Routing fundamentals (for example, dynamic compared with static, BGP)

Can you explain the difference between static and dynamic routing?

Static routing involves manually configuring routes in a router’s routing table. It is simple to implement in small networks but doesn’t automatically adjust to network changes, making it less scalable. Dynamic routing, on the other hand, uses protocols such as OSPF, EIGRP, or BGP to automatically discover network destinations and determine the best paths based on the current network state, making it more adaptable and scalable for larger, changing networks.

What are the key characteristics of Border Gateway Protocol (BGP)?

BGP is a path vector protocol that manages how packets are routed across the internet. It’s widely used for routing between autonomous systems (ASes). Key characteristics include its use of TCP for reliable session establishment, its emphasis on policy-based routing decisions, and its scalability to handle millions of routes.

How does BGP select the best path to a destination?

BGP selects the best path based on attributes such as path length (AS_PATH), origin type, local preference, MED (multi-exit discriminator), and others. The decision process is hierarchical, starting with preferring routes with the highest local preference, then the shortest AS_PATH, followed by the lowest origin type (IGP is preferred over EGP, which is preferred over Incomplete), and so on.

What are the benefits and drawbacks of using static routing in a network?

Benefits of static routing include predictability, low overhead on router resources, and no additional protocol configuration or complexity. Drawbacks are lack of scalability, manual intervention for network changes, poor fault tolerance, and it is not suitable for large or complex networks.

What is route summarization, and why is it significant in dynamic routing protocols?

Route summarization is the process of consolidating a set of routes into a single summary route. It is significant because it reduces the size of routing tables, decreases routing update traffic, and improves convergence times by abstracting smaller networks into larger network representations.

Can you explain the term “autonomous system” in the context of BGP?

An autonomous system (AS) is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the internet. In BGP, ASes are identified by unique AS numbers (ASNs) used for BGP routing.

What is the purpose of an AS_PATH in BGP, and how does it prevent routing loops?

The AS_PATH is an attribute that lists the ASes a BGP route has traversed. It serves two main purposes: to select routes based on policy (such as preferring shorter paths) and to prevent routing loops. If a BGP router receives a route advertisement that includes its own ASN in the AS_PATH, it knows that the route has made a loop and will discard it.

Can you describe route propagation in BGP and how it’s related to route filters or policies?

Route propagation in BGP refers to the distribution of BGP routes between peers. Route filters or policies can be applied to influence this propagation, allowing network administrators to control which routes are advertised or accepted, which can affect traffic flow and ensure compliance with business or technical objectives.

How does BGP differ from interior gateway protocols (IGPs) such as OSPF or EIGRP?

BGP is an exterior gateway protocol (EGP) designed for routing between ASes on the public internet, focusing on policy-based routing and scalability. IGPs like OSPF or EIGRP are used within a single AS for routing based on metrics such as bandwidth or hop count, with faster convergence times and automatic network topology discovery.

Why is it important to understand the concept of route preference when configuring routing protocols on AWS?

Understanding route preference, also known as route priority or administrative distance, is important because it determines which routes are used when multiple paths to the same destination exist. On AWS, route preference is crucial for ensuring predictable traffic flow, minimizing latency, and improving redundancy and failover capabilities.

What are the primary factors affecting BGP route selection in an AWS environment?

Primary factors affecting BGP route selection in AWS include BGP attributes such as local preference, AS_PATH length, origin type, MED, and the TCP connection state between BGP peers. AWS-specific considerations also include route propagation settings in Virtual Private Gateways, routing priorities in route tables, and network ACLs.

In an AWS environment, how does BGP contribute to high availability and fault tolerance?

BGP contributes to high availability and fault tolerance by enabling redundant connections to the AWS cloud across multiple locations. It facilitates automatic failover and load balancing, as BGP will direct traffic to the best available path based on current network conditions and policies, ensuring reliability and consistent network availability.