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LDP retention and distribution modes

MPLS protocol uses labels to forward traffic between point A and B. These labels are binded to FECs and distributed on the network by means of different protocols like (LDP, RSVP, BGP-LS, SPRING).

LDP (Label Distribution Protocol “RFC5036”) is still by far the widely used protocol among them and was developed to do label distribution unlike RSVP and BGP.

In this article we will discuss how LDP binds labels to  FECs and how it distributes these labels on the network.

As stated above the main goal of LDP is to distribute labels on the network so once activated on the routers LDP will try to discover directly connected peers by sending multicast messages to UDP port number 646 but it allows for discovery of remote peers using targeted hello messages that span multiple hops for some specific use cases.

Once a peer is discovered, a TCP session is established to it using the port 646 in a way to stat the exchange of Label/FEC bindings based on the features and mode that are negotiated between the peers.

To establish LSPs, LDP relies on IGP protocols and LSPs are always following the best path to destination which means that the two protocols have to work on tandem and kept synchronized to avoid any  traffic loop or blackhole situation.

 

Label retention mode:

Label Retention means what labels a router have to conserve for a given FEC. Should a router conserve labels coming from different routers even if are not all in the forwarding path? or should it conserve only labels from the IGP best next hop router to destination and discard the rest?

The first option is called Liberal Retention and the second option is called Conservative label retention.

 

To explain the difference between these two modes, let’s use the diagram below:

Assuming that all links have the same cost hence to reach Loopback 0 from R1 the path “R1″>”R2″>”R4″>”R5” will be used.

  • “R5” associate “L1” label to the connected loopback interface and send it to its neighbor “R4”
  • “R4” binds label “L2” and send it to both “R2” and “R3”
  • “R3” binds label “L3” and send it toward “R2”
  • “R2” binds label “L4” to the FEC represented by the Loopback0.

 

From here we can raise to points :

  • R2 receive two labels to the same FEC,one from “R4” and the other from “R3”
  • Each router advertises a Label to its peers and expects receiving the flow tagged with that label. This method is called downstream distribution

 

The first remark is about retention mode. In LDP we have to mode:

  • liberal retention mode: on which “R2” will retain label from “R3” and from “R4” even if in normal circumstance the LSP will be establish through trough the short path “R1″>”R2″>”R4″>”R5” this mode is resource consumer as the router will install in the Label Information Base some labels that won’t be used immediately but in case of “R2” “R4” link failure the label will switch swap rapidly to the label received and installed from “R3” earlier to repair the LSP.
  • Conservative retention mode: on which R2″ and on the reception for label “L3” from “R3” it will discard it. Which make sense in case you have to preserve resources and uses them for other processes but this method comes at cost since unwanted labels are discarded,if the link between “R2” “R4” fails the traffic will be blackholed until the reception and the installation of the Label from “R3”.

Within today network, Routers come with large memory capacity to handle a large label space so the Liberal retention mode is the widely used mode.

The second remark is about the mode of label distribution:

Based on RFC3031: ” the decision to bind a particular label L to a particular FEC F is made by the LSR which is DOWNSTREAM with respect to that binding.  The downstream LSR then informs the upstream LSR of the binding.  Thus, labels are “downstream-assigned”, and label bindings are distributed in the “downstream to upstream” direction ” If we apply the RFC above to our diagram when “R4” receives label “L1” to loopback 0 from “R5” it assigns a label “L2” and sends it downstream to “R3” and “R2” then R2 expect that for all traffic going to Loopback 0 be tagged with “L2” label in the incoming interface before it swaps the label to “L1” and send it upstream to “R5”. This method is called downstream allocation  In the other side should “R4” send label for the Loopback0 to “R3” and “R2” (unsolicited label distribution) or only for routers ask for them (On demand label distribution)?. Having known about the unwanted blackhole situation that will be raised if the second option has been used. The vast majority of vendors prefer the first mode of label distribution.  So, the label distribution mode used by LDP is called downstream unsolicited.

Orhan Ergun 2 Comments

Microwave or Fiber which one is faster ?

Microwave or Fiber which one is faster ? I will explain  the faster connectivity option and some of the use cases for each, deployment considerations a bit in this post. Why latency is important for some special businesses  ? Have you heard about HFT (High Frequency Trading) ? If you like the discussion points, after reading the post, share your comment, let me know it so I can write more on this subject.

 

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What is convergence in different field of networking ?

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This term has different meaning for different networking groups. In this post, I will explain the two most common usage. This kind of knowledge definitely will differentiate you from the others 🙂 Read more

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Orhan Ergun Youtube Channel

Orhan Ergun Youtube Channel

I have been sharing technology based videos on my youtube channel for some time. I will hopefuly share more often public videos out there !

I just don’t want to share technical videos, but my training updates, bootcamp announcements, success stories and the things which make me happy or mad !

If you want to follow me on Youtube, click on below link and subscribe to my channel !

Youtube Channel Link

 

 

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2G is still the most common Mobile network technology

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Istanbul/Turkey Onsite CCDE Training – 50% OFF until Sep-15, 2017

Istanbul/Turkey Onsite CCDE Training will be held between October 30 – November 4.

 

Course will be in English as usual, everyday will be between 9am – 6pm, 9 hours.

 

I am going to extend my CCDE Materials for this course as there was new scenarios and the technologies after August 29, 2017 CCDE Practical/Lab exam.

 

When you register for this course, you will have Self Paced CCDE Training and Self Paced Service Provider Training for free.

 

Also attendees will have two of my books; CCDE In-Depth and Service Provider Networks.

 

You probably heard my free retake offering. When you register for Istanbul Onsite training, you will be able to attend other Online CCDE Trainings such as October 16 – 25 Webex, for free.

 

Please know that, four days after this training, there is a CCDE Practical/Lab exam and you can attend the exam in Istanbul. Let me help you to pass !

 

Just for Istanbul/Turkey CCDE Training, until September 15, 50% discount is valid. Please don’t ask for a discount after September 15, 2017.

 

For more information :  sales@orhanergun.net

 

Course agenda and registration please click here

 

Orhan Ergun’s CCDE Training is the most recommended CCDE Training in the World. You can talk with the existing CCDEs for the feedback.