Inter AS Option B – Design Considerations and Comparison

Inter-AS Option B is a highly scalable, reasonably secure, but operationally complex inter-autonomous MPLS VPN architecture.

For those who want to understand the easiest, most flexible, and most secure Inter-AS solution, which is Inter-AS Option A please click here.

I am explaining this topic in deep detail in my Onsite CCDE, Live/Webex CCDE, and Self-Paced CCDE course.

First, I will explain how Inter-AS Option B works. After that,

I will cover the design considerations for Inter-AS Option B. I will use fig.1 to explain Inter AS Option B.

Inter AS Option B

Figure 1 : Inter AS Option B

In the topology shown above in fig.1, AS10 and AS20 are the service providers.

There are two customers, VPNA and VPN B

NOTE: All the customers of the service providers could run different routing protocols at different sites. For example, while customer VPNA connected to AS10 is running OSPF, VPN A connected to AS20 could run EIGRP.

In the service provider network, PE routers are connected to the customer locations. PE routers run MP-BGP with route reflectors to advertise VPNv4 prefixes.

PE router that connects the two providers at the edge is known as ASBR (Autonomous System Boundary Router).

Since ASBR runs MP-BGP with RR as well, ASBR learns all the VPN routes from Inter-AS customers.

As for Inter-AS Option A, separate sub-interfaces are enabled, and IGP protocols run between ASBRs. MP-BGP is redistributed into IGP.

Inter-AS Option B does not require separate sub-interfaces and different IGP protocols or BGP per sub-interfaces between the ASBRs.

Between the ASBRs, MP-BGP enables the VPNv4 address family.

ASBRs (Autonomous System Boundary Routers) advertise the customer prefixes which are learned from the local BGP route reflector to the other service provider through MP-BGP VPNv4 session.

You do not need to keep the VRF table for the customers on the ASBRs. That is why there is no VRF on PE, as shown in fig.1.

You just need to keep the customer prefixes in the LFIB database.

CPU and memory requirements for Inter-AS Option B are much less than those of Inter-AS Option A.

PE devices have BGP VPNv4 sessions with the Service Provider's Route Reflectors. PE redistributes customer VPN prefixes fromPE-CE routing protocol to MP-BGP and vice versa.

Route-target extended community is placed on the VPNv4 update. Route Target helps to place the VPN prefixes in the appropriate VRF.

When PE receives the customer IP prefixes, it changes next hop as themselves, Customer IP prefixes are made VPN prefixes by adding Route Distinguisher and a VPN update is sent to the VPNv4 route reflector.

The route reflector does not change the next hop; rather, it only reflects the route to the ASBR. (This is normal BGP RR behavior but it is different in Option C)

ASBR does not place the MP-BGP route into VRF, since it does not have to keep the VRF table but to maintain the VPNv4 BGP table in Inter-AS Option B.

By changing the next hop, ASBR from SP-A sends VPNv4 prefixes through the MP-BGP session to SP-B ASBR.

SP-B ASBR sends the customer prefixes to its local route reflector.

The next hop is changed on ASBR. (Normally EBGP to IBGP, next hop doesn't change).

Thus, either ASBR to ASBR link is redistributed into the global IGP table of service providers, or "next-hop-self" is enabled in order to change the next hop as ASBR.

Route reflector in the SP-B domain reflects the prefixes as is, send to the PE which is connected to Customer A2 location.

SP-B PE sets the next hop again and sends the prefixes to the customer A2 router.

As shown in the service provider domains, there are three LSP.

Because changing the BGP next hop terminates the LSP, a new VPN label is assigned on that router for all VPN prefixes.

Caveat :

Normally ASBR would not accept the VPN prefixes because they do not have VRF for the customers.

Since ASBRs do not have a VRF and the Route Target is not associated with the VRF in Option B, they would reject the prefixes.

As shown in fig.1, "no bgp route-target filter" configuration knob is enabled.

This configuration command allows the ASBRs to accept the VPN prefixes even though they do not have a route target associated with VRF.

Inter-AS Option B requires MP-BGP between autonomous systems. The VPN address family is enabled on the MP-BGP session.

Inter-AS Option B does not require LDP or IGP protocols; thus service providers do not need to understand the internal addressing structure of one another.

LSP is not end-to-end but it is terminated at many points; as a result, it is very difficult to manage.

Similar to Inter-AS Option A, you do not need to redistribute VPN prefixes at the ASBR.

Route reflectors store all the VPN routing information for each customer, and they advertise those prefixes to ASBR.

Operators need to manage MP-BGP on the ASBRs as well as on the route reflectors.

What if by removing those prefixes from ASBRs, can we advertise all the VPN prefixes directly between route reflectors? To answer this question, peruse the next article on Inter-AS Option C.

To have a great understanding of SP Networks, you can check my newly published SP Book. It covers the SP network Technologies with also explaining in detail a factious SP network.

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

MPLS Zero to Hero Training

30:07:04 Hours
51 Lectures
Intermediate

$250

Cisco CCIE Service Provider Training

108:43:00 Hours
258 Lectures
Expert

$995

MPLS VPN Zero to Hero Training

15:12:01 Hours
24 Lectures
Intermediate

$100