How many labels for VPN in MPLS ?
How many labels for VPN in MPLS ?
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
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.
From here we can raise to points :
The first remark is about retention mode. In LDP we have to mode:
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.
What is deadlock situation in MPLS Traffic Engineering ? What happens when deadlock occurs ? Is there any mechanism to prevent deadlock ? I will explain all the details in this post.
What is MP, Merge Point in MPLS Traffic Engineering ? Understanding Merge Point in MPLS Traffic Engineering will help you to understand the MPLS TE – Fast Reroute
Bin Packing Problem ? What is Bin Packing ? I will explain in this post Bin Packing Problem in MPLS Traffic Engineering.
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.
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.
Unique RD per PE per VRF is a deployment option in MPLS Layer 3 VPN.
What is PLR, Point of Local Repair in MPLS Traffic Engineering ?
MPLS Traffic Engineering Fast Reroute Link Protection – I will explain it in plain English and the most important point of MPLS TE FRR Link Protection among other Fast Reroute mechanisms.
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
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.
What makes for a successful protocol ? Which protocol is successful and why ?
Have you ever been asked these questions ?
Is MPLS mandatory for Traffic Engineering? Read more
What is VPWS , VLL , EoMPLS ? Read more
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
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.
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.
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.
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
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
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
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.
Figure -1 MPLS network PE, P and CE routers Read more
MPLS Design Question – MPLS is one of the most commonly used encapsulation method today. Especially on Wide Area Networks of the Service Providers, Large Enterprises and some datacenters.
Service Providers sell MPLS services to customers for decades. When customers want to have Service Provider redundancy so two MPLS circuit from different Service Providers, Inter AS MPLS service is created between those providers.
Or customers have a location where their MPLS service provider don’t have there but customer wants to have end to end MPLS connectivity between their all locations, again Inter MPLS Service is created between the providers.
If you are a visual learners and want to understand the basics of Inter AS MPLS VPN Options, check my youtube channel and subscribe for the more design videos.