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MPLS (Pop tag, stacked label, LSP, FEC) ??

Hello People,

I am studying EIGRP configurations and verifications and I come accross a sub topic on EIGRP Over MPLS.  I take this to mean that I need to understand the rudiments of MPLS in order to be able to configure EIGRP over it.  However, I do not know MPLS.  I am trying to read materials on MPLS, but the topic has been rather intractable and frustrating.

I have attached a Microsoft Word document containing the first few areas of confusion.  Please, oblige me with your comments.




vivek srivastava
Rising star

it will be good if you copy0paste the quarries directly on the portal.

\from your doc I tried to ans the qus as much I can, but yes they are many ways to explain so actually it depends on you in-terms of what you understands. Below are the remarks:


Please, I need clarifications on the following:

  1. How is the label stack formed with respect to my network diagram above?

[ans] since you are simulating communication from RA to RB, so just assume (like we did in physics classes i.e. let’s assume a variable x and bla .. bla) what R3 does when attached to network.  In MPLS network, the LSP formation is actually in reverse to the traffic flow. Here R1 knows about the network 172.x.x.x and he advertise to R2 that he (R1) can reach 172.x.x.x and if R2 want to reach 172.x.x.x then he (R2) has to attach a label of 15 (outgoing tag of R2) and send the packet from R2 to R1. Same is applicable from R3 to R2 as well.

  1. Under which condition is more than one label pushed onto the label stack?

[ans] there are 3-4 ways.

If you are using VRFs then an additional label is attached (inside the MPLS label which you know in above example). It will be looked only when outer label is removed

  1. Under which condition does the pop operation take place?

[ans] in your example, R1 knows that 172.x.x.x is directly connected to it. So R1 advertise a POP label to R2 that 172.x.x.x network is known as “directly connected” to R1. Then R2 will remove the tag-15 and sends a normal IPv4 packet to R1. R1 looks LFIB and POPs the packet.

Though practically, lable 0-16 are reserved and you will not see in without any tuning.

  1. What does the "Pop tag" in the table above mean?

[ans] refer above. POP tag means that the network is directly reachable via the next MPLS HOP.

  1. What would be the Label Switch Path from R1 to R3?

[ans] refer 1st ans. it is actually reversed i.e. R2->R2->R1

Thanks, but I am still confused, really.  Look at the inserted image below.  The routes that have the "Pop Label" are not directly connected to R1.


Again, its atually in reverse-direction.

Since you have PEs connected directly then there will be no lable swaping. remember PHP ??

For R1-R3, and are directly connected to R3 so it will send a Implicit Null label to R1. So when you see LFIB on R1 it will show a POP label.

I think you need to refer below example:


Thanks Vivek, I do not understand the explanation of directly connected network.  I have followed the link you recommended and I could not find explanation relating to why some entries in the LFIB may have the "pop tag" in the outgoing tag.  Please, look at the image inserted below.

Networks,,,, are all directly connected.

In the LFIB, networks and do not have the "pop tag".


Also, I do not know the meaning of PHP.  Nice if you could expatiate a bit.



Like Vivek mentioned, each LSR while label for prefixes in routing table (directly connected prefixes or IGP learnt) will assign label 3 if the prefix is directly connected. This information will be seen in "show mpls ldp binding" output. For example, On Pomero1, if you execute this command, you should see local binding as "imp-null" which is label 3.


This will be advertised to neighbors via LDP. Any LSR idrectly connected to Pomero1 will have "PoP" as action in LFIB table. This information will be seen in "show mpls forwarding-table" output.


For your queries above:


Networks,,,, are all directly connected.

<Nagendra>These prefixes are 1 hop away and not attached to Pomero1. So LFIB table will have POP action as those neighbors advertise label 3 for these prefixes. 


In the LFIB, networks and do not have the "pop tag".


<Nagendra> Yes. They are 2 hops away. So for, on Pescara "show mpls ldp binding" will show local binding as imp-null, Pauillac will show "show mpls forwarding-table with outgoing tag as PoP".