MPLS Label Operations: Pushing, Swapping, and Popping Explained

Hello! This is Lazarus at telecom.Tech, where  telecom and networking technologies... are simply   explained! Today we'll be simply explaining  the pushing, swapping, and popping of MPLS   labels. As always, don't forget to subscribe,  enable notifications, and like this video if  

you like this video! And if you have questions or  feedback, please write them in the comments below!   I'm always happy to read them and to respond. Last  time, we took a closer look at how LDP operates   within the framework of MPLS. In this video,  we'll take a look at the mechanisms involved  

in adding switching and removing LDP labels.  In MPLS terminology, this is called pushing,   swapping, and popping labels. Now it's important  to understand the fundamentals of this process now   because in later videos we'll be talking about  how MPLS interacts with other features such as  

Layer 3 VPNs. In these cases you can have label  stacks where you can push multiple labels onto a   packet to provide traffic segmentation based on  customer networks. We'll talk more about that in   a future video, but to understand these concepts,  we'll first need to know how pushing, swapping,  

and popping labels takes place. Here we have two  customer devices, CE1 and CE2, and we have an MPLS   infrastructure that's being used by the ISP to  interconnect these two sites. In our scenario,   some host on the 192.168.11.0/24 network is  sending data to the 192.168.12.0/24 network.  

Remember the CE routers know nothing about MPLS,  so they're going to send their data based on the   routing protocol that's being run between the CE  and PE routers here. So the packet arrives at PE1.   PE1 looks at the destination IP address and does  a lookup in its LFIB. It sees that the local label  

is 101 and the outgoing label of the next hop node  which is P1 is 201. Now at this pointm I'd like to   say a few words about the actual label values. The  label is a 20-bit field within the MPLS header.   Label values 0 to 15 are reserved for special  purposes and cannot be assigned to specific  

prefixes. If you're interested in finding out more  about those reserved labels take a look at a link   to the related RFC in the description below.  So by default, in most vendor implementations,   LDP labels for MPLS are assigned starting with  a value of 16. This is certainly the case for  

Cisco devices while other vendors may choose  a different implementation. Since labels are   locally significant, it doesn't really matter  what actual values are used. So if you leave   all configurations at their default you will most  likely see that the same label is being used by  

multiple routers along the path. Now because we  humans need some way to differentiate between   the actions taken by each node in the path, it's  always a good idea to change this default behavior   such that we can more easily determine which  labels are assigned by which nodes. For example,  

you can make PE1 here use a range of labels from  100 to 199, make P1 here use a range of labels   from 200 to 299, and P2 to use a range of 300 to  399, and so on, and you can do that for all their   label assignments. That way you can know which  node generated which label. For now, let's say  

that we've already done this for our topology. In  a later video, when we get into configurations,   we'll show how this is actually implemented.  For now, let's take a look at what the PE1 node   does with this packet. As it sends the packet  out, it will push a label onto it. Remember,  

pushing means adding a label. Specifically,  label 201 is pushed. You can see that value   in the outgoing label column of the LFIB in PE1.  How did PE1 know to push that label? Because P1   shared with PE1 that its label for that particular  network is 201. We can also see the exit interface  

as well as the next hop IP of P1 within the LFIB.  Remember that the next hop IP is the IP addressing   that's used in the underlay network that supports  the MPLS infrastructure. Now if we look inside the   MPLS header, we'll see that label 201 has been  added or pushed into the appropriate field. Now  

you'll note that PE1 here has a local label for  the 192.168.11.0/24 network that local label is   actually never pushed onto the packet. In this  specific scenario, this is because the PE1 router   is the first MPLS router that is encountered along  the path so that label is never pushed. Only the  

remote label, that is, the label advertised by  P1, is pushed in order to forward the packet   appropriately. However as we will see later on,  this label will be used when MPLS is employed with   Layer 3 VPNs, but at this point, I digress. Let's  go on to what we're looking at in this video. So  

that was label pushing. Now P1 will take a look  at its own LFIB and we'll find the destination   prefix, and we'll see that the defined outgoing  label is 301. We also see the exit interface and   the next hop IP of P2 of the underlay network,  which is actually our next hop node. This label  

and the additional information was learned via  LDP, so P1 will perform label swapping at this   point. It will remove its own label of 201  which is how the packet arrived and, it will   swap it with the remote label, or outgoing label  for this particular destination which is 301,  

and it will send it to P2, which is the next hop  node. Now once the packet reaches the P2 router,   it will look up the destination in the LFIB and it  will see an outgoing label of "Pop label." This is   the penultimate hop popping process that we talked  about in the previous video. The last P router in  

the path, which in this case is P2, will pop  the label, or in other words, remove the label   before it is sent to the final PE router which  is PE2. As you can see, the MPLS header here is   removed completely during the popping process and  thus the data arrives at PE2 with no label. That  

means that PE2 here will simply use traditional  routing to get that packet to the destination,   which is behind find this C2 router here. So  that covers the fundamentals of label pushing,   swapping, and popping, all fundamental operations  found within MPLS. I hope you found this video  

useful, and if you have, please click that thumbs  up button! If you have any questions feel free to   leave a comment, and please subscribe to get  updates to newly published videos. I'm Lazarus   at telecom.Tech, I hope this has been helpful  for you and I'd like to thank you for watching!