VPN Technologies

What this chapter covers

VPN Technologies is about carrying private traffic across a shared network, using either a provider MPLS core or tunnels over the internet.

The chapter has three topics. Two describe how MPLS works and how it supports Layer 3 VPNs, and one shows how to configure a single hub DMVPN.

The MPLS topics are descriptive, so you should be able to explain how the pieces work and read basic output. The DMVPN topic is hands on, so you should be able to configure and verify it.

How the topics fit together

MPLS and DMVPN solve the same problem in different ways.

The first page, MPLS operations, explains label switching and LDP. It is the base for the second page, which adds VRFs, route distinguishers, route targets and MP-BGP to deliver a private Layer 3 service.

The DMVPN page is separate. It builds tunnels yourself over any IP network, and adds NHRP and IPsec to make them dynamic and secure.

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.

Read the MPLS pages in order, because the VPN page uses the label terms from the first. For DMVPN, practise the build in a lab and check each stage with the verification commands.

Keep a small notes sheet of the terms that are easy to confuse. Examples are RD and RT, LSR and LER, and the hub and spoke roles in NHRP. Write one line for each, and test yourself on it before you attempt practice questions.

The MPLS topics reward careful reading of diagrams. Trace a packet from the customer edge to the far site, and say out loud which label is pushed, swapped or popped at each router.