Basic MPLS Question

[Andrewcpp]

Hi all,

I have a question regarding MPLS. I just started to learn about MPLS and everything is good but this question just sticks into my mind and I couldn't find any question on forums or books/videos.

Let's assume we have a network with 3 routers, like below: https://excalidraw.com/#json=OpWYAl0BLNQRk07o0id47,XywKafB5vC5Tko8zN1fROwhttps://excalidraw.com/#json=2kKdcxeOdzrGIocR0zROH,U4k4PnJKTgK1imgw2UZdaQ

The problem I have is on router R3. He will assign local labels for every route that is locally known. Let's say R3 assigns local label of 16 for his loopback 3.3.3.3/32 But it will also receive a REMOTE LABEL. Let's say it receives the label 19 for 2.2.2.2/32 and 20 for 1.1.1.1/32

That Remote Label of 19, is sent by R2 and is used only by Router R2 or is also used by Router 1? Because the route 2.2.2.2/32 is reachable through both R1 and R2. Why isn't he receiving two remote bindings? Because that route is accessible via both of the Routers R1 and R2.

Thank you!

[MHM Cisco World · Accepted solution]

MPLS use IGP

R3 have only path to 2.2.2.2 which is toward R2 not via R1 and hence it add 2.2.2.2 with label egress interface is toward R2.

Same is for 1.1.1.1 the R3 add label and egress interface toward R1 only not toward both R1 and R2.

MHM

[Andrewcpp]

So, if that link goes down, so does the remote binding I received from that router, and the second after that, I receive another remote binding from R1?

[MHM Cisco World]

Yes correct 

MHM

[Harold Ritter]

Hi @Andrewcpp ,

I agree with @MHM Cisco World  that the label switch path (LSP) will follow the IGP path. The labels though will be received from both neighbors.

Let's say, R3 will receive a label for 1.1.1.1/32 from R1, but also from R2. If the direct path to R1 is up, the label received via R2 is not used, but as soon as this direct path goes down, R3 already has the label to 1.1.1.1/32 via R2 and will start using it as the IGP converges.

Regards,

[Andrewcpp]

So, you are basically saying that the router has in the cache or somewhere in the memory both labels for the same route? If so, how do I see all labels that are waiting to be used in case of an emergency issue?

Thank you!

[MHM Cisco World]

Check by 

Show mpls ldp binding 

This give you all label router send receive in router'

MHM

[Harold Ritter]

Hi @Andrewcpp ,

What you call cache is actually referred to as Label Information Base (LIB). This is where the locally allocated labels and the LDP received labels are kept, regardless if they are currently used or not. These labels can be seen with the "show mpls ldp binding" command.

Once an IGP best path has been selected, the associated label is installed in the Label Forwarding Information Base (LFIB) and the Forwarding Information Base (FIB) and can be displayed using with the "show mpls forwarding" command for the LFIB and the "show ip cef" command for the FIB.

Regards,