BGP Issue: Loopback Interface Connectivity

When using loopback interfaces for eBGP peering, it's crucial to understand that by default, eBGP sessions are designed to form only with directly connected neighbors (i.e., within one hop). If you try to peer using a loopback address without additional configuration, the session will remain in the Idle or Active state.

Key Points:

• Symptom:
  The eBGP session remains in Idle or Active state when using loopbacks for peering.

• Root Cause:
  By default, eBGP expects a directly connected neighbor. A loopback interface, which is not directly connected, requires additional configuration for eBGP to recognize it as a valid peer.

• Solution:
  To successfully establish an eBGP session using loopback interfaces, you must either:

  1. Configure ebgp-multihop:
     This command allows eBGP sessions to be established with peers that are more than one hop away.

  2. Ensure proper reachability:
     Implement static routes or use an Interior Gateway Protocol (IGP) to reach the neighbor’s loopback interface.

Example Scenario:

Incorrect Configuration (Using a Loopback without ebgp-multihop):

Router0(config)# router bgp 5110
Router0(config-router)# neighbor 1.1.1.1 remote-as 2640

In this setup, if the IP address 1.1.1.1 is associated with a loopback, eBGP will not form a session because it expects the neighbor to be directly reachable.

Corrected Configuration with ebgp-multihop:

Router0(config-router)# neighbor 1.1.1.1 remote-as 2640
Router0(config-router)# neighbor 1.1.1.1 ebgp-multihop 2
Router0(config-router)# neighbor 1.1.1.1 update-source loopback1

Explanation:

• Using ebgp-multihop:
  The ebgp-multihop command tells the router to allow eBGP sessions with neighbors that are more than one hop away. In the example, setting it to 2 hops provides a sufficient range for the session to establish using a loopback address.

• Update Source Command:
  The update-source command specifies that the BGP session should use a loopback interface (in this case, loopback1) as the source IP address, ensuring consistency in the session's establishment and routing.

• Alternative Approach:
  If you prefer not to use ebgp-multihop, you can alternatively peer using a physical interface that is directly connected, bypassing the need for additional configuration.

By ensuring that these configurations are correctly applied, you can successfully establish an eBGP session using loopback interfaces and maintain a robust routing setup.