Knowledge Base

What is BGP and Why is it Used?

Border Gateway Protocol (BGP) is a protocol used to exchange routing information between different autonomous systems (AS) on the internet. Essentially, it forms the backbone of the internet and determines how different networks will communicate with each other. If you are wondering What is ASN?, you have taken an important step in understanding how BGP works. Because BGP announces the paths between ASNs and enables the routing of data traffic through these paths.

Why is BGP Used?

• Scalability: BGP is designed to cope with the size and complexity of the internet.
• Policy-Based Routing: BGP provides network administrators with the flexibility to define and implement routing policies. This makes it possible to route traffic through specific ASs or block traffic from specific ASs.
• Redundancy and Reliability: BGP allows multiple paths to be announced, ensuring that traffic is automatically routed over another path in the event of a path failure.
• Internet Standard: BGP is a widely used and accepted routing protocol on the internet.

Important Note: Although BGP is not a distance vector protocol, it is considered a path vector protocol. This means that each route announcement includes a list of ASs that the packet must pass through. In this way, routing loops can be prevented.

How Does BGP Work?

BGP works by establishing neighbor relationships. A BGP router establishes TCP connections with other BGP routers (neighbors) and shares routing information (prefixes and attributes) over these connections. This shared information creates the BGP routing table. The BGP router uses this table to select the best path and adds this path to the routing table.

Basic Steps:

1. Establishing Neighborhood (Peering): BGP routers establish neighbor relationships over TCP port 179. This relationship is configured manually.
2. Routing Information Exchange (Updates): Neighbors send each other information about their own networks and routes to other networks. This information is carried in "BGP Update" messages.
3. Best Path Selection (Path Selection): BGP uses the received routing information to select the best path. This selection is based on a number of attributes (weight, local preference, AS path length, etc.).
4. Updating the Routing Table: The selected best path is added to the routing table and data traffic is routed over this path.

Example BGP Configuration (Cisco IOS):

router bgp 65001
 neighbor 192.168.1.2 remote-as 65002
 neighbor 192.168.1.2 update-source Loopback0
 network 10.0.0.0 mask 255.255.255.0

This configuration configures a router with ASN 65001. It establishes a BGP session with the neighbor with IP address 192.168.1.2 (ASN 65002). Updates are sent via the Loopback0 interface, and the 10.0.0.0/24 network is advertised.

Important Note: Correct configuration of BGP is critical for the stability and performance of the network. Incorrect configuration can lead to routing loops, instability, and performance issues.

What are BGP Attributes and How are They Used?

BGP attributes are properties that carry routing information and are used in best path selection. Different attributes allow different routing policies to be applied.

Basic BGP Attributes:

• Origin: Specifies the origin of the route (IGP, EGP, Incomplete).
• AS Path: Contains the list of ASes the route has passed through. This attribute helps prevent routing loops, and the route with the shortest AS path is preferred.
• Next Hop: Specifies the next IP address to go to in order to reach the route.
• MED (Multi Exit Discriminator): Specifies which point to prefer when there are multiple entry points to the same AS.
• Local Preference: Specifies which route to prefer among routers within an AS. The route with the higher local preference is preferred.
• Weight: A Cisco-specific attribute that is configured locally. The route with the highest weight is preferred.
• Community: Allows tags to be added to routes. These tags can be used in the application of routing policies.

Attribute Usage Example:

A company connects to the internet through two different internet service providers (ISPs). The company primarily wants to exit to the internet via ISP1. In this case, a higher local preference value can be assigned to routes coming from ISP1. In this way, the BGP routers within the company will prefer routes coming from ISP1.

router bgp 65001
 neighbor 192.168.1.2 route-map SET-LOCAL-PREF in
!
route-map SET-LOCAL-PREF permit 10
 set local-preference 150

This configuration assigns a local preference value of 150 to routes coming from address 192.168.1.2.

Important Note: Correct understanding and configuration of BGP attributes is critical to ensure that the network operates as desired. Incorrect configuration of attributes can lead to unexpected routing behavior.

How to Establish a BGP Neighbor Relationship (Peering)?

BGP peering is a TCP connection established between two BGP routers to share routing information. This relationship is configured manually and requires correct configuration on both sides.

Step-by-Step BGP Peering Setup:

1. ASN Determination: The ASN of both parties must be determined. If you do not yet have an ASN, you can check the How to Get an ASN? guide.
2. IP Address Determination: The IP addresses that both parties will use for the peering relationship must be determined. These IP addresses must be directly connected.
3. Configuration: The BGP router is configured on both sides. The configuration includes the neighbor's IP address and ASN.
4. Verification: It is verified that the peering relationship has been established. This can be done using the "show ip bgp summary" command.

Example BGP Peering Configuration (Cisco IOS):

Router 1 (ASN 65001):

router bgp 65001
 neighbor 192.168.1.2 remote-as 65002
 neighbor 192.168.1.2 update-source Loopback0

Router 2 (ASN 65002):

router bgp 65002
 neighbor 192.168.1.1 remote-as 65001
 neighbor 192.168.1.1 update-source Loopback0

In this configuration, Router 1 establishes a BGP session with the neighbor (ASN 65002) with the IP address 192.168.1.2. Router 2 establishes a BGP session with the neighbor (ASN 65001) with the IP address 192.168.1.1. Both routers send updates over the Loopback0 interface.

Important Note: When establishing a peering relationship, care should be taken to ensure that IP addresses and ASNs are configured correctly. It is also important that firewall rules are configured to allow BGP traffic.

What is a BGP Route Reflector and Why is it Used?

A BGP route reflector (RR) is a solution used to reduce the number of BGP routers within an AS and eliminate the need for a full mesh. A full mesh means that each BGP router must establish a peering relationship with all other BGP routers. In large ASs, this can lead to a large number of peering relationships and high processing load.

How a Route Reflector Works:

The route reflector reflects (reflects) routes learned from other BGP routers (clients) to other clients and non-clients (other BGP routers). In this way, it is sufficient for clients to establish a peering relationship only with the route reflector.

Advantages of a Route Reflector:

• Scalability: Route reflectors enhance the scalability of BGP in large ASes.
• Ease of Management: Route reflectors simplify BGP configuration and ease its management.
• Resource Saving: Route reflectors reduce the processing load on routers and save resources.

Route Reflector Configuration Example (Cisco IOS):

router bgp 65001
 neighbor 192.168.1.2 remote-as 65001
 neighbor 192.168.1.2 route-reflector-client

This configuration configures the neighbor with the IP address 192.168.1.2 (within the same ASN) as a route reflector client.

Important Note: The correct positioning and configuration of route reflectors are critical for the network's performance and stability. Incorrect configuration can lead to routing loops and instability.

How to Ensure BGP Security?

Since BGP forms the backbone of the internet, its security is of great importance. Ensuring the security of BGP sessions helps prevent malicious individuals from manipulating routing information and redirecting network traffic.

BGP Security Measures:

• MD5 Authentication: MD5 authentication can be used to ensure the integrity of messages sent between BGP sessions. This prevents unauthorized individuals from interfering with BGP sessions.
• Prefix Filtering: Routers are only allowed to announce prefixes they are authorized for. This prevents the propagation of incorrect or malicious prefix announcements.
• Route Filtering: Routers are only allowed to accept routes they are authorized for. This prevents incorrect or malicious routes from entering the network.
• RPKI (Resource Public Key Infrastructure): RPKI is a system used to verify the ownership of IP addresses and ASNs. RPKI increases routing security by checking the validity of BGP prefixes.
• BGPsec (BGP Security): BGPsec provides cryptographic signing and verification of BGP messages. This significantly increases the security of BGP sessions.
• Surveillance and Monitoring: BGP sessions and routing information should be continuously monitored, and abnormal situations should be detected.

MD5 Authentication Configuration Example (Cisco IOS):

router bgp 65001
 neighbor 192.168.1.2 password MYSECRET

This configuration configures MD5 authentication with the password "MYSECRET" for the BGP session with the neighbor with the IP address 192.168.1.2.

Important Note: BGP security is a continuous process and should be updated regularly. As new security threats emerge, new security measures may need to be taken.

Common Issues and Solutions in BGP Configuration

BGP configuration can be complex and lead to various issues. Here are the most common problems and possible solutions:

• Failed to Establish Neighbor Relationship:
  • Causes: Incorrect IP address, incorrect ASN, firewall blocking, MD5 authentication error.
  • Solutions: Check IP addresses and ASNs, check firewall rules, configure MD5 authentication correctly.
• Routes Cannot Be Learned:
  • Causes: Prefix filtering, route filtering, AS path filtering, route reflector configuration error.
  • Solutions: Check filters, check AS path filtering, check route reflector configuration.
• Routing Loops:
  • Causes: Incorrect AS path configuration, route reflector configuration error.
  • Solutions: Check AS path configuration, check route reflector configuration, use loop prevention mechanisms.
• High CPU Usage:
  • Causes: Too many routes, frequent route updates, complex route maps.
  • Solutions: Reduce the number of routes, optimize route updates, simplify route maps, upgrade hardware.
• Memory Issues:
  • Causes: Too many routes, insufficient memory.
  • Solutions: Reduce the number of routes, increase the amount of memory.

Troubleshooting Tips:

• Use the "show ip bgp summary" command to check BGP neighbor relationships.
• Use the "show ip bgp" command to check the BGP routing table.
• Use the "debug ip bgp updates" command to monitor BGP updates.
• Use the "ping" and "traceroute" commands to test the connection.

BGP and Data Centers

BGP also plays an important role in answering the question of what a modern What is a Data Center? is. Data centers must connect to the internet with high bandwidth and low latency. BGP allows data centers to connect to multiple internet service providers (ISPs) and choose the best path. In addition, customers who receive What is Co-location? service can integrate into the data center network using their own ASNs and BGP configurations.

BGP Usage Scenarios in Data Centers:

• Multi-homing: Data centers establish connections with multiple ISPs, ensuring redundancy and high availability. BGP automatically routes traffic through the best path.
• Traffic Engineering: Data centers can use BGP attributes (MED, Local Preference) to route traffic through specific ISPs. This can be used to optimize costs or improve performance.
• Protection Against DDoS Attacks: BGP can be used to detect and mitigate DDoS attacks. For example, it can protect the data center network by routing attack traffic through different ISPs.

Real-Life Example:

An e-commerce company operates a website with high traffic volume. The company hosts the website in two different data centers to ensure that the website is always accessible. Each data center is connected to two different ISPs. The company uses BGP to automatically route traffic through the best path and ensures that the website operates without interruption in the event of a failure in any data center or ISP.

BGP Statistics and Comparisons

The following tables present some important statistics related to BGP and comparisons of different routing protocols.

Routing Protocols Comparison:

Important Note: Given the complexity and resource consumption of BGP, it should only be used when truly necessary. In small and medium-sized networks, simpler routing protocols such as OSPF or EIGRP may be preferred.