Technical Tip: Next-Hop behavior in BGP

[Anonymous]

Description	This article describes the next-hop behavior in BGP.
Scope	FortiGate.
Solution	Refer to the topology below: eBGP Next-Hop Behavior: In eBGP, when a route is advertised between different ASes, the next-hop is automatically set to the IP address of the eBGP neighbor that sent the update. iBGP Next-Hop Behavior: When a route is advertised within the same AS (iBGP), the next-hop IP is not changed by default. That means Router B will advertise 172.16.0.0 to Router C, keeping the next-hop as 10.10.10.3 (which belongs to Router A). The Problem with the Default Behavior: Router C is in AS 65000 and does not have a route to reach 10.10.10.3, since 10.10.10.3 belongs to AS 64520 (outside its IGP domain). Therefore, Router C cannot reach the next hop, and the route becomes unusable until the IGP advertises that network. The Problem with the Default Behavior: Router C is in AS 65000 and does not have a route to reach 10.10.10.3, since 10.10.10.3 belongs to AS 64520 (outside its IGP domain). Therefore, Router C cannot reach the next hop, and the route becomes unusable until the IGP advertises that network. Solutions to the Next-Hop Reachability Issue: Option 1: Use an IGP (OSPF, EIGRP, etc.) Configure an IGP within AS 65000 to carry reachability information for 10.10.10.3. This allows Router C to resolve the next-hop address and successfully reach 172.16.0.0. Option 2: Configure the next-hop-self command on Router B. Option 3: Use a Route Reflector. In larger iBGP deployments, a Route Reflector (RR) can be used to reduce the requirement for a full iBGP mesh. The RR reflects routes between iBGP clients within the same AS.