Introduction to BGP

Hi Rene
if we wanted to send 80% of the outgoing traffic on the primary link and 20% down the backup link? That’s not going to happen here but with BGP it’s possible.” then yeah?Can you show us in practical LAB.


Hello Mohamad

The process of optimizing and controlling traffic flow within a network based on the BGP protocol is called BGP traffic engineering. It is possible with various techniques to achieve very granular manipulation of traffic including getting an 80/20 split of traffic across two links.

There are several ways you can approach this. The simplest is probably using AS-Path prepending.

You can configure a primary and secondary BGP peering with two neighbors, create a route-map that will prepend the local AS number multiple times, and apply the route-map to the secondary BGP peering. Now the traffic behavior will depend upon how many times you prepend your own AS. If you prepend it four times, the secondary path will appear less preferable to other ASes, and they will prefer the primary path. The result is an approximation to an 80/20 split of traffic.

Keep in mind that BGP traffic engineering depends on the routing decisions of other ASes, so it’s not possible to guarantee an exact 80/20 split. You’ll need to monitor the traffic and adjust the configuration as needed to achieve the desired distribution. Additionally, always coordinate with your neighboring ASes to make sure your traffic engineering policies are effective and aligned.

Another way is to use the dmzlink-bw feature that Rene mentioned in his post. It allows you to configure BGP to distribute traffic proportionally to the bandwidth of external links. This command is configured for multipath load balancing between directly connected external BGP (eBGP) neighbors. You can find out more information about how it can be implemented at this Cisco command reference.

I hope this has been helpful!



I have a quick question about these three options.
"We can use BGP to advertise our address space to the ISPs but what are the ISPS going to advertise to our customer through BGP? There are a number of options:

1. They advertise only a default route.
2. They advertise a default route and a partial routing table.
3. They advertise the full Internet routing table.

Is this something that we would have to discuss with our ISP and inform them about what exactly we want them to advertise?

Hello David

Yes, this is something that you’ll discuss with your ISP. Typically, the ISP will have a default behavior that they configure on all their implementations, and that is typically delivering just a default route. This is the simplest and most basic configuration. However, any deviation from that will be discussed further with your ISP.

BGP is a protocol that requires communication between the entities to which particular ASes belong, whether those ASes belong to entities on the Internet or private enterprises.

I hope this has been helpful!


Hello Laz, I apprecitate your help here!

I have one more question.

Instead of using public IP addresses from the ISP we will get our own public IP addresses.The IP address space is maintained by IANA (Internet Assigned Numbers Authority – ). IANA is assigning IP address space to a number of large Regional Internet Registries like RIPE or ARIN. Each of these assign IP address space to ISPs or large organizations.

So instead of using a public IP from an ISP, we can just register our own block from the RIRs. However, from what I’ve heard, aren’t most of these RIRs already out of IP addresses due to the IPv4 address shortage problem? If RIRs like ARIN are out of IP addresses, how do we get our own IPv4 address block that we can advertise?

Thank you in advance for your help.

Kind regards,

Hello David

Indeed, RIRs are running out of IPv4 addresses. Today it is very difficult to get one let alone a block of IPv4 addresses. I know of one particular company that had asked for a /24 IPv4 block from ARIN, and it took over two years to fulfil their request. And they paid A LOT for the range. This was back in 2020. I doubt you can register such a large block today.

But the logic of the text can be applied to IPv6 addresses as well, which are freely available, so it still stands.

I hope this has been helpful!


Can i use same BGP AS number but with different network(advertise) from different independent locations and connect to a BGP network Cloud

Hello Shaji

What you are describing is called a discontiguous AS. In BGP, discontinuous ASes refer to the situation where an AS is separated into multiple non-contiguous segments. These segments are not directly connected to each other and may be separated by networks belonging to other ASes just like in your diagram. This scenario can occur due to various reasons such as mergers and acquisitions, strategic networking decisions, or geographic distribution.

Managing such setups requires certain considerations when designing the network including:

  • Routing Policies: Careful design of routing policies is essential to ensure consistent and efficient routing between the different segments of the discontiguous AS.
  • Inter-AS Connectivity: Connectivity between the different segments of a discontiguous AS is typically established through other ASes. This requires appropriate peering or transit arrangements with the ASes that lie in between.
  • BGP Configuration: The BGP configuration must account for the fact that the AS segments are not directly connected. This includes the use of BGP attributes like AS_PATH and NEXT_HOP to manage how routes are advertised and received.
  • Potential Challenges: Discontiguous ASes can present challenges in terms of routing complexity, increased dependency on intermediate ASes for connectivity, and potential for suboptimal routing paths.

Although technically speaking such a setup is feasible, it is generally avoided because of the added complexity of network management.

I hope this has been helpful!


Hi Rene,
I have a quick question for you. Please find the picture that I had drawn, before answering the questions…

Suppose Intel has 5 different branches. Each of the Intel branches are connected to their local ISP.
Intel-Ireland, Intel-Arizona, Intel-China, Intel-India — they all should have same AS number or different AS number?

Question-2:>>>> When Intel-Ireland will communicate with Intel-Arizona, they will use IGP protocol (like OSPF/EIGRP/IS-IS) or not?


In OSPF, we divide into areas to seperate LSA flooding zones.
The areas will be like:
Intel-Ireland - Backbone area (area0)
Intel-Arizona - area1
Intel-China - area2
Intel-India - area3

OR Intel-Ireland divided into different areas?

We use iBGP (Interior BGP) mainly inside transit AS… right? like transit ISP?
I know, but not 100% sure. please comment on it. I want to make sure whether my understanding is correct or not.

Hello Koushik

If the ISP is providing you with public ASNs, then it’s not a good idea to have the same ASN in all locations, because public ASNs should be geographically specific. There may be inefficient routing issues if you do that. However, if you’re using private ASNs then you can have the same ASN at each location if you choose to. Whether you do it or not depends on what you want to achieve. In this lesson, you can see an example of an MPLS topology where the two remote sites use the same ASN. In this case, you must use the Allow-AS-In feature of BGP. In this lesson, you have a similar topology but with different ASNs at each location.

I assume your question has to do with the internal network of the Intel organization, correct? You will typically have an IGP configured at each location, however, your routes will be shared with eBGP. In addition, the MPLS network uses an IGP as well, it can use OSPF however, IS-IS is the more common IGP used by ISPs. It may also be configured with BGP as well, although that is less common. So to answer your question, it depends on what IGP is configured and where that IGP is configured, but yes an IGP will typically be configured for this purpose.

I assume again that you are talking about the routing within the Intel organization (and not that on the MPLS network). If that is the case, and you want to use OSPF, the best practice is typically to create a different OSPF instance at each remote location and not multiple OSPF areas of the same OSPF instance. The OSPF instance in Ireland will redistribute its routes into eBGP, and so will each OSPF instance at each location. So there is no direct exchange of OSPF LSAs between locations. You can see an example of such a topology in this lesson.

iBGP is used within an AS. It is by definition BGP peerings that take place between BGP routers with the same AS, regardless of whether it is a transit AS or not.

Much of what you are asking is covered in the MPLS course which includes topologies similar to the one you describe in your post. I suggestion going over these lessons to more fully understand the details of such topologies.

I hope this has been helpful!