MPLS and Segment Routing

What this chapter covers

This chapter explains how a provider core forwards traffic by label or by segment.

It starts with classic MPLS and LDP, then adds RSVP traffic engineering and fast reroute. It then moves to segment routing, which can do the same jobs with less state in the core.

The later pages cover SR traffic engineering, TI-LFA, path computation, Flexible Algorithm and SRv6.

You do not need to memorise every command. Focus on what each technology does, which problem it solves, and how IOS XR and IOS XE differ in naming. Pay attention to the show commands, because many exam questions ask you to read their output and decide what is wrong.

How the topics fit together

Each page solves a limit of the page before it.

The topics build on each other in this way:

• MPLS with LDP builds label switched paths that follow the IGP shortest path, and unified MPLS joins several domains.
• MPLS TE chooses the path on purpose, and uses RSVP to reserve bandwidth, with fast reroute for protection.
• Segment routing basics removes LDP and RSVP, because the IGP distributes the segments.
• SR-TE and advanced SR add policies, TI-LFA protection, PCE, Flexible Algorithm and SRv6.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

Compare each SR feature with its older equivalent. For example, TI-LFA does the job of MPLS TE fast reroute, but with no tunnels. The comparison helps you remember what each one adds.