/0/0
 ip router isis 1

segment-routing
 mpls
  set srgb 16000 23999
  node-sid 16001

---

6. Conclusion

Segment Routing provides a more scalable, flexible, and efficient alternative to traditional MPLS-LDP. By eliminating LDP and RSVP-TE, SR-MPLS simplifies operations, reduces control plane overhead, and enables advanced traffic engineering. Migrating to SR can be done in phases to ensure smooth adoption without disrupting services.