meaning of Label Stack in show mpls forwarding table

[peymank64]

I am trying to understand the output of 'show mpls forwarding-table detail' command on cisco 7200 routers. For each entry in the MPLS table, I can see a set of 'label stacks'; example below:

18         35         10.0.1.1/32   0             Gi1/0      10.0.5.1 

    MAC/Encaps=14/18, MRU=1500, Label Stack{120, 35}

Does this mean that if a packet with top of stack label 18 arrives, it will go out with labels 35 and 120? What if the incomming packet has more than one label? Are all those labels removed? 

Thanks!

[Sandip Rathod · Accepted solution]

Hi

Packet is coming from lable 18 and swapping to lable 35 and going to out interface Gig1/0

Lable 120 is VPN lable, if you have VPN topology

Look at figure 27 in below document, you will understand how this works.

http://books.google.co.in/books?id=cBzK2cLtwgMC&pg=PA29&lpg=PA29&dq=label+stack+in+show+mpls+forwarding-table+details&source=bl&ots=MoXbvoRZdm&sig=aQtDWJSiIYD7_3oQLmNJrsVbjTI&hl=en&sa=X&ei=bZQmU-ONFMa2kgWnq4HIDQ&ved=0CHAQ6AEwCA#v=onepage&q=label%20st...

Let me know if you need further clarification

Regards,

Sandip

[peymank64]

Thanks Sandip. I was wondering what the behavior would be if a packet arrives with let's say labels 100 and 18, with 18 being the top-of-stack label? It technically matches on this entry, right? now what will be the outgoing label(s) look like?

[Muhammad Thanveer]

Hello,

Up to my knowledge.

If the router is a LER it will pop lable 18 and will check the purpose of the other label in the stack whether it is a VPN label or TE label and deliver it accordingly.

If the rotuer is a LSR it will swap the label 18 with the label as per the LFIB and forward it accordingly.

Regards

Thanveer

[peymank64]

Hi Mohammad,

I think the MPLS entry, regardless of where/how the router is deployed should determine the behavior. Based on the information in this entry, does a incomming packet with labels {X, 18} go out as {X, 120, 35} or just {120, 35}? 

Thanks

Peyman

[Muhammad Thanveer]

Hello,

As an example we are running vrf at R2 for R1 and Vrf for R5 at R4.

R1---->vrfR2---->R3---->R4vrf---->R5

If R1 wnats to reach R4,

R2 and R3 have a vpnv4 enabled ,think that X is an MPLS label  in the label stack and 18 being VPNV4 label at R2{X,18} then R3 swaps it to Y as per the LFIB and then sends it to R4. then R4 checks it and POPS the label Y and finds the VPNV4 label and then it takes the decission as per the label to send the ip traffic to proper vrf instance.

Hope you understand

[peymank64]

Thanks for you example. I understand this case. Now imagine you are in a carrier network with 2 layers of MPLS and a VPN label (a total of 3 labels)

I am going to modify your example as follows:

R1---->vrfR2---->[R3---->[  R4  ]--->R5]---->R6vrf---->R7

where R3 and R5 have an MPLS+VPN label and R4 has 2 MPLS + 1 VPN label.

Now if R3 has the MPLS entry 18-->{35,120}  that I should at the beginign of this discussion; and it receives a packet with [VPN label, 18] it should send the packet out with three labels: [VPN label, 120, 35], right? In that case how the router knows the full label stack? IMO, it should only know the top two outgoing labels and can only show them in the output of 'show mpls forwarding-table detail'. therefore, 'label stack' part of the output only refers to the top of the stack, not all the stack.

[Sandip Rathod]

Hi

Your packet will be swapped with top lable, which is 18 in your case

Ougoing lable will be [MPLS LABLE | VPN LABLE | IP | DATA ]

Regards,

Sandip

[Sandip Rathod]

Hi

Your packet will be swapped with top lable, which is 18 in your case

Ougoing lable will be [MPLS LABLE | VPN LABLE | IP | DATA ]

Regards,

Sandip