This scenario is a network with four routers in a square; each has a connection to a different transit provider with a full BGP table from each.
R1----R2 | | | | R3----R4
How do full BGP feeds affect the memory requirement of the routers in the ring, other than the router receiving the full feed?
All four routers here take full feeds from different providers, and are iBGP meshed (you can imagine diagonal lines join R1 & R4, and R2 & R3, my ASCII art isn't brilliant).
iBGP peers send each other all eBGP routes so, R1 receives full table from upstream1. R2 receives a full feed from upstream2. R2 will send R1 all its external routes and vice versa. For any received routes that are better via upstream2, R1 will place into it's FIB, and send traffic to R2. And so on and so fourth, for all four routers and upstream providers. Now, for traffic that has a better path via upstream3, it will be sent via R3, even when it comes from a device attached to R1. That is the beauty of BGP.
Lets just use R1 and R2 to keep it simple for now, and they are booth bringing up BGP peerings for the first time;
When R1's BGP session with upstream1 first establishes, it receives BGP updates from upstream1, it processes the routes and they progress from BGP RIB into the FIB.
All of these routes on R1 are now passed to R2 via iBGP, and R2 pushes them all from its BGP RIB into FIB (these are the only external routes it has at present).
R2's session with upstream2 comes up and upstream2 sends BGP updates to R2. At this point, does R2 compare the routes in it's BGP RIB, keeping the better ones, drop the lesser ones, than it has from upstream1, and push the replaced better ones into FIB (assuming no better routes in FIB from other routing protocols or sources). Does R2's BGP RIB still only contains one full BGP table at this point, but its a mix of both upstream providers table views? Is the correct?
The same now happens in reverse, with R2 sending updates to R1. R1 ends up with the same BGP view.
Am I correct that unless
soft-reconfiguration inbound or something similar is configured, any router in this square only holds the equivalent of one full table? What change in memory requirements would enabling
soft-reconfiguration inbound here make, would each router hold 4 full tables and need 4 times as much memory?