MPLS Design Case Study

MPLS Design – MPLS Design Case Studies are the useful resources to learn the design details of MPLS.

You will be provided a brief information about the business and technical requirements. You can share your answer at the comment section below.

Note: This is only one of the case studies in the DesignWorld.

DesignWorld provides video and written content for the network designers.

Smallnet is a fictitious VPN Service Provider which provides Business Layer 2 and Layer 3 VPN services to their customers.

Bigger com is a transit service provider of Smallnet which provides layer 3 service for the Smallnet customers.

Smallnet wants to carry all its customer prefixes through BGP over Biggercom infrastructure.

Biggercom’s SLA allows only 1000 prefixes from the Smallnet but Smallnet has around 3200 customer prefixes and the number of prefixes grow rapidly.

Please provide a scalable solution for the Biggercom.

Should Smallnet change the routing protocol from BGP to IGP  ?

Is your solution provides scalability for the Smallnet as well ?

Is there any drawback with your design ? Please explain.

Most scalable solution for the Biggercom would be Layer 2 point to point or Layer 3 VPN service which has small amount of customer sites and small amount of prefixes from that customer.

In this post, I choose Biggercom to provide Layer 3 VPN service to Smallnet. But based on the given information Layer 2 VPN  would be a solution as well.

If Biggercom provides a Layer 3 VPN service, 3200 customer prefixes of Smallnet is too much and it is not in the SLA of Biggercom (1000 customer prefixes, this is given in the real life as well )

Smallnet only can carry 3200 prefixes by hiding them from the Biggercom.

Hiding is done through MPLS encapsulation between Smallnet and Biggercom.


Carrier Supporting Carrier

Figure-1 Carrier Supporting Carrier Architecture

If MPLS is enabled between two providers then the solution is called Carrier Supporting Carrier.You can hear it as Carrier of Carrier as well.

This architecture is covered in detail here.

Should Smallnet change the routing protocol from BGP to IGP ?

Smallnet uses BGP to carry customer prefixes and IGP for the Infrastructure. In the Service Provider environment this is always the case.

IGP is used to carry transport addresses such as PE loopbacks and BGP is used to carry Internet or VPN prefixes.

That’s why answer is no.

Note: BGP could be used to carry PE loopbacks if the MPLS is extended to the access domains, since the number of access devices (PE in this case) loopback addresses would be huge. This architecture is called Seamless MPLS and I explained it in detail here.

Is Carrier Supporting Carrier provides scalability for the Smallnet ?

Yes it does. Smallnet only advertises its PE and/or Route Reflector loopbacks to Biggercom and when two discontiguous sites of Smallnet reach each other, MP-BGP neighborship is setup. Over MP-BGP VPN prefixes of the Smallnet customer is sent.

Is there any drawback with this design ?

Yes there is. Actually there are a lot but let me mention couple of them. Since with this architecture, customer carrier (Smallnet in our example) doesn’t control the path in the Service Provider domain, end to end SLA should be negotiated between Smallnet and Biggercom very carefully.

Biggercom may not be able to provide IPv6, Multicast and QoS services to Smallnet or cannot provide the required level of flexibility. (Smallnet might provide BGP FRR but Biggercom may not)

Many other problems can be discussed with the L3 nature of Carrier Supporting Carrier.

If Smallnet would receive an L2 transport from the Biggercom, IPv6 , QoS , Multicast and other services would be transparent to Biggercom and Smallnet would manage its Wide Area Routing itself, would have better protection for the critical traffic and so on.


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