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targetted ldp


how does label distribution happens in a targeted ldp session..especially if two hops are there between the targetted ldp neighbours..

consider the below scenario..

VRF1a.Router A---B---C--D.vrf 1d---------- targetted LDP between A and D.

it forms a session , exchanges label mappings for each route in igp routing table, so a label is advtsed to A for loopback ip of B , C and D

so if traffic from vrf 1 a to vrf 1 d comes..whether label for lo of d is used between a and D (as LDP session is between these two), or label swapping

happens in B and C..?

Thanks in advance,



Re: targetted ldp

I must say a very good question.

Its purely a question for which the answer revolves around the control and the forwarding plane. This is what will happen in this scenario.

1) A will receive Label Bindings for all the IGP routes from two neighbors, B and D. (Different Labels for the same prefixes from two different neighbors)

2) D will receive Label Bindings for all the IGP routes from two neighbors C and A.(Different Labels for the same prefixes from two different neighbors)

3) These Label Bindings will create our LIB, which is nothing but the Label Information Base, which contains LDP ID to Label to Prefix Mapping per neighbor. (You can view this by "show mpls ldp bindings")

4) Now what happens with these LIB is not all the received bindings are used.Because when the PE learns a label for a prefix from an LDP peer, it must be able to determine whether that peer is currently a next hop for the prefix to determine whether it needs to start using the newly learned label when forwarding packets that match the prefix.

So what happens is the Label which was received from B at A will be used. Also in case of D the same would happen and it will use the label bindings received from C and not from A.

So for the traffic received from lets say VRF at A which needs to go to D will go Hop by Hop with normal label swapping.

This happens as the next hop for a prefix should be equal to the LDP ID from which the label binding was received. Other bindings are just preserved in the LIB.

Once this is clear the next question which may come to mind is how it happens for ATOM, which also uses directed LDP.

Some food for thought :-)

Happy Learning!!



Re: targetted ldp

IF you have anymore queuries regarding the same, feel free to revert back.



New Member

Re: targetted ldp

Hi Swaroop,

Thanks for your reply..I think you are mentioning about NHLFE ,here in your first reply...

I have two more queries..

Pls refer the below diagram..

Vrf1a..Router A ---B ---C--Vrf1d

I am running targetted LDP between the lo A and Loopback C..

1st scenario :

1) Running an RSVP beteen A and B (physical links )

2) Also running RSVP between B and C (physical links).

3) No LDP between A and B and B and C.

Question : For a traffic between A and C ,what will be the label stack..?

Is it RSVPlabel--LDP label--BGP label

or LDP label--BGP label.

Scenario 2:

1) Digaram same..

Instead of single hop RSVPs,If I am running RSVP between A physical link to C physical link ,constrained to pass through the loopback of B ..what will be the label stack..?

Scenario 3:

Its pretty straightforward.

Same as in scenario 1 ,RSVP single hop between A and B , B and C,

and if I am having LDP also between A and B and B and C ,along with targeted LDP between A and C ..

I think label stack wll be , LDP label--BGO label..

Any thoughts / answers are welcome !!

Thanks in advance ,


Re: targetted ldp

Wishing all a Very Happy Dusshera!!

Ok now lets break this down.

NHLFE is all about how you take your forwarding decisions based on elgible received Label bindings. What i tried to mention in my previous post was what is considered eligible.

(This refers to RFC 3036 - Section 2.7)

Now, coming to your scenarios, if you go by basics the answers would be very clear,

As they all are Label ditribution protocols. A Stack will only be created if one protocol is using the LSP created by another protocol to learn the label information from its peer.

Just running multiple label distribution protocols together wont give a Label Stack,

1) In this again you will not use your directed LDP labels as the LDP ID cannot be reached as the next hop. The traffic will go out with a Tunnel Label only. As your RSVP can generate labels

using the TE entension to create LSP Tunnel so you get a Tunnel Label. Refer to RFC 3209.

"""VPN LSP Path Break."""

2) Again there is no label stack. As the directed LDP wont be established in this case.

As the RSVP tunnel doesnt support LDP by default.

Having said that if you enable RSVP tunnels for LDP then you will get a stack if there is anything downstream your router C. Or else it would be a PoP label for C loopback. with the Tunnel Label. In this Case its VPN will go with a """VPN + TE label."""

3) In this scenario even though for LDP the neighbors are connected directly. Because of the RSVP tunnels the sessions between each neighbors would be a directed session.

Again there wont be any stack if your tunnels are not enabled for LDP or TDP.

If enabled then the stack would be your LDP/TDP label + Tunnel Label.

"""VPN LSP Path Break."""

End of the day, we need to understand the basic thing that all RSVP-TE, LDP, BGP are label distribution mechanisims. If one protocol provides an existing LSP for another protocol to

ride over and learn some more information with a certain peer, then only a stack comes into picture. Or else only Tunnel Label prevails as that is the only thing which figures up in your

forwarding table. And in these examples we havent considered your end VRF's. which is bound to suffer in these 3 scenarios

PS: If you plan to run a full network without LDP, then you should not run tunnels Hop by Hop basis. You sould run then across edges in a full mesh.

Refer to this link as well it has numerous such combinations.

If you have any more questions :-), feel free to revert.

Happy Learning!!



Re: targetted ldp

Hows the learning going.

If you have had any doubts feel free to revert. Or let me know that you understood, that should be good.



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