BGP Mechanics — Sessions, Updates, and Path Selection

Now that you understand Autonomous Systems and why BGP exists, let's look under the hood. How do BGP routers establish sessions? How do they exchange routing information? And when multiple paths exist to the same destination, how does a router pick the best one? These mechanics are essential for understanding NOC incidents involving routing changes, session flaps, and traffic shifts.

BGP Sessions — TCP Port 179

Unlike IGP protocols that use multicast or broadcast for neighbor discovery, BGP peers must be explicitly configured. Each BGP session runs over a TCP connection on port 179.

Session Types

Type	Description	Typical Use
eBGP (External BGP)	Between routers in different ASes	Telnyx ↔ upstream provider
iBGP (Internal BGP)	Between routers in the same AS	Telnyx router ↔ Telnyx router

┌──────────────────┐          ┌──────────────────┐
│   Telnyx (AS46887)│   eBGP   │  Cogent (AS174)  │
│                  │◄────────►│                  │
│  Border Router   │ TCP:179  │  Border Router   │
└──────────────────┘          └──────────────────┘
        │
        │ iBGP (TCP:179)
        │
┌──────────────────┐
│  Telnyx Internal │
│  Route Reflector │
└──────────────────┘

💡 NOC Tip: a BGP session won't establish, check: (1) TCP 179 connectivity — firewalls often block it, (2) matching AS numbers in the config, (3) correct neighbor IP. Use telnet <peer-ip> 179 as a quick reachability test.

BGP Finite State Machine

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