5.4 BGP: the Border Gateway Protocol

Summary

This video delves into the Border Gateway Protocol (BGP), the crucial inter-domain routing protocol that connects the internet's autonomous systems. It explains BGP's role in both routing and policy management, highlighting its ability to control data flow based on factors beyond performance, such as geographic or ISP restrictions. The video compares BGP with OSPF, focusing on policy-driven decisions in BGP and its use of path advertisements. The discussion also touches on BGP's message types, hot potato routing, and the potential of Software Defined Networking (SDN) to centralize control in future network architectures.

Takeaways

• 😀 BGP (Border Gateway Protocol) is a critical inter-domain routing protocol that connects different networks, enabling the internet to function as a global system of networks.
• 😀 Unlike OSPF, BGP uses policy-based routing, allowing network operators to define routing decisions based on criteria beyond just performance, such as cost, security, or geopolitical factors.
• 😀 BGP routers exchange routing information via semi-permanent TCP connections using port 179, with four main types of messages: Open, Update, Keepalive, and Notification.
• 😀 BGP advertises two key things: the destination network (e.g., a network prefix) and the path attributes, including the AS path, which lists the autonomous systems that the route passes through.
• 😀 Path advertisements in BGP are subject to policy decisions, such as rejecting paths that pass through certain ISPs or countries, highlighting BGP's emphasis on policy control over routing decisions.
• 😀 eBGP (External BGP) is used for communication between routers in different autonomous systems, while iBGP (Internal BGP) is used for communication within the same autonomous system.
• 😀 In BGP, policy decisions play a significant role in whether a router accepts or advertises a particular path, and this control allows network operators to manage traffic flow according to their business interests.
• 😀 The concept of Hot Potato Routing refers to forwarding packets to the closest exit point in the network, prioritizing getting the packet out of the network as quickly as possible rather than considering the best global path.
• 😀 BGP's scalability is a key feature, allowing it to handle the vast number of networks on the internet while minimizing the propagation of internal routing information.
• 😀 The major difference between intra-domain protocols like OSPF and inter-domain protocols like BGP is that OSPF focuses on performance metrics like latency, while BGP emphasizes scalability and policy control over performance.
• 😀 While BGP’s routing decisions prioritize policy over performance, its focus on scalability ensures that routing decisions can be made for millions of networks without overwhelming the internet's infrastructure.

Q & A

What is BGP, and why is it often referred to as the glue that binds the internet?

BGP (Border Gateway Protocol) is the de facto inter-domain routing protocol used in the internet today. It is referred to as the 'glue that binds the internet' because it connects different autonomous systems (ASes) and enables routing between them, facilitating communication across the global network of networks.

How does BGP differ from OSPF in terms of routing methodology?

While OSPF (Open Shortest Path First) uses Dijkstra's link-state routing algorithm, BGP is based on the distance-vector algorithm. BGP is more focused on policy-based routing and path advertisements rather than solely calculating the shortest path.

What role does policy play in BGP routing decisions?

In BGP, routing decisions are not just based on path cost but also on various policies. For example, a network may choose not to use a path passing through a certain ISP or country, or it may avoid advertising certain paths to its neighbors to control traffic flow and reduce unwanted transit traffic.

What are the two main types of BGP and how do they differ?

The two main types of BGP are eBGP (external BGP) and iBGP (internal BGP). eBGP runs between routers in different autonomous systems, while iBGP runs between routers within the same autonomous system. Both types of BGP are crucial for the propagation of routing information across and within networks.

What are the main BGP message types and their functions?

BGP uses several message types: OPEN (to establish a session), UPDATE (to advertise or withdraw paths), KEEPALIVE (to maintain the session), and NOTIFICATION (to close a session in case of error). The UPDATE message is the most critical as it conveys the advertised paths and any changes to them.

What does the AS path attribute in BGP represent?

The AS path attribute in BGP lists the autonomous systems a packet will pass through when traveling to a destination. It helps in loop prevention and policy-based routing, as routers can use the AS path information to decide whether to accept or advertise a path.

How can BGP path advertisements be used to control routing policy?

BGP path advertisements allow network operators to control the flow of traffic by deciding which paths to advertise to other networks. For example, an ISP may advertise only paths that lead to its own customers, avoiding the advertisement of transit paths that could carry traffic for other ISPs.

What is the concept of 'hot potato' routing in BGP?

'Hot potato' routing is a policy where routers forward packets to the closest gateway to quickly exit the network, without considering the overall cost of reaching the destination. It aims to minimize the time the packet spends inside the network, even if this results in suboptimal routing.

What challenges does BGP face in terms of scalability?

BGP faces scalability challenges due to the sheer number of networks and routes it must manage. To address this, BGP avoids propagating internal routing information between autonomous systems, and it uses strategies like hierarchical routing within autonomous systems to limit the scope of information.

Why does BGP prioritize policy over performance when making routing decisions?

BGP prioritizes policy because it allows network operators to implement routing strategies that align with business needs, such as controlling transit traffic or limiting routes based on economic considerations. While performance (e.g., minimizing path costs) is important, policy often takes precedence to ensure that routing aligns with the operator's objectives.