# IBGP Source Interface Selection Still Requires Configuration

DevFeed: [IBGP Source Interface Selection Still Requires Configuration](<https://devfeed.tech/articles/ibgp-source-interface-selection-still-requires-configuration-11104.md>)

Original publisher: [Read original article](<https://blog.ipspace.net/2024/12/ibgp-source-interface-trivia/>)

Published: 2024-12-10T07:32:00Z

Content type: article

Language: en

Sources: [ipSpace.net blog](<https://devfeed.tech/sources/ipspace-net-blog.md>)

Topics: [BGP](<https://devfeed.tech/topics/bgp.md>), [networking](<https://devfeed.tech/topics/networking.md>), [configuration](<https://devfeed.tech/topics/configuration.md>)

Tags: [bgp](<https://devfeed.tech/tags/bgp.md>), [configuration](<https://devfeed.tech/tags/configuration.md>), [ip](<https://devfeed.tech/tags/ip.md>), [networking](<https://devfeed.tech/tags/networking.md>), [spoofing](<https://devfeed.tech/tags/spoofing.md>), [tcp](<https://devfeed.tech/tags/tcp.md>)

## AI overview

The article explains that FRRouting does not automatically select the correct source interface for IBGP sessions. Because BGP runs over TCP and IBGP commonly uses loopback addresses, the router must be configured to send TCP SYN packets with the appropriate loopback source address.

## Source excerpt

A fellow networking engineer recently remarked, "FRRouting automatically selects the correct [IBGP] source interface even when not configured explicitly." TL&DR: No, it does not. You were just lucky. Basics first1. BGP runs over TCP sessions. One of the first things a router does when establishing a BGP session with a configured neighbor is to open a TCP session with the configured neighbor's IP address. Read more ...