Understanding iBGP for IPv4

iBGP Overview

• Same ASN: In iBGP (Internal Border Gateway Protocol), all peers belong to the same Autonomous System Number (ASN).
• Direct Connection Not Required: iBGP peers do not need to be directly connected to each other. The Time to Live (TTL) is typically set to 255.
• Loopback Addresses: Peering is often done via loopback addresses for redundancy and stability.

Static and IGP Considerations

• Layer 3 Reachability: iBGP still requires Layer 3 reachability, which can be achieved through static routes or an Interior Gateway Protocol (IGP).

iBGP Advertisement Rules

• Update Perspective: iBGP advertisement rules depend on how the update was learned:
  • eBGP to eBGP: Updates are passed along.
  • eBGP to iBGP: Updates are passed along.
  • iBGP to iBGP (Not Allowed): This prevents routing loops and is a fundamental part of iBGP operation.
• Exceptions: Route Reflection and Confederations are used to relax these rules in specific scenarios.

Full Mesh Considerations

• iBGP Full Mesh: A full mesh topology in iBGP ensures reachability and complete knowledge of the network topology.
• Optimal Routing: Full mesh enables optimal routing and hot potato routing.
• Number of Connections: The number of connections in a full mesh equals n(n-1)/2, which can grow rapidly with the number of peers.

Route Advertisements & iBGP

• Update Sources: Route advertisements depend on the source:
  • Learned from an eBGP peer: Both iBGP and eBGP peers receive the update.
  • Learned from an iBGP peer: The update is sent to eBGP peers but not to other iBGP peers.

iBGP Validation & Verification

• Validation Commands: iBGP validation and verification can be performed using commands such as:
  • show bgp ipv4 unicast summary: Provides a summary of BGP information for IPv4 unicast routes.
  • show bgp ipv4 unicast neighbor [neighbor IP]: Displays details of a specific BGP neighbor for IPv4 unicast routes.
  • show ip bgp: Shows BGP information for IPv4.
  • show ip bgp neighbor [neighbor IP]: Offers detailed information about a specific BGP neighbor in IPv4.

Understanding iBGP for IPv4 and adhering to its rules is essential for building and maintaining a robust internal BGP network.