Why does BGP require an IGP to function?

Why BGP Requires an IGP or Static Routing Inside an AS

The video explains the necessity of having an Interior Gateway Protocol (IGP) or static routing within an Autonomous System (AS) for Border Gateway Protocol (BGP) to function properly.

BGP Overview and Types

BGP operates with two types of peerings:

• iBGP (internal BGP): Peerings between routers within the same AS.
• eBGP (external BGP): Peerings between routers in different ASes.

Prerequisite for BGP Operation

Routing must already be established inside the AS using an IGP (such as OSPF, EIGRP) or static routing. This ensures that all routers can reach each other, which is essential because:

• iBGP requires a full mesh of peerings between all iBGP routers inside the AS.

Role Differentiation Between IGP and BGP

• IGP: Handles routing within the AS, ensuring internal connectivity.
• BGP: Responsible for learning and advertising routes external to the AS.
  • eBGP advertises routes between ASes.
  • iBGP propagates these external routes inside the AS.

Routing Advertisement Examples

• An edge router (e.g., R1) learns an external network (e.g., 30.30.30.0/24) via eBGP from another AS and then distributes this route internally via iBGP.
• Internal networks (e.g., 20.20.20.0/24) are not propagated by iBGP for internal routing purposes but may be shared for advertisement to other ASes.

Why Not Use BGP for Internal Routing?

• Although technically possible, using BGP for internal routing is not best practice.
• BGP converges slower than IGPs and may cause unexpected behavior.
• The hierarchical approach—IGP for internal routing and BGP for external routing—is standard and recommended.

Summary

• IGP or static routing must be configured first inside the AS.
• iBGP requires full mesh connectivity, which depends on internal routing.
• BGP’s main function is to distribute external routing information, not to handle internal routing.

The video serves as a clear tutorial explaining the interplay between BGP and IGP, emphasizing best practices in network design and routing protocol deployment.

Main speaker/source: Lazarus from telecom.Tech