Technical Tip: Rules followed when BGP routes are being advertised to peers (iBGP or eBGP)

[fwilliams]

Description	This article describes how BGP routers determine what route to advertise to any other BPG router it has BGP peering with.
Scope	FortiGate.
Solution	Note: This article assumes no filter is applied on R0. When a single router has multiple BGP peering with other routers (some iBGP and some eBGP), it follows certain rules to determine what it advertises to every other BGP peer. The following diagram illustrates this:   Router R0 is at the edge of the company network, running iBGP with R1 (internal router), and eBGP with R_ISP1 & R_ISP2. Below is what R0 advertises to each BGP router: R0 will advertise routes learned from R_ISP1 and R_ISP2 (eBGP peers/eBGP routes) to all BGP routers (i.e. both iBGP and eBGP routers). R0 will advertise routes learned from R1 (iBGP peer) to R_ISP1 and R_ISP2 (eBGP peers), but not to other iBGP peer/s, if that exists. Any time R0 or another router in the diagram, for instance, establishes a new session with a BGP peer, it will advertise all of the routes (prefixes) that match condition 1 and 2 above to that peer. Advertisements made to these BGP peers after this time will be only incremental updates. R0 or any other BGP router in the diagram will only advertise routes that it uses (i.e. routes which are installed in its table).