Right now, as you read this, millions of data packets are racing across the internet to load this webpage. But how did they know exactly where to find you?
The answer is BGP, the Border Gateway Protocol. It's the invisible system that makes the internet work.
Think of it this way: Without BGP, the internet would be like a massive city with no street signs, no GPS, and no addresses. Your email would never reach its destination. Your video calls would fail. Websites would be impossible to find.
In this guide, you'll learn what BGP is, how it works, and why it matters for your business.
What is BGP (Border Gateway Protocol)?
BGP is a routing protocol that provides rules and directions for data transmission. BGP works like the internet's GPS, helping autonomous systems (large networks) share routing information and determine the best paths for data to travel between different networks.
When you visit a website, the Domain Name System (DNS) first translates the domain name (like google.com) into an IP address. BGP determines the best route for your request to reach that IP address across the internet's network of networks.
Real-World BGP Examples
When You Stream Netflix
When you click play on Netflix, here's what BGP does behind the scenes:
- Your request travels through your Internet Service Provider’s (ISP) network
- BGP finds the fastest route to Netflix's servers
- If the main route is slow or broken, BGP automatically picks a backup path
- Your show streams smoothly because BGP chose the best available route
The Facebook Outage of 2021
Remember when Facebook, Instagram, and WhatsApp all went down for six hours? That was a BGP problem.
Facebook accidentally withdrew its BGP routes, essentially telling the internet "we don't exist anymore." – it was like removing all road signs pointing to Facebook's offices. The outage cost Facebook over $60 million and affected 3.5 billion users worldwide.
How ISPs Use BGP
ISPs rely on BGP routing to:
- Connect to other internet providers worldwide
- Find the cheapest routes for customer traffic
- Automatically reroute traffic when networks fail
- Manage bandwidth and prevent slowdowns
- Deliver content from the closest servers
Key Components of BGP (Border Gateway Protocol)
Before diving deeper, let's define the important pieces:
Autonomous Systems (ASes)
An AS is a large IP network or collection of IP networks under one network operator with a single routing policy. Think of it like a country with its own postal system. Examples include:
- Your internet provider's network
- Google's network
- Amazon's network
- University networks
Each AS gets a unique number (called an ASN) for identification.
BGP Peers (Neighbors)
BGP routers that talk to each other are called peers or neighbors, and in large networks, there can be multiple BGP speakers exchanging routes simultaneously.
- Internal peers: Routers within the same organization
- External peers: Routers in different organizations
Path Attributes
Each BGP route has a set of path attributes that help determine the best route. Examples of path attributes include:
- How many networks the data must cross
- Which route the local network prefers
- The origin of the route information
- Traffic engineering preferences
Why is BGP (Border Gateway Protocol) Important?
At its core, BGP is the protocol that allows different ASes to connect and communicate with each other through a process called peering. Without BGP, peering would not be possible.
As such, BGP enables the internet to be a network of networks, allowing autonomous systems to exchange routing information. This is essential for global connectivity, allowing data and information to travel from one part of the world to another seamlessly.
Besides facilitating global connectivity, BGP also helps with the following:
- Routing efficiency – Chooses the best route for data by evaluating multiple path attributes, improving performance and speed.
- Real-time route updates – Keeps networks informed of changes so data always follows the most optimal path
- Policy control – Lets administrators shape traffic flow, enforce security, and optimize resources through custom routing rules
- Scalability – Handles massive and growing numbers of routes without degrading performance
- Redundancy and failover – Maintains multiple routing options so traffic can be quickly rerouted if one path fails
How Does BGP (Border Gateway Protocol) Work?
BGP peers connect with each other, with each peer exchanging network layer reachability information so routers know which IP prefixes can be reached and through which paths. This information is stored, enabling routers to analyze various peering options and select the most efficient routing path.
BGP peers have four key functions that help facilitate this process.
1. Route discovery
- BGP routers introduce themselves to neighbors
- They exchange information about which networks they can reach
- Authentication ensures only legitimate routes are accepted
2. Route Storage
- Each router maintains a global routing table of possible paths
- Tables are constantly updated as network conditions change
- Invalid routes are removed automatically
3. Path Selection
- When multiple paths exist, BGP picks the best one
- Selection considers speed, reliability, and business policies
- The chosen path is installed in the router's forwarding table
4. Route Propagation
- The best route is shared with neighboring routers
- Information spreads across the internet
- All routers eventually learn about network changes
BGP routers send "keepalive" messages to maintain connections. If a router goes silent, its routes are automatically removed to prevent traffic from entering a black hole.
BGP Attributes: How Routes Get Chosen
When multiple paths exist to reach a destination, BGP uses a priority system (called attributes) to pick the best one. Think of it like choosing which highway to take – you consider speed limits, traffic, tolls, and distance.
BGP checks these attributes in order, stopping at the first difference it finds. Here's BGP's decision process, in order of priority:
1. Weight (Highest Priority)
A local preference set by network administrators. Higher weights win.
2. Local Preference
Used within an autonomous system to prefer certain routes. Higher values are better.
3. Originate
Routes that originated from your own network are preferred over routes learned from others.
4. AS Path Length
The number of autonomous systems (networks) a route crosses. Shorter paths usually mean faster delivery.
5. Origin Type
How the route was learned. Possible values include IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), or Incomplete. IGP-originated routes are preferred over EGP and Incomplete.
6. MED (Multi-Exit Discriminator)
A hint about which entry point to use when multiple options exist. Lower values are preferred.
7. External vs Internal BGP
Routes learned from other organizations (eBGP) are preferred over internal routes (iBGP).
8. IGP Metric to Next Hop
The cost to reach the next hop router determined by the IGP. Lower costs win.
9. Path Age
If routes are otherwise equal, older paths are preferred for stability.
10. Router ID and IP Address
Final tiebreakers using router identifiers and IP addresses. The path with the lowest router ID and lowest neighbor IP address is preferred.
Note: It’s possible to tailor attributes to your liking for more granular control over network traffic flow.
What are the 2 Types of BGP (Border Gateway Protocol)?
There are two types of BGP: internal BGP (iBGP) and external BGP (eBGP).
Both share the core principles of BGP but operate in different contexts:
- Internal (iBGP) – between routers in the same autonomous system, e.g., internal network
- External (eBGP) – between routers in different autonomous systems, e.g., the global internet
eBGP must be used to connect your organizational network to the global internet. However, iBGP is not required internally; you can use any routing protocol to connect routers on your internal network.
Think of it as an international postal service where each country (autonomous system) has its own rules and guidelines for mail service (iBGP). However, if you want to post something internationally, it must meet global standards to ensure your mail reaches its destination country (eBGP).
How Does BGP Handle Scale?
The internet has grown from thousands to billions of connected devices. BGP handles this massive scale through several clever mechanisms.
1. Decentralized Management
Instead of one central authority managing all routes, each network manages its own piece. This distributes the complexity and prevents single points of failure.
2. Hierarchical Structure
- Tier 1 providers: Major internet backbones that connect globally
- Tier 2 providers: Regional networks that connect to Tier 1s
- Tier 3 providers: Local ISPs that connect to Tier 2s
3. Scaling Technologies
Route Aggregation
- Multiple small ASes can be announced as one large block
- Reduces the size of internet routing tables
- Example: Instead of announcing 256 individual /24 networks, announce one /16
Route Reflectors
- Special routers that reduce the number of connections needed
- Act as hubs for route information sharing
- Prevent the need for every router to connect to every other router
Confederations
- Large ASes can be subdivided internally into smaller sub-ASes
- Appears as one AS to the external peers (the outside world)
- Reduces complexity
Incremental Updates
- BGP only shares changes, not complete routing tables
- Keeps network traffic manageable
- Reduces processing requirements
BGP vs OSPF
BGP and OSPF (Open Shortest Path First) are two of the most popular routing protocols. ISPs and large networks typically use BGP due to its scalability and flexibility, whereas OSPF is used more commonly with internal networks. Here’s a side-by-side comparison of both protocols.
| Feature | BGP (Border Gateway Protocol) | OSPF (Open Shortest Path First) |
|---|---|---|
| Primary Use | Routing between different Autonomous Systems | Routing within a single Autonomous System |
| Implementation | Complex | Simpler |
| Scope/Routing Domain | Primarily inter-domain or inter-AS routing | Intra-domain routing |
| Type | Path Vector Protocol | Link-State Protocol |
| Protocol | TCP | IP |
| Convergence | Slower convergence | Faster convergence |
| Metrics | Uses path attributes like AS path length, Local Preference, MED | Uses cost based on link bandwidth |
| Resource Requirements | Proportional to the size of the routing table | Memory- and CPU-intensive |
| Network topology | Mesh, but can be modified using a route reflector | Hierarchical |
| Scalability | Highly scalable, designed for large-scale networks | Scalable within an AS up to several hundred routers |
| Route Calculation | Based on path attributes and policies | Automatically calculates shortest paths based on bandwidth/cost |
| Use Case | - Internet backbone - ISPs - Large enterprise networks - Cloud providers |
- Internal network routing within enterprises - Data centers - Campuses |
What is the difference between BGP and DNS?
DNS servers find the IP address, whereas BGP provides the most optimal path to reach it. In other words, if finding an IP address is a destination, BGP is the GPS that provides the directions for the most efficient route.
Final Thoughts
BGP is the invisible force that makes the internet work. Every website you visit, every app you use, and every video you stream relies on BGP to deliver data efficiently and reliably.
While you don't need to manage BGP directly, understanding it helps you make better decisions about internet services, hosting providers, and network security.
Frequently Asked Questions (FAQs)
What happens when BGP fails?
When BGP fails, parts of the internet become unreachable. We saw this with major outages affecting Facebook, Cloudflare, and other services. BGP failures can make websites, apps, and online services inaccessible.
Do small businesses need to worry about BGP?
Most small businesses don't directly manage BGP – their ISP handles it. However, understanding BGP helps you choose better hosting providers and understand why internet outages happen.
How is BGP different from your home router?
Your home router uses simple protocols to direct traffic within your home network. BGP operates at a much larger scale, routing traffic between major networks across the globe.
Can BGP be hacked?
BGP has security vulnerabilities, including route hijacking and route leaks. However, modern security measures like RPKI and careful route filtering help protect against these threats.
