Orhan Ergun 2 Comments

Different IGP and BGP Methodologies of Multi National Service Providers. I mentioned two different IGP and BGP design approaches for two different Multi National Service Providers in my last CCDE course

 

Both are these operators in Africa and one of them has an operation in 4 and another has an operation in 5 countries.

 

I will not share the topologies of them but just want to mention their IGP and BGP design , challenges they face , advantages and the disadvantages of each method.

 

For better understanding let’s call them Operator 1 and Operator 2.

 

Both are using as an IGP IS-IS. Flat , Single level , Level 2 IS-IS network. They have LDP in their network for MPLS control plane but their MPLS design is out of scope, otherwise post would be too complex to understand.

 

Operator 1 is using single IGP and same BGP AS across all 4 countries. Which mean, IS-IS is extended over entire company network. So, the routers in different countries have exactly the same reachability and topology information in Operator 1 network. And same BGP Autonomous System, which mean between the countries , they have an IBGP.

 

Operator 2 is using separate IGP and unique BGP AS per country. Which mean, IS-IS LSPs are not exchanged between the countries and each country has unique BGP Autonomous System. I need to share a bit more detail about Operator 2’s BGP design. They have an operation in 5 countries but they don’t have International Peering and Transit capacity in each country. Instead, they send the International traffic from some countries to the countries which have an International Peering and Transit arrangements (Based on cost and availability of submarine cable system)

 

If something is not clear so far or want to know little bit more about their design, please let me know in the comment section below.

 

Both of these operators have an MPLS VPN customers. Some of their MPLS VPN customers have their locations in different countries.

 

This is just a normal Intra AS MPLS VPN deployment for Operator 1 , as Operator 1 has single IGP and same BGP on different countries.

 

But Operator 2 needs to deploy Inter-AS MPLS VPN solution. I will not explain the details of Inter-AS MPLS VPNs as I explained in many posts on the website as well on my YouTube channel but just know that Operator 2 has an Inter-AS Option A.

 

If you have been following so far, and hopefully no problem , let’s have a look at their challenges.

 

Operator 1 complains that, they cannot use very low end device in their IGP domain, They have around 200 device in IGP domain and they have a problem with flooding , SPF calculation and route churn. Thus they want to use separate IGP domain in each country.

 

My take on Operator 1 Request :

 

Valid reason and in fact they will separate the IGP domain and whenever they need and end to end reachability, they may leak the required prefixes. But separating IGP domains, will prevent end to end topology view. If they need end to end between the countries a Traffic Engineering, they need a node which has a Global View. (PCE in MPLS)

 

The new design will allow them to continue using low end routers in each country even the number of prefixes grow per country.

 

New design might even allow using different IGP per country. For example they may use in Country 1 , IS-IS as an IGP and Country 2 , OSPF as an IGP.

 

We can definitely find more advantages and disadvantages with new design of Operator 1 but let’s just look at what is the Operator 2’s challenge.

 

Operator 2 complains that they get a lot of request from their customers for MPLS VPN across the countries. As they have unique BGP AS per country, they need to deal with Inter-AS policy per customer. They are considering to use single BGP AS per country. Though they will continue to use separate IGP per country.

 

My take on Operator 2 Request

 

They currently have EBGP between the countries, they send customer VPN information over EBGP as well as default route into Global routing table as there is no International connection from each and every country as I mentioned before.

They should continue to use separate IGP and I mentioned the benefits of using separate IGP per country for Operator 1 , same applies here.

 

If there is too much request for Inter-AS MPLS VPN , Option B and Option C are more scalable and less effort is required per customer when Inter-AS Option B or C is used.

 

On the other side, EBGP providers better tool for traffic engineering comparing to IBGP.

 

If they convert their BGP design to IBGP, they need to deploy either full mesh IBGP , BGP Confederation or BGP Route Reflector.

 

If they deploy BGP RR , Optimal routing is should be always one of the first considerations for the designers. Especially for IP traffic , not the VPN. I know so many Service Provider networks , as of 2018 , they use regional RR. In Operator 2 case, for IP Traffic , either they need to deploy BGP RR in each country and then full mesh IBGP between RRs or hierarchical RR design , or , they need to send all International exits to each country , or they need to check Optimal RR design approach (In their case there will not be end to end visibility as they have separate IGP per country , that’s why IGP assisted Optimal RR wouldn’t work)

 

Better for them to continue using EBGP but considering different MPLS VPN approach for Inter-AS.

 

So although problem seems having different BGP AS per country and using same AS wouldn’t require Inter-AS operation, they can solve their challenge by looking at different layer , which is MPLS in this case.

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