Why do we use BGP with MPLS – Cisco?
In any possible article where you have read about MPLS, you will find that it has been mentioned somewhere that one of the major benefits of MPLS is having a BGP-free core network. At some point, you will get an explanation of why, and in most cases, you won’t know the importance BGP holds.
But, in order to understand the statement properly, it is vital to remove the MPLS from the core network and then see the adjustments that you have to make in order to get transit traffic to the destination.
The role of BGP played in the MPLS network:
BGP offers high-end scalability for the MPLS networks by playing a major role in separating the controlling plane from the forwarding plane. Using the proper labels for aggregating forwarding information while trying to maintain various routing hierarchies will allow the network to scale proficiently. It is because various parts of the network will just carry only information needed to perform specific functions.
- For example, you have the core routers, which will need to hold that label information and the internet network information within internal gateway protocols.
- Moreover, VPN information will further get distributed to the edge routers only. This pointer has something to do with those with the VPNs only and will not focus on the edge routers.
- BGP is noted to be a protocol vital for carrying external routing information. Some of the examples are internet routing info or the customers’ routing information.
- Whenever you are towards the MPLS network for providing internet services and the L3 VPN services, you can see the role of BGP in carrying the internet routing table. It further carries the VPNv4 routing information and the IPv4 or IPv6 customer’s routing information with VPN labels.
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The tunneling mechanism from MPLS:
The MPLS tunneling mechanism will allow the core routers to forward the packets using labels only without the proper need to look up their current destinations in the present IP routing tables.
- Here, only the edge routers will be the ones to forward packets by checking out the destinations in the current routing table.
- It means that the edge routers will be the only routers that need to have this said information. So, it is vital to run the BGP over here.
The information that BGP carries:
From the points mentioned above, you might have understood the importance that BGP holds in MPLS. Now, the real question is to understand the type of information that BGP carries through MPLS.
- At first, it will focus on the customers’ routing information.
- BGP will further handle the internet routing information.
- Then you have the VPNv4 routing information with the said VPN labels piggybacked straight from BGP.
- In some of the selected MPLS applications, the BGP is widely used for distributing label information, which will be piggybacked in the chosen updates.
Now, for a second, just imagine that you got the MPLS removed from the core! Then what will be the results to deal with? Let’s find out.
- The core routers will no longer be using the labels for forwarding packers, and they further need to look up the destination addresses of these transit packets.
- For that, you need to run the BGP in core with all the complex issues in it, along with the full meshing, configuration overhead, route reflector bottlenecks, network maintenance, and confederation.
- Or, you can try to redistribute the BGP into IGP, which is rare to find in real life.
- Once you have removed MPLS from the core, then the MPLS VPN labeled packets will not be forwarded by core routers. It results in some other historical complexities as well.
In the end, it is easy to say that you cannot imagine a network without MPLS. If so, most modern-day services would probably not even exist. Moreover, MPLS and BGP are two major companions, and MPLS is scaling only because of how the MPLS engineers are utilizing and using the power of BGP in their said networks!