BGP specifies that routes learned using Interior BGP should never be learned by other IBGP peers. Because of this rule, BGP requires that all IBGP networks to be complety fully meshed as shown in the picture above. Therefore if you had just 13 routers in your AS running IBGP, you would need 78 total connections in order for all 13 routers to connect to every other router! This causes a big problem with bandwidth due to sending redundant data across all of the routers at the same time.
To over come this, the creation of Route Reflectors (RR) were created. Route Reflectors allows an AS that's running IBGP to not have to use a complete full-mesh topology. Instead you can creat whats called clusters which can group sets of routers together. You can think of a cluster as a mini network that sits inside of your AS. But instead of a full-meshed topology, the cluster is designed in a hub and spoke fashion with one router being designated the Route Reflector (Hub) and the other routers being the spokes that connect to the RR. The Route Reflector then passes its updates to the AS, other clusters, or even other AS's depending on the configuration. This saves on the number of BGP TCP sessions that must be maintained and and also reduces the BGP routing traffic!
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