How BGP Routing Protocol Works

Understanding How BGP Routing Protocol Works for Security


BGP is an Internet routing protocol that allows networks to exchange information about the best route to a destination. Routers then use this information to forward traffic to the appropriate network. Engineers can influence which routes get preference by manipulating attribute values. Changing these settings can significantly impact a network’s routing topology and performance.

Route Selection Process

When a packet is destined for a network, the BGP routing protocol determines the best route to send it over by iterating through its routing policy database (RPDB). It first looks for an entry that matches the destination IP address in the RPDB. If there isn’t one, it looks for a matching entry in each of its consulted routing tables.

Each BGP speaker also advertises a list of routes it knows about to its peers. BGP uses special techniques to ensure the routing information it propagates is valid and accurate.

As a result, the rate at which a router receives routing updates can exceed its ability to process them in real-time. This is called out-of-sync, which can cause the router to propagate a ‘ghost’ route that is no longer reachable.

Routing Tables as the Brain of a Router

The routing table acts like the brain of a router, guiding data packets through the labyrinth of networks. When a router receives an IP data packet, it consults its routing table to determine the best path for forwarding. Each route entry in a routing table contains information about how to reach a particular network destination. This includes the destination network address, subnet mask, and a next-hop router to reach the destination. The routing table also stores routing metrics such as administrative distance and the cost of a given route.

When a router first connects to a new network, it may not have a complete routing table. In this case, it can advertise its routes to other routers by using routing protocols. Eventually, these other routers will add these routes to their routing tables. This process is called dynamic routing. Routing tables improve network performance by expediting the decision-making processes within routers. The more accurate a routing table, the fewer network hops required to deliver data packets.

Routing Policy in BGP

BGP routers exchange routing information and reachability data among Autonomous Systems (ASes) on the Internet to establish end-to-end connectivity. This ensures that data packets are reliably routed to their intended destination.

Each router manages a routing table for its network and the routes it learns from external peers. The routing table is updated based on path information that the BGP peers advertise, including MED, weight, origin, and local preference. Several extensions can be added to the basic BGP standard, enabling a traffic-sharing ratio based on bandwidth values configured on individual links. The route selection process takes business considerations into account as well. Competing businesses often own autonomous systems that levy fees on each other for network throughput. 

Handling Route Flapping

BGP’s role is to ensure data reaches its destination swiftly and efficiently. It does this by checking all the routes available to deliver a packet of data and picking the best one. In some cases, these routes pass between autonomous systems.

A network administrator can configure a router to filter certain routes based on various factors. For example, a network may want to filter routes based on its internal routing policy, or it might prefer routes with a lower cost. This allows the network to optimize its connectivity and improve performance.

Another way that BGP can optimize its route selection is through a process called route aggregation. This involves combining multiple links with the same prefix into a single link and then advertising this as one route to other routers on its local network. This improves efficiency and reduces routing updates.

The Internet is a vast network with many different autonomous systems, each operating its network of routers. As such, each autonomous system has rules for how BGP routes can flow. These rules sometimes conflict, resulting in routing changes known as flapping. These changes can cause traffic to be unreliable and slow to stabilize, leading to Internet outages. A solution to this problem is a feature known as route flap dampening, which helps reduce the effects of flapping on network stability.

Security Considerations in BGP

BGP is the language that stitches together all of the networks that deliver what we think of as the Internet. Networks share information about the fastest-to-reach routes so that data can be sent to where it needs to go.

BGP does not include security mechanisms but is based on the trust that networks will configure their systems correctly and not send incorrect routing information to their neighbors. This is an important part of what makes the Internet work, but it also leaves the system prone to mistakes and malicious attacks.

One common problem is route flapping, where a router announces a route but then withdraws and re-announces it rapidly. This can cause other routers to become overloaded with the routing update. BGP uses route flap dampening to combat this, which blocks IP prefixes that flap too often in a short period. 


How does BGP influence route preference?

Engineers can manipulate attribute values to influence route preference in BGP, impacting the network’s routing topology.

What is the purpose of routing tables in BGP?

Routing tables act as the brain of a router, guiding data packets through the network by containing information about reaching specific network destinations.

How does BGP handle route flapping?

BGP uses route flap dampening to mitigate the effects of route flapping, blocking IP prefixes that flap too often in a short period.


In conclusion, BGP plays a crucial role in determining the best routes for data packets across the Internet. Understanding its route selection process, the significance of routing tables, and the impact of routing policies is essential for optimizing network performance. Despite its effectiveness, BGP lacks inherent security measures, making it susceptible to issues like route flapping. By incorporating features like route flap dampening, network administrators can enhance BGP’s stability and reliability in the ever-evolving landscape of the Internet.


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