MPLS Labels and Devices

In my introduction to MPLS I explained why we use MPLS and explained some of the basics of how we use labels for forwarding decisions. In this lesson, we’ll take a closer look at the MPLS labels, the devices that we use and how IP packets travel through the MPLS network.

MPLS Label Format

The MPLS header has been standardized, you can find it in RFC 3032. The header is pretty simple, here’s what it looks like:

MPLS Label Header

Here’s what the different fields are used for:

  • Label value: the name says it all, this is where you will find the value of the label.
  • EXP: these are the three experimental bits. These are used for QoS, normally the IP precedence value of the IP packet will be copied here.
  • S: this is the “bottom of stack” bit. With MPLS it’s possible to add more than one label, you’ll see why in some of the MPLS VPN lessons. When this bit is set to one, it’s the last MPLS header. When it’s set to zero then there is one or more MPLS headers left.
  • TTL: just like in the IP header, this is the time to live field. You can use this for traces in the MPLS network. Each hop decrements the TTL by one.

The MPLS header is added in between the L2 and L3 header:

MPLS Header Between Layer 2 3

That’s why we call it a “layer 2.5” protocol. Here’s an example of what it looks like in wireshark:

wireshark capture mpls header

Above you have an example of the MPLS header in between the Ethernet and IP header. You can also see the different fields, this header uses label value 16. We don’t use QoS and since there is only one MPLS header, the bottom of label stack bit has been set.

Where did the label value come from? MPLS uses a protocol called LDP (Label Distribution Protocol) for this. You will learn about it in the next lesson.

MPLS Devices and Operations

Now you know what the MPLS labels look like, let’s talk about a bit about the different devices you will encounter in a MPLS network. Here’s an overview:

MPLS Network Topology

Above you will find three different routers:

  • CE (Customer Edge): this device is the last device in the customer’s network, it could be a L2 or L3 device. In my picture I used a router but for example, it could be a switch. This device does not use MPLS.
  • PE (Provider Edge): this device is owned by the ISP and sits at the edge of the ISP’s network. It has an important role…it receives packets or frames from the customer and will then add a MPLS label to it and forwards towards the core. Another common name for this device is LER (Label Edge Router).
  • P (Provider): this device connects to PE routers and other P routers. It has a simple job, it switches packets based on their labels or removes the labels. Another common name for this device is the LSR (Label Switch Router) or transit router.

There are three actions we can perform with labels:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You’ve Ever Spent on Your Cisco Career!
  • Full Access to our 740 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

556 Sign Ups in the last 30 days

satisfaction-guaranteed
100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Tags:


Forum Replies

  1. Rene,

    Great lesson however, I have a question about MPLS TTL =254. It means there are 254 hops from CE1 to CE2 for each hop TTL decreases by 1 is that correct? How do we determine or define TTL value?

    Thanks

    Hamood

  2. Hi Hamood,

    That’s right, the TTL for the MPLS label works the same as the TTL for the IP packet.

    When the PE router receives an IP packet then it will copy the TTL of the IP packet into the MPLS label TTL. Within the MPLS Core, the TTL of the label decreases at each hop. When the MPLS label is popped, it’s TTL value is copied into the TTL of the IP packet.

    Rene

  3. Hi Rene,

    Thank you for the excellent explanation.

    Question is regarding the frame-size, with the MPLS header and the ethernet frame the MTU would be maximum 1504 bytes.

    It this going to be fragmented?

    Rgd,

    Hans

     

  4. Hi Hans,

    Most interfaces nowadays support Jumbo frames (9000 bytes) so the extra label(s) from MPLS shouldn’t be a problem. You should check this though and increase the MTU if required.

    Rene

  5. Dear Rene,

    What is the best practice to configure the MTU for MPLS? I see some need 1504, some 1508, some 1522 or 1526.

    Davis

38 more replies! Ask a question or join the discussion by visiting our Community Forum