Inter AS Option C – Design Considerations and Comparison

Inter AS Option C is the most complex, insecure, uncommon, but extremely scalable inter provider MPLS VPN solution.

I am explaining this topic in deep detail in my Self Paced CCDE course.

In this post, I will explain how service providers can use Inter AS Option C to assist customers to have an end-to-end MPLS VPN service.

In the Inter AS Option B post, I explained that ASBR routers between the service providers do not keep a VRF table for the VPN customers.

As depicted in the fig.1 (shown below), as for Inter AS Option B, MP-BGP VPNv4 session is set up between service providers’ ASBR PEs.

Inter-AS Option B

Figure 1: Inter-AS Option B

As for Inter AS Option B, ASBR routers – the provider-edge devices between the service providers – maintain only the VPN prefixes of the customers in the BGP table.

In fact, I have shown that VPNv4 BGP session has been set up between the ASBRs.

The high-level operational differences between Inter AS Option C and Inter AS Option B are in two folds: one is that ASBRs do not have VRF table; the other is that unlike Inter AS Option B, Inter AS Option C do not keep VPN prefixes for the customer on the ASBR.

ASBR is the edge routers which are used to connect the Service Provider each other.

As depicted in fig. 2 (shown below), as for Inter AS Option C, the VPNv4 BGP session is set up between route reflectors of the service providers.

Inter AS Option C

Figure-2 Inter AS Option C

In fig.2, there are two service providers: service provider A and service provider B.

Service Provider A:

Service provider A has two customers: customer A and B

For scaling purposes, all the provider-edge routers run VPNv4 BGP session with VPNv4 route reflector.

Service provider B:

Service provider B has two customers: customer A and B

A and B are companies which require Inter AS MPLS VPN service.

Service provider B runs IS-IS internally ( It could run other IGP protocols as well for sure)

PE-CE routing protocols are enabled on the VRF; thus service provider A has two separate VRF table.

For scaling purposes, all the provider-edge routers run VPNv4 BGP session with VPNv4 route reflector.

Inter AS Option C runs by enabling VPNv4 BGP session between the Route Reflectors of the Service Providers.

Provider-edge routers of service providers do not set up VPNv4 neighborship neither with AS nor between ASes.

In order to use the route reflectors to setup VPNv4 neighborship, they should move towards each other. i.e. BGP runs over TCP.

For reachability, route reflector loopback interfaces are advertised on the global routing table of service providers.

As for AS Option C depicted in fig. 3, ASBR PEs of the service providers run IPv4 BGP session between them.

Over IPv4 BGP session, loopback interfaces of route reflectors and internal PEs are advertised. Next, I will explain why you need to advertise internal PE loopback addresses, not just route reflectors.

ASBR PEs know the loopback address of Route reflector through OSPF in the Service Provider A network, through IS-IS in the Service Provider B network.

Since the VPNv4 EBGP neighborship is set up between VPNv4 RR of the service provider, the next hop for the VPN route is the route reflector.

Traffic trombones on the RR unless ” no bgp next-hop unchanged ” configured on the Route Reflectors. This is one of the important caveats in Inter AS MPLS VPN Option C.

Once this feature is enabled, route reflector does not change the next hop to itself even though the peering is EBGP VPNv4, between the RRs. ( Whenever there is an EBGP session between two BGP speaker, next hop is changed since they are not client of each other but regular EBGP neighborship, otherwise RR doesn’t change the next hop in IBGP)

In this case, because internal PEs continue to be a next hop for the customer prefixes, internal PE loopback is advertised on IPv4 BGP session between ASBRs.

As for the VPN next hop, transport label should be assigned to MPLS VPN to work efficiently, and MPLS label is assigned to the IPv4 unicast routes, which are the loopback of RRs and Internal PEs of ASBR.

Inter-AS Option C Design Objectives:

  • Inter AS Option C is unique, since it requires internal addressing advertisement between the service providers.
  • Those addresses are leaked into the global routing table of the providers, a process that providers dislike.
  • It can still be used in large scale design if the company has multiple autonomous system. (Not between the service providers).
  • This is just a network design option. Please note that it is not the only option.
  • Inter AS Option C is very difficult to implement because it requires meticulous redistribution at the edge of the networks, VPNv4 neighborship between the route reflectors, IPv4 BGP neighborship between the ASBRs, label advertisement, route target agreement between the service providers, and so on.
  • It is insecure because the internal address leaks between the providers.

I know you must have thought whether AS design is common between the service providers in real life.

In reality, I helped two times Inter-AS MPLS VPN design of the Service Providers, both of them concluded with Inter AS Option A.

To have a great understanding of SP Networks, you can check my new published “Service Provider Networks Design and Architecture Perspective” Book.

Created by
Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

View profile

Daniel Lardeux
Daniel Lardeux Senior Network Consultant at Post Telecom

I passed the CCDE Practical exam and Orhan’s CCDE course was very important contributor to my success. I attended the CCDE course of Orhan Ergun in July and it was exactly what I needed, Orhan is taking the pain to break down the different technologies.

Roy Lexmond
Roy Lexmond Senior Network Designer at Routz CCDE #20150017 & CCIE R&S; #26557

After I attended Orhan Ergun’s CCDE course I passed the CCDE practical exam.I really enjoyed the course a lot ...

Nicholas Russo
Nicholas Russo Network Consulting Engineer (CCDE/CCIEx2), Cisc

I signed up for Orhan’s CCDE training. This training is very technically detailed and the use-cases, quizzes, scenarios, and mind maps are all great resources in the overall training program. Orhan teaches his students to think like a network designer ...

Slide Heading
Slide Heading Network Systems Engineer at Conscia A/S CCIE #42544 (SP) & CCDE #20160015

Orhan is forcing you to take off the implementation hat that most of us have been wearing for many years, instead he is providing a new fancy design hat, which makes you see and deal with the issues presented ...

Kim Pedersen
Kim Pedersen CCIE in RS and SP (#29189) CCDE#20170021

I’ve used Orhan’s self-paced CCDE training material. If you are interested in knowing how all the technologies go together in a coherent design i can highly recommend it.I also enjoyed the Quizzes which helped pick out my weak spots in selecting ...

Laurent Metzger
Laurent Metzger 3xCCIE/CCDE Senior Network Architect

Hi Orhan. I passed the CCDE exam on February 22. I read everything that you put on your Self Paced CCDE Training course and it was very helpful in my success. Thank you very much.

Martin J. Duggan
Martin J. Duggan Network Architect at AT&T;, Ciscopress Author CCDE #20160006 & CCIE#7942

I attended Orhan’s April 201610 days CCDE Bootcamp. I am CCDE now !

You can tell Orhan has a great deal of experience, it really comes through when he presents his design case studies and the CCDE Practical scenarios.

Muhammad Abubakar
Muhammad Abubakar Lead Network Architect – CCDE #20160016 2xCCIE #26693 2xJNCIE VCIX

Your excellent CCDE materials and amazing Bootcamp helped me tremendously through my learning journey.Also thank you very much for being available whenever I have a design question or a complex design topics. I can’t compare your design skills ...

Jennifer Pai
Jennifer Pai Network/Security Engineer at KNET Technology

Thanks Orhan very much for this course. It helped strengthen my “Network design mind”.

Ruslan Silyayev
Ruslan Silyayev Solution Architect at R.I.S.K Company

Training by Orhan is not a CCDE preperation training only. It will be useful for engineers which are dealing with design. You want to pass CCDE exam or learn network design, then don’t look at anywhere else!

Sameer Meher
Sameer Meher Solutions Architect at 23 Wards/Japan

Orhan Ergun’s CCDE course was really very good. CCDE Level Intelligence was delivered very well and with very useful case studies and the scenarios, I am thankful to Orhan for all his help!

Ken Young
Ken Young Senior Technical Architect Province of Nova Scotia, 2xCCIE #41597 | CCDE #20170047

If anyone wants to understand network design and architecture, also pass CCDE exam , I recommend you to attend Orhan’s online courses! I am a CCDE now but learning is a journey, we will be together in your other courses too Orhan!

Matt Cross
Matt Cross Technical Architect at Heartland – CCDE #2019::7

Orhan did an excellent job of filling in the gaps of knowledge that I had that took me to the finish line of the practical exam CCDE. The community of people that Orhan facilitates are both engaging and supportive of the journey to CCDE. Orhan ...

Shiling Ding
Shiling Ding Sentinel Technologies – CCDE #2019::12

Just passed the CCDE Practical exam! I attended Orhan Ergun’s CCDE training program , used Orhan’s Instructor Led and Self-Paced CCDE training and Online CCDE Practical Scenarios during my CCDE journey. Orhan’s CCDE In Depth book is an excellent summary ...

Abelardo Basurto
Abelardo Basurto Solutions Architect at Cisco Systems – CCDE 2018::6

Hi everyone, I’ve just passed the CCDE Exam. My Number is CCDE 2018::6 I attended to Online CCDE Bootcamp of Orhan. I want to thank Orhan not only for the great book and bootcamp, but also for his commitment, availability and willingness to assist the ...

Hady Mohamed Abdellah
Hady Mohamed Abdellah Network Architect Hamad International Airport Qatar – CCDE 2018::1

Hi guys, I’m so happy that I passed the exam. I’ve already got my number CCDE 2018::1. Thanks to Orhan for being the best CCDE instructor in the world. I highly recomend Orhan’s CCDE Training and In-Depth-CCDE ...

Bryan Bartik
Bryan Bartik Sr. Systems Engineer at CompuNet – CCDE 20170059

Hi Orhan I passed CCDE Practical exam on November 2017 ! I really enjoyed your materials and quizzes and use cases. They were definitely helpful in my preparation. Thanks a lot !

Giedrius Trapkauskas
Giedrius Trapkauskas Network Solutions Architect at Liberty Global – CCDE 20180004

I attended Orhan’s CCDE Training in Istanbul and it was very helpful in my preparation. I passed the exam recently and I want to say Thank you Orhan! For those who want to pass the CCDE exam, definitely start with ...

Alaa Issa
Alaa Issa Sr.Solutions Architect – CCDE#20180033 3xCCIE ( Collab|DC|Security )#27146

I registered to Orhan’s training in Feb 2017. From that time, I attended Orhan’s training several times. The depth of knowledge which Orhan has is amazing, and how to present such consistent knowledge to the ...

Mazin Ahsan Design Lead Engineer | Solutions Engineer | CCDE License # 20160030 | CCIE Licence # 23892

I passed the CCDE Practical Lab exam on November 17,2016 from supplications of elders and dedication from my Sensei Mr. Orhan Ergun I took different CCDE bootcamps in the past. Orhan has the most depth and expertise ...

Jeff Patterson CCDE# 2018::11

Hi Orhan I wanted to pass along my appreciation for the outstanding training material. I used the online CCDE training provided by Orhan as well as the In-Depth-CCDE book and passed the exam in February 2018. Thank you Orhan!

Mehdi Sfar
Mehdi Sfar Network and Security Architect / CCDE #20210003 | CCIE R&S; #51583

I signed up for Orhan’s CCDE Self paced Course. This course, along with the CCDE In Depth book, helped me for my CCDE Practical as well as Written exams. It pushed me to ask the "WHY" questions and allowed ...

Related courses

Cisco CCDE v3 Training

71:39:34 Hours
225 Lectures
Expert

$995

MPLS Zero to Hero Training

30:07:04 Hours
51 Lectures
Intermediate

$195

Cisco CCIE Service Provider Training

108:43:00 Hours
258 Lectures
Expert

$995