Understanding Cisco Segment Routing: What It Is and Why You Need to Know It

Right, let’s talk about segment routing. Now, I know what you’re thinking – “Great, another technology to learn.” But here’s the thing: segment routing is actually going to make your life easier, not harder. It’s Cisco’s answer to the mess that traditional MPLS networks can become when you’ve got LDP, RSVP-TE, and your IGP all trying to play nicely together.

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Think of it this way – you know how in traditional MPLS, you need LDP to distribute labels, RSVP-TE for traffic engineering, and then your IGP for basic routing? It’s like having three different sat-navs in your car all giving you different directions. Cisco segment routing basically says “sod this” and puts everything into the IGP itself. Much cleaner.

What Actually Is Cisco Segment Routing?

Let me explain this without the marketing waffle. Segment routing is a way of telling packets exactly which path to take through your network by putting a list of instructions in the packet header itself. It’s called “source routing” because the source router decides the entire path.

Imagine you’re giving someone directions to your house. Instead of calling every junction along the way to coordinate the route, you just write down the complete directions and hand them over. They follow each instruction until they arrive. That’s exactly what cisco segment routing does with packets.

The clever bit is that it uses your existing IGP – either IS-IS or OSPF – to distribute all the information needed. No more LDP sessions to worry about, no more RSVP-TE tunnels to maintain. Just your IGP doing what it’s always done, but with some extra information piggybacked on top.

How This Differs from What You Know

If you’ve worked with traditional MPLS, you’ll know the pain of keeping LDP and your IGP synchronised. When a link goes down, you’ve got to wait for both protocols to converge before traffic flows again. With segment routing, there’s only one protocol to converge – your IGP. Everything else just follows.

The operational difference is huge. When you’re troubleshooting a problem in a traditional MPLS network, you’re checking IGP adjacencies, LDP sessions, RSVP-TE tunnel states, and forwarding tables. With segment routing, you check the IGP and the forwarding table. Done.

Understanding the Different Types of Segments

Now, segments are basically instructions for routers. There are different types, and you need to understand what each one does because the exam will definitely test you on this.

Global Segments and the SRGB

Global segments are understood by every router in your segment routing domain. These come from something called the Segment Routing Global Block (SRGB). By default, Cisco uses labels 16000 to 23999 for this – notice how it doesn’t overlap with the LDP range of 24000+.

Here’s how it works: when you assign index 2 to router PE2, every router in the domain knows that label 16002 (that’s 16000 + 2) means “send this packet to PE2 via the shortest path.” Simple.

# Setting up SRGB on IOS-XE
PE2# configure terminal
PE2(config)# segment-routing mpls
PE2(config-sr-mpls)# global-block 16000 23999

# Same thing on IOS-XR  
RP/0/0/CPU0:PE4# configure
RP/0/0/CPU0:PE4(config)# segment-routing
RP/0/0/CPU0:PE4(config-sr)# global-block 16000 23999
RP/0/0/CPU0:PE4(config-sr)# commit

The golden rule here: keep your SRGB ranges the same across all routers. If you don’t, you’ll have a right mess trying to build end-to-end paths.

Prefix Segments (Node SIDs)

Prefix segments, also called Node SIDs, represent the shortest path to a specific router. When you configure a Node SID on a router’s loopback, every other router learns how to reach it via the IGP.

The beauty is that this leverages ECMP automatically. If there are multiple equal-cost paths to the destination, the router will load-balance across them. You get traffic engineering without having to explicitly engineer anything.

Adjacency Segments

Adjacency segments let you force traffic over specific links, regardless of what the IGP thinks is best. These are locally significant – each router assigns its own adjacency segment labels to its interfaces.

Why would you want this? Maybe you need to route certain traffic over encrypted links, or you want to balance load manually across parallel connections. Adjacency segments give you that granular control.

Local Segments

Local segments are only understood by the router that creates them. They’re useful for local policy decisions – like choosing between multiple egress interfaces based on criteria other than IGP cost.

Control Plane: How Segment Routing Actually Works

The control plane is where segment routing gets clever. Instead of inventing new protocols, Cisco extended the existing ones. This means you don’t need to learn completely new show commands or troubleshooting techniques.

IS-IS Extensions

IS-IS supports cisco segment routing through additional TLVs (Type-Length-Value fields). The existing protocol structure carries new sub-TLVs that contain segment routing information alongside the normal routing data.

# Basic IS-IS segment routing config on IOS-XR
RP/0/0/CPU0:PE4# configure
RP/0/0/CPU0:PE4(config)# router isis CCNP
RP/0/0/CPU0:PE4(config-isis)# net 49.0001.0000.0000.0004.00
RP/0/0/CPU0:PE4(config-isis)# address-family ipv4 unicast
RP/0/0/CPU0:PE4(config-isis-af)# metric-style wide
RP/0/0/CPU0:PE4(config-isis-af)# segment-routing mpls
RP/0/0/CPU0:PE4(config-isis-af)# exit
RP/0/0/CPU0:PE4(config-isis)# interface Loopback0
RP/0/0/CPU0:PE4(config-isis-if)# passive
RP/0/0/CPU0:PE4(config-isis-if)# address-family ipv4 unicast
RP/0/0/CPU0:PE4(config-isis-if-af)# prefix-sid index 4

The brilliant thing about this approach is backward compatibility. Routers running older software that don’t understand segment routing will still forward the TLVs to their neighbours. This “opaque flooding” means segment routing information reaches all routers in the domain, even during software upgrade cycles.

OSPF Extensions

OSPF does the same thing using Opaque LSAs – specifically Type 10 LSAs that flood within an area. This maintains OSPF’s area structure whilst adding segment routing capabilities.

# OSPF segment routing on IOS-XR
RP/0/0/CPU0:PE4# configure
RP/0/0/CPU0:PE4(config)# router ospf CCNP
RP/0/0/CPU0:PE4(config-ospf)# router-id 10.1.100.4
RP/0/0/CPU0:PE4(config-ospf)# segment-routing mpls
RP/0/0/CPU0:PE4(config-ospf)# area 0
RP/0/0/CPU0:PE4(config-ospf-ar)# interface Loopback0
RP/0/0/CPU0:PE4(config-ospf-ar-if)# passive enable
RP/0/0/CPU0:PE4(config-ospf-ar-if)# prefix-sid index 4

OSPF’s segment routing support includes penultimate hop popping (PHP) and explicit-null signalling, so it plays nicely with existing MPLS forwarding behaviour.

BGP for Segment Routing

Now, BGP as a control plane for segment routing isn’t common in traditional service provider networks, but it’s worth knowing about because it’s used in large-scale data centres.

BGP works well in symmetric topologies like Clos architectures where you’ve got loads of parallel paths. Its simpler state machine and constrained flooding make it attractive when you’re trying to minimise complexity.

Segment Routing Traffic Engineering (SR-TE)

Right, this is where things get interesting. Traditional traffic engineering with RSVP-TE is a right pain – you’ve got complex signalling, state maintenance at every hop, and limited scalability. SR-TE throws all that out the window.

With SR-TE, you define policies that specify explicit paths using sequences of segments. The key difference is that there’s no per-flow state at intermediate routers. The path information travels with the packet.

SR Policies Explained

An SR policy is uniquely identified by three things:

• Headend: The router where the policy starts
• Tailend: The destination router
• Colour: A number that lets you have multiple policies between the same routers

The colour bit is crucial. Say you need to route voice traffic over low-latency paths and data traffic over cost-optimised paths between the same two routers. You’d use different colour values to distinguish between them.

# Basic SR policy configuration
RP/0/0/CPU0:PE2# configure
RP/0/0/CPU0:PE2(config)# segment-routing
RP/0/0/CPU0:PE2(config-sr)# traffic-eng
RP/0/0/CPU0:PE2(config-sr-te)# policy VOICE-PATH
RP/0/0/CPU0:PE2(config-sr-te-policy)# color 100 end-point ipv4 10.1.100.7
RP/0/0/CPU0:PE2(config-sr-te-policy)# candidate-paths
RP/0/0/CPU0:PE2(config-sr-te-policy-path)# preference 100
RP/0/0/CPU0:PE2(config-sr-te-pp-info)# explicit segment-list VOICE-SEGMENTS

Path Computation Element (PCE)

Now, PCE is where segment routing gets properly clever. Instead of each router computing paths independently, you can have a centralised controller that has a complete view of the network and can compute optimal paths based on real-time conditions.

The PCE receives topology information via BGP-LS and can make traffic engineering decisions that individual routers simply can’t make because they don’t have the full picture. This is particularly valuable in multi-domain scenarios.

Think of PCE as a sat-nav for your network – it knows the current traffic conditions and can route around congestion in real-time.

Real-World Examples Where This Actually Matters

Let me give you some proper examples of where cisco segment routing makes a real difference, because understanding the use cases helps everything else make sense.

Service Provider Networks

Large SPs have thousands of routers and millions of traffic flows. In traditional MPLS networks, you’d need thousands of LDP sessions and potentially thousands of RSVP-TE tunnels. That’s a lot of state to maintain and a lot of things that can go wrong.

Take a national SP with regional data centres that need guaranteed bandwidth for cloud services. With traditional MPLS, you’d create RSVP-TE tunnels with bandwidth reservations. Works, but it’s operationally complex.

With segment routing, you achieve the same result using SR policies, but without all the tunnel state. When links fail, you only wait for IGP convergence – no LDP re-establishment, no RSVP-TE resignalling.

The downside? You need modern hardware that can handle deep label stacks, and your operations team needs retraining. Also, not all vendors support segment routing equally well, so multi-vendor networks can be challenging.

Data Centre Interconnects

Modern hyperscale data centres use segment routing for DCI because it scales better than traditional approaches. When you’re connecting hundreds of data centres with thousands of tenant requirements, maintaining individual RSVP-TE tunnels becomes impossible.

A global cloud provider might use cisco segment routing to ensure premium customers get dedicated low-latency paths whilst standard customers use best-effort routing. SR policies enable this differentiation without separate overlay networks.

The challenge here? Coordinating segment routing policies across multiple data centres requires sophisticated orchestration platforms. You can’t do this manually at scale.

Mobile Network Transport

5G networks introduce network slicing where different services get guaranteed network resources. Autonomous vehicle communications need ultra-low latency, mobile broadband needs high throughput, and IoT needs massive scale but low bandwidth.

Cisco segment routing enables this by providing deterministic paths through the transport network. Each slice gets specific SR policies that guarantee its requirements.

The problem? 5G slicing requirements change dynamically, so you need automated policy provisioning. Manual configuration doesn’t cut it.

What You Actually Get from Using Segment Routing

Let’s talk benefits, but I’ll give you the real story, not the marketing version.

Protocol Simplification

The biggest win is getting rid of LDP and RSVP-TE. That’s roughly 40% fewer protocols to configure, monitor, and troubleshoot. When something breaks, you’re only checking IGP and forwarding tables instead of multiple protocol states.

In my experience, this alone justifies the migration for most networks. The operational simplicity is genuinely significant.

Better Traffic Engineering

You get traffic engineering capabilities that would require complex tunnel arrangements in traditional networks. Need traffic to visit specific service appliances? Just include them in your segment list. Simple.

The stateless nature means you can implement traffic engineering policies that would be impossible with RSVP-TE because of scaling limitations.

Improved Scalability

Traditional RSVP-TE networks don’t scale well because every tunnel needs state at every hop. Segment routing scales linearly because forwarding uses standard MPLS label operations.

SDN Integration

Segment routing plays nicely with SDN controllers and automation platforms. The programmatic interfaces enable zero-touch service provisioning that traditional tunnel-based approaches struggle with.

The Challenges You’ll Face

Let me be honest about the problems you’ll encounter, because every technology has them.

Hardware Requirements

Segment routing can require deep label stacks – sometimes 6-9 labels or more. Older hardware might not support this, meaning equipment upgrades. That’s expensive.

Also, not all platforms support all segment routing features equally. Check your hardware capabilities before committing to specific features.

Migration Complexity

You can’t just flip a switch and move from traditional MPLS to segment routing. It’s typically a parallel deployment where both run together until you can cut over services.

Service migration is particularly tricky – L3VPNs and L2VPNs that currently use LDP transport need careful transition to segment routing transport.

Skills Gap

Your operations team needs to learn new troubleshooting techniques. The concepts are different enough that you can’t just wing it. Budget for training.

Monitoring tools also need updating. Traditional MPLS monitoring focuses on tunnel states and LDP sessions. Segment routing monitoring is about segment utilisation and policy effectiveness.

Label Depth Limitations

Complex deployments might hit hardware label depth limits. You need to balance segment routing flexibility with platform constraints, which sometimes means you can’t implement the ideal solution.

How to Check If It’s Working

Once you’ve got segment routing running, you need to know how to verify it’s working properly. Here are the commands that’ll save your bacon.

Basic Verification

# Check segment routing state
PE2# show segment-routing mpls state
PE2# show mpls forwarding-table labels 16000-23999
PE2# show isis segment-routing prefix-sids

# On IOS-XR, same commands work
RP/0/0/CPU0:PE4# show segment-routing mpls state
RP/0/0/CPU0:PE4# show mpls forwarding labels 16000:23999
RP/0/0/CPU0:PE4# show isis segment-routing prefix-sids

SR Policy Verification

# Check your SR-TE policies
RP/0/0/CPU0:PE2# show segment-routing traffic-eng policy all
RP/0/0/CPU0:PE2# show segment-routing traffic-eng policy color 100
RP/0/0/CPU0:PE2# show segment-routing traffic-eng topology ipv4

Troubleshooting Common Issues

When segment routing breaks, it’s usually one of three things:

1. SRGB mismatch – Check that all routers have the same SRGB range
2. Missing prefix-SIDs – Verify that Node SIDs are being advertised properly
3. Forwarding table problems – Make sure labels are being installed correctly

The systematic approach is: check SRGB consistency first, then prefix-SID advertisement, then forwarding table installation.

Integration with Services

Cisco segment routing doesn’t exist in isolation – it provides transport for actual services. The good news is that existing services work fine over segment routing transport.

L3VPN Services

Your L3VPNs can use segment routing for transport without any service-level changes. The VPN labels stack on top of the segment routing transport labels. Everything works exactly as before, but with better traffic engineering.

EVPN Integration

EVPN particularly benefits from cisco segment routing’s path control. In data centre interconnect scenarios, you often need different tenant traffic to follow specific paths. SR policies make this straightforward.

The combination of EVPN’s service flexibility with segment routing’s transport optimisation creates some powerful capabilities for modern networks.

Look, segment routing isn’t a magic bullet, but it does solve real operational problems with traditional MPLS networks. The protocol consolidation alone makes it worth considering for most service provider environments.

The key is understanding that this isn’t just about replacing LDP with something else – it’s about simplifying your entire control plane whilst gaining better traffic engineering capabilities. That’s a win-win if you can navigate the migration challenges.

For your CCNP studies, focus on understanding the segment types, how the control plane works, and the verification commands. The exam will test your knowledge of how segments are advertised and how forwarding decisions are made.

The technology is here to stay, and networks are increasingly being built with segment routing from day one rather than migrating from traditional MPLS. Understanding it properly puts you in a good position for the future of service provider networking.

External Link: Cisco Learning Network CCNP Service Provider Community – excellent resource for additional study materials and community discussions about service provider technologies.