Orhan Ergun No Comments

Segment Routing and MPLS VPN

MPLS VPN and Segment Routing. Many people have been asking what are the differences between MPLS VPN and Segment Routing.

 

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Make before break and Break before make

Make before break and break before make. I shared many posts so far which was covering the terms used in different field of networking. This one is one of them. Also I will introduce, probably to many of you, a new terminology ‘ Break before make ‘  Read more

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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 No Comments

What does P router mean in MPLS ?

What does P router mean in MPLS ? It is used in MPLS VPN networks mostly but can be used with any MPLS application, use case.

 

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What is Attachment Circuit in MPLS VPN ?

What is attachment circuit in MPLS VPN ? Definitions are important in networking, if there are alternative usages of the definition, better to know them all for effective communication.

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Orhan Ergun 2 Comments

Is Inter-AS MPLS VPNs commonly deployed ?

Is Inter-AS MPLS VPNs commonly deployed ? In real-life deployment which Inter-AS MPLS VPN Option is most common ? What are the use cases of Inter-AS MPLS VPNs ? This is not a theory post , I will share practical information with you. Read more

Orhan Ergun 4 Comments

What is MPLS Traffic Engineering and Why do you need MPLS-TE ?

MPLS Traffic Engineering is a mechanism that provides cost savings in an MPLS networks.

 

How cost saving can be achieved  ?  How traffic is steered to the paths which wouldn’t be used in normal circumstances ?  I will explain in this post.

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Orhan Ergun 1 Comment

MPLS Layer 3 VPN Deployment

MPLS Layer 3 VPN Deployment

In this post I will explain MPLS Layer 3 VPN deployment by providing a case study. This deployment mainly will be for green field environment where you deploy network nodes and protocols from scratch. This post doesn’t cover migration from Legacy transport mechanisms such as ATM and Frame Relay migration as it is covered  in the separate post on the website. Read more

Orhan Ergun 2 Comments

MPLS Transport Profile (MPLS-TP) Basic Explanation and Key Points

MPLS Transport Profile (MPLS-TP)

Multi-Protocol Label Switching Transport Profile (MPLS-TP) is a new technology developed jointly by the ITU-T and the IETF. The key motivation is to add OAM functionality to MPLS in order to monitor each packet and thus enable MPLS-TP to operate as a transport network protocol.

 

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Orhan Ergun 1 Comment

MPLS over IP Encapsulations and Comparison between MPLS over LSP

Do you need an LSP for MPLS ?

In this post, I will go through below topics. This is one of the points which network engineers struggle to understand as I have seen.

  • What is an LSP (Label Switched Path) ?
  • What was the purpose of having LSP in the first place?
  • Do we need an LSP for MPLS and MPLS Applications such as 2547 VPNs ?
  • MPLS over LSP vs. MPLS over IP Encapsulations
  • MPLS VPN  infrastructure in 2017

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Orhan Ergun No Comments

Benefits of MPLS – Why MPLS is used ? – MPLS Advantages

Benefits of MPLS, Why MPLS is used on today networks and the Advantages of MPLS will be explained in this post.

As an Encapsulation and VPN mechanism, MPLS brings many benefits to the IP networks. In this article most of them will be explained and design examples will be shared by referring more detailed articles on the website as well.

Below list shows the benefits of MPLS. Although this may not be the full list, comment section always open for your comment and questions.

 

  • Faster packet processing with MPLS compare to IP

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Orhan Ergun No Comments

BGP in MPLS Layer 3 VPN – BGP as a PE-CE Routing Protocol

BGP can be used as a PE-CE routing protocol in MPLS Layer 3 VPN. Also Service Providers run MP-BGP (Multiprotocol BGP) if they have MPLS Layer 3 VPN.

In this article, MP-BGP will not be explained since it has been explained here earlier in detail.

When BGP is used as a PE-CE routing protocol between the customer and its MPLS Layer 3 VPN Provider, it is important to say that there is no need to redistribute on the Service Provider PE. All the other routing protocol require redistribution from MP-BGP into the routing protocol and vice versa.

In this post, I will explained what are the design considerations when BGP is used as PE-CE routing protocol in MPLS Layer 3 VPN and the how you can mitigate possible routing loop problems.

Last but not least, I will share when CE (Customer Equipment) is multihomed to two PEs (Provider Edge Device), what would be the design considerations if BGP is used in MPLS Layer VPN. Read more

Orhan Ergun 6 Comments

Inter-AS Option AB (a.k.a Option D)

Inter-AS Option AB is also known as Option D or Hybrid Inter AS Option. It is called Hybrid because Inter-AS Option B uses the best capabilities of Inter-AS Option A and Inter-AS Option B. These capabilities will be explained throughout this post.

MPLS VPN providers often need to inter-connect different ASes to provide VPN services to customers.

Inter-AS Option AB first deployed by Cisco but today many vendors including Juniper provides Inter-AS Option AB feature.

But what are the best capabilities of the Inter-AS Option A and Inter-AS Option B ? Read more

Orhan Ergun 1 Comment

What is MPLS tunnel label and why it is used ?

In networking we use many times different terms to define the same thing. MPLS tunnel label or transport label are just two of those.

Not only transport and tunnel labels but also other terms are used to define the same thing which these labels provide.

Let me explain first why and where MPLS tunnel label is used. Read more

Orhan Ergun 8 Comments

What does PE-CE mean in MPLS ?

What does PE-CE mean in the context of MPLS ? What is CE , P and PE device in MPLS and MPLS VPN ?

 

These are foundational terms and definition in MPLS.

 

MPLS is one of the most commonly used encapsulation mechanism in Service Provider networks and before studying more advanced mechanisms, this article is must read.

 

In order to understand PE-CE, we need to understand first what are PE and CE in MPLS.

 

Let’s take a look at below figure.

 

Note: If you are looking for a much more detailed resource on this topic, please click here.

 

MPLS VPN PE-CE

Figure -1 MPLS network PE, P and CE routers Read more