# Concepts

## Upstream Provider

An ISP or carrier that offers IP Transit services to another network. It connects you to the global Internet.

## Downstream Customer

The network or organization that buys IP Transit to access the Internet.

## BGP (Border Gateway Protocol)

The routing protocol used to exchange routing information between different networks (Autonomous Systems). It allows IP Transit providers and customers to communicate routing updates.

## Autonomous System (AS)

A collection of IP networks managed by a single organization with a unified routing policy. Each AS is assigned a unique AS number (ASN).

## Delivery Mode

### Routing Mode (Static / BGP)

Specifies how routing information is exchanged between your network and the transit provider:

* **Static Routing**\
  Routes are manually configured and fixed. Simple but not scalable.
* **BGP (Border Gateway Protocol)**\
  A dynamic routing protocol that automatically exchanges routes between Autonomous Systems (ASes). Scalable and flexible, commonly used for IP Transit.

### Gateway Mode

Refers to the device or role acting as a gateway between your network and the upstream transit provider. It handles traffic forwarding, routing decisions, and possibly BGP sessions.

## BFD (Bidirectional Forwarding Detection)

A fast failure detection protocol used with routing protocols (like BGP) to quickly detect link or path failures. Helps minimize downtime by triggering fast rerouting.

* **Transmit Interval**: How often BFD packets are sent (e.g., 300 ms).
* **Receive Interval**: Expected interval at which BFD packets are received from the peer.
* **Detection Multiplier**: Number of missed packets before considering the link down. For example, multiplier = 3 means the link is declared down if 3 consecutive packets are missed.

## Inbound Routing Type

Specifies what kind of route information you receive from the transit provider:

* **Full Route**:\
  You receive the entire global BGP routing table (all internet routes). Offers full control over path selection but requires more memory and CPU.
* **Default Route**:\
  You receive a single default route (`0.0.0.0/0`) that sends all non-local traffic to the provider. Simpler and less resource-intensive.
* **Full Route + Default Route**:\
  You get the full routing table and a default route. Allows flexible fallback if specific routes are withdrawn or filtered.