What is the meaning of BGP free core?
BGP refers to an Internet protocol used between different Autonomous System on the Internet. The purpose of this post is not to explain the fundamentals of BGP, as I believe that readers are already familiar with the basic of BGP and IP routing operation. To understand the rudimentary aspect of BGP, click here to peruse articles on BGP.
Let’s look at the topology shown below to understand the BGP operation and IP destination-based lookup.
BGP Free Core
In the above topology, when CE1 wants to reach CE2 at the other side of the network, the packet becomes either R1 or R2. If there are no tunneling mechanisms such as MPLS, GRE, or any other mechanisms, R1 or R2 makes IP destination-based lookup and sends packets to R3 or R4.
If the prefixes on CE2 is learned by BGP in the middle of the network, all the routers have to do an IP destination-based lookup to see if there is a route for the CE2 prefixes in the routing table from BGP. Every router – R1, R2, R3, R4, R5, and R6 – has to run BGP.
If the Layer 3 overlay tunnelling technology runs in the network, then the routers in the middle, which are R3 and R4, do not have to keep the CE1 and CE2 prefixes.
R3 and R4 keep only the routing information on the edge nodes. As a result, R3 and R4 are used for reachability between R1, R2, R5, and R6.
Since MPLS is a tunnelling mechanism that provides Layer 2 or Layer 3 overlay, if MPLS is used in the middle of the network, R3 and R4 as a core node do not have to run BGP.
R1, R2, R5, and R6 are called edge nodes, and R3 and R4 are known as core nodes.
That’s why you can have BGP free core network if MPLS is used in the networks.