2.1 Explain characteristics of routing technologies
Dynamic routing
📘CompTIA Network+ (N10-009)
1. What is BGP?
BGP is the routing protocol that allows large networks—such as internet service providers (ISPs), data centers, cloud providers, and big enterprises—to share routing information with each other.
In simple terms:
- Inside a company network → internal routing protocols (e.g., OSPF, EIGRP, RIP).
- Between company networks → BGP.
Because the internet is made of thousands of networks, BGP ensures that these networks can reach each other.
2. Why BGP is Important
For the exam, understand that BGP is used for:
✔ Internet routing
BGP decides the best path for traffic across different organizations on the global internet.
✔ Connecting ISPs to each other
ISPs use BGP to exchange routes with other ISPs.
✔ Multi-homed enterprise networks
If a company connects to two or more ISPs, BGP is used to manage failover and load sharing.
✔ Cloud connectivity
Organizations use BGP to connect their on-premises network with cloud providers like AWS, Azure, or Google Cloud.
3. Autonomous Systems (AS)
BGP uses the concept of Autonomous Systems (AS).
What is an AS?
An AS is a group of IP networks under a single administrative control.
Examples:
- An ISP is an AS.
- A large enterprise with multiple sites may be an AS.
- Cloud providers (AWS, Azure) have many AS numbers.
ASN (Autonomous System Number)
Each AS is assigned an ASN.
There are two types:
| ASN Type | Purpose |
|---|---|
| Public ASN | Used on the internet between organizations |
| Private ASN | Used internally or in private hosted environments |
Exam note:
Public ASN range = 1–64511
Private ASN range = 64512–65534 (16-bit) and 64512–65535 (extended)
4. Internal vs. External BGP
There are two types of BGP:
4.1 eBGP (External BGP)
- Runs between different Autonomous Systems.
- Example: ISP A ↔ ISP B, or enterprise ↔ ISP.
- Used for internet routing.
Exam key point:
eBGP neighbors usually sit on directly connected networks.
4.2 iBGP (Internal BGP)
- Runs inside the same organization’s AS.
- Used in large enterprise or ISP networks to distribute BGP routes internally.
- Helps propagate external routes across a company’s internal routers.
Exam key point:
iBGP neighbors do not need to be directly connected. They form relationships over internal paths.
5. BGP as a Path-Vector Protocol
Unlike OSPF (link-state) or RIP (distance-vector), BGP is a path-vector protocol.
What does this mean?
BGP makes routing decisions based on:
- Path attributes (metadata about the route)
- Policies configured by the administrator
- AS-Path (the list of ASNs a route travels through)
BGP is highly tunable, meaning network engineers can control:
- Preferred paths
- Backup paths
- Load sharing across multiple upstream providers
This is why BGP is used for complex routing at internet scale.
6. BGP Routing Attributes (Important for the Exam)
BGP uses many attributes to select the best path.
For Network+, only the key ones are required:
6.1 AS-Path
- Shows the list of ASNs a route passed through.
- Shortest AS-Path is usually preferred.
Used to avoid loops.
6.2 Next-Hop
- The next router used to reach the destination network.
Critical in multi-ISP environments.
6.3 Local Preference
- Used inside an AS.
- Higher local preference = more preferred path.
Used to choose the preferred exit point of a network.
6.4 MED (Multi-Exit Discriminator)
- Suggests to external ASes which path is preferred.
- Lower MED = better.
6.5 Origin
- Indicates how the route was learned.
7. How BGP Forms Neighbor Relationships (Peering)
BGP routers form a BGP session, also called a peering or neighbor relationship.
Key requirements:
- Manual configuration of neighbor IP addresses
- Exchange of keepalive messages
- Use of TCP Port 179 (important exam point)
BGP uses TCP (Transmission Control Protocol)
Why?
- Reliable delivery
- Large routing tables
- Stability for internet routing
8. BGP Route Advertisement
A BGP router advertises networks that it knows about.
Networks are usually advertised:
- From internal routing tables
- From static routes
- From connected networks
- Or from cloud connections
Each advertisement has attributes (e.g., AS-Path) that influence routing decisions.
9. Common BGP Uses in an IT Environment (Real IT Examples Only)
Example 1: Enterprise with Two ISPs (Multi-homing)
A company connected to ISP-A and ISP-B uses BGP to:
- Receive internet routes
- Automatically failover if one ISP link fails
- Control which ISP to send outgoing traffic through
Example 2: Connecting to Cloud Providers
When a company connects to AWS Direct Connect or Azure ExpressRoute:
- BGP is used to exchange routes
- Cloud routes and on-premises routes are automatically updated
Example 3: ISP Backbone Routing
ISPs use iBGP internally to ensure every router knows:
- Which external AS provides the best internet path
- How to reach customer networks
10. Advantages of BGP
| Advantage | Explanation |
|---|---|
| Highly scalable | Handles extremely large routing tables used on the global internet |
| Policy-based routing | Administrators can control path selection |
| Supports multi-homing | Essential for redundancy with multiple ISPs |
| Loop prevention | Uses AS-Path to avoid routing loops |
| Stable | TCP-based and designed for long-lived connections |
11. Disadvantages of BGP
| Disadvantage | Explanation |
|---|---|
| Complex to configure | Not as simple as OSPF or RIP |
| Slow convergence | Takes longer to update routes compared to interior protocols |
| Requires skilled management | Misconfiguration can cause large internet outages |
| Not ideal for small networks | Too powerful and complex for simple environments |
12. Important Exam Facts to Memorize
Here is a summary of must-know BGP concepts for Network+:
- BGP = path-vector dynamic routing protocol
- Only major Exterior Gateway Protocol (EGP)
- Uses TCP port 179
- Exchanges routes between Autonomous Systems
- Two types: eBGP and iBGP
- Uses ASNs
- Key attributes: AS-Path, Next-Hop, Local Preference, MED
- Used for internet routing, ISPs, cloud connections, multi-homing
Final Summary
BGP is the routing protocol that makes the internet work.
It connects thousands of organizations, ISPs, and cloud networks together.
It uses AS numbers, path-vector logic, and policy controls to choose the best internet paths.
For CompTIA Network+, focus on the purpose, characteristics, and basic operation of BGP—not deep configuration.
