Is BGP RR used in EBGP ? You might think I mean IBGP , not EBGP or I mean BGP Route Server, not BGP Route Reflector but no I really mean BGP RR (Route Reflector).
If you are reading this post, for sure you know what is BGP Route Reflector and it is used for BGP Scalability. Session and configuration scalability point of view, it is critical element in IBGP (Internal BGP)
But those who know what is Inter-AS MPLS VPN Option C, probably will immediately understand why I said, is BGP RR used in EBGP.
If you are not familiar with Inter-AS MPLS VPN Option C, please read this post first.
Just as a reminder, let me share quickly the important point of Option C , related with this post.
In order to send VPN prefixes between different BGP Autonomous system, in Inter AS Option C , VPN address family neighborship is created between the Route Reflectors of the BGP Autonomous Systems.
Reason of using BGP RR of the Autonomous System to exchange VPN prefix is scalability. Since BGP RR knows VPN prefixes anyway, why to extend all VPN prefixes to ASBRs (Option B). I know this sentence might look complex, that’s why in the beginning I recommended you to read that post.
Anyway, you understood why BGP RR is used to exchange the VPN prefixes between the Autonomous Systems but if you haven’t noticed, when you do this, in fact, you use BGP RR in EBGP.
Why that is important ?
As you know, when BGP RR is used in IBGP, next hop is not changed.
But, whenever there is an EBGP session between the devices, BGP next-hop is changed.
That’s why, even though BGP RR is used to setup a BGP session, since the type of BGP is EBGP in Inter-AS VPN Option C, next hop is changed, when the prefix advertised to neighbor AS.
Changing the next-hop can cause sub optimal routing. That’s the reason. And if you wonder, how it can cause sub optimal routing , this will be third time , but I recommend you to read that post 🙂