Driss Jabbar No Comments

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 No Comments

What is convergence in different field of networking ?

What is convergence ?


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

Orhan Ergun No Comments

Turkiyede CCDE Egitimi

Bu Turkce paylastigim ilk post olacak. Heyecanliyim. Ama daha cok , Turkiyede ve Turkce CCDE Egitimi verecek olmaktan dolayi heyecanliyim.

Takipcilerim bilirlerki 2 yildan fazla bir suredir Cisco CCDE Egitimi vermekteyim ve egitimlerime Dunyanin her yerinden 100 lerce kisi katilmistir.

Cogunlukla Online/Live olmakla birlikte, Amerikada, Dubai de , Afrika da , Qatar ve Avrupada Onsite egitimler de veriyorum.

Insanlar, basta network design ogrenmek amaciyla bu egitime katiliyorlar. Tabiki ogrendikleriyle birlikte CCDE Egitimini de gecmeleri mumkun oluyor. Iki yil icerisinde 30 dan fazla ogrencim CCDE numaralarini aldi bile. Read more

Orhan Ergun No Comments

OSPF Multi-Area Adjacency | OSPF Area Placement | RFC 5185

OSPFv2 by default setup only one adjacency over a single link. But this can be an issue some time and as a network designer you should understand the consequences and know the available solutions.

Placing a link in wrong OSPF area can create an OSPF sub optimal routing especially in hub and spoke topology.

In IS-IS or OSPFv3, this wouldn’t be an issue because IS-IS and OSPFv3 allow link to be placed in more than area or level. (In IS-IS, area is assigned to the router, not to the link. Thus I use level keyword)

Let’s look at the below network to understand what is the issue and how OSPF multi-area adjacency can solve the sub optimal routing problem. Read more

Orhan Ergun No Comments

More network design resources are available for subscribers !

As a reader of this blog, you can access all of the posts on the website for free. But if you don’t know yet, this website has a membership area.

When you become a member you get access to 50+ hours network design videos which will help you in Cisco CCDE exam as well as real-life network design. There are technology mind maps, design quizzes, design e-books and technology comparison charts.

Imagine that you have to select a technology for a particular design but you don’t know all the available tools. Comparison charts will teach you the pros and cons of each technology, protocol and where to use them in a network design.

Below resources have been recently added to the members area of www.orhanergun.net


Mind maps will help you to study technologies as well as will be quick refresher from the design point of view. More mind maps will be added shortly, stay tuned.

IPv6 Mind Map
QoS Mind Map
Multicast Mind Map
VPN Mind Map
General Design Mind Map
BGP Mind Map
GETVPN and BFD Mind Maps


Comparison Charts

New comparison charts as below.

OSPFv2 vs OSPFv3
MPLS QoS- Uniform vs Short Pipe vs Long Pipe
BGP Route Reflector vs Confederation
PIM ASM vs SSM vs Bidir



You will find questions and answers of the below quizzes in the membership area.

IPv6 Design Quiz
OSPF Design Quiz
CCDE Demo Scenario : This is similar to CCDE Practical exam. 2 hours long demo scenario.You should test yourself to see whether you are ready for CCDE Practical as soon as possible.

If you are not a subscriber yet, you can start today your membership !


If you have any question regarding subscription send an email to orhan@orhanergun.net

Orhan Ergun No Comments

50 000 Page views of orhanergun.net between March-May 2015

This blog received 50 000 page views between 1 of March and 1 th of May.I shared you couple more metrics from the site stats in addition to Pageviews.

Since at the same time two classes I teach ( Pre-CCDE and CCDE ) in addition to my other jobs, I couldn’t update the blog since a month too much. But still very good numbers and I thank all of you.


orhanergun.net statistics


A picture is worth a thousand words.

Orhan Ergun 39 Comments

ASK your questions and SHARE your opinion

I receive lots of questions from my students, readers, customers, followers on network design.

I try to  answer as quickly as possible and in detail.

Thanks to all of them ! I receive a lot of kind emails, messages from them to put my effort on this blog as well.

But for many reasons, I have created this category.

  • Those questions and answer can be beneficial to many other people if I post them here.
  • It is hard anymore to reply every email individually, at least timely manner.
  • When I share a post on social media, I receive a lot of feedback and have a good conversations there. Please share them here as well 🙂

Thus, please ask your questions, give your suggestions for the questions, share your opinion for the blog,recommend new blog categories,

So do you like this blog ? Share in the comment box below if you liked 


Orhan Ergun 6 Comments

Do you really need Network Training ?

Certification is cool, opens the door to new companies, positions , maybe to the better roles,you can proud that you have CCIE, JNCIE and so on.

Cisco , Juniper , HP, every major companies have certification program and network training is a big business. We all know that !

Have you ever asked yourself, Do you really need Network training ?

Read more