2.2.9 Border Gateway Protocol (BGP) 101

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Questions and Answers

What primary function does the Border Gateway Protocol (BGP) serve?

  • To control how data flows across different autonomous systems. (correct)
  • To provide error correction and flow control over UDP.
  • To manage local network traffic within a single autonomous system.
  • To automatically configure network peering relationships.

Over which transport protocol does BGP operate?

  • ICMP
  • UDP
  • TCP (correct)
  • IPSec

What is the purpose of an Autonomous System Number (ASN) in BGP?

  • To prioritize traffic based on quality of service (QoS) metrics.
  • To encrypt data exchanged between routers.
  • To dynamically allocate IP addresses to devices.
  • To identify different entities within the network, such as different peers. (correct)

What does it mean when an Autonomous System is described as a 'black box' from a BGP perspective?

<p>Its internal routing details are irrelevant to external BGP operations. (D)</p> Signup and view all the answers

What is the primary function of the AS Path attribute in BGP?

<p>To track the autonomous systems a route advertisement has traversed. (B)</p> Signup and view all the answers

In the context of the Brisbane network example, what does a 'Next Hop' IP address of 0.0.0.0 indicate in the route table?

<p>The destination network is directly connected to the local router. (B)</p> Signup and view all the answers

When Brisbane advertises its network to Adelaide, what action does Adelaide take concerning the AS Path?

<p>Adelaide prepends its own ASN to the AS Path. (D)</p> Signup and view all the answers

Why does BGP, by default, prefer the shortest AS Path when selecting a route?

<p>Shorter AS Paths usually indicate a more direct route, reducing the number of hops. (D)</p> Signup and view all the answers

What is AS path prepending, and why would a network administrator implement it?

<p>A process of artificially lengthening the AS Path to influence route selection. (A)</p> Signup and view all the answers

Which of the following best describes the function of BGP in a network?

<p>It facilitates inter-domain routing between autonomous systems. (A)</p> Signup and view all the answers

Which organization allocates Autonomous System Numbers (ASNs)?

<p>IANA (A)</p> Signup and view all the answers

What is the range of private ASNs?

<p>64512-65534 (B)</p> Signup and view all the answers

What is the key difference between Internal BGP (iBGP) and External BGP (eBGP)?

<p>iBGP handles routing within an AS, while eBGP handles routing between ASs. (C)</p> Signup and view all the answers

Which of the following is NOT a characteristic of BGP?

<p>Automatic peering configuration (A)</p> Signup and view all the answers

How does BGP contribute to the construction of a network topology map?

<p>By exchanging network reachability information between autonomous systems. (A)</p> Signup and view all the answers

If a network engineer wants to ensure that the satellite connection between Alice Springs and Brisbane is only used as a backup, what BGP technique should they implement?

<p>AS path prepending (B)</p> Signup and view all the answers

Which of these statements is true regarding private ASNs?

<p>They can be used internally without registration. (B)</p> Signup and view all the answers

What does the 'I' in the ASPATH column of a BGP route table signify?

<p>The network was learned from a locally connected network. (C)</p> Signup and view all the answers

Which of the following protocols is BGP most closely related to?

<p>OSPF (Open Shortest Path First) (D)</p> Signup and view all the answers

What characteristic differentiates BGP from protocols like OSPF or RIP?

<p>It considers a variety of factors including policy and autonomous systems traversed. (C)</p> Signup and view all the answers

What does it mean when BGP is described as a 'path-vector' protocol?

<p>It exchanges the best path to a destination between peers, not every path. (A)</p> Signup and view all the answers

If the network administrator for Alice Springs configures AS path prepending, what is the likely outcome?

<p>Traffic from Brisbane is more likely to use the path via Adelaide. (C)</p> Signup and view all the answers

What is the most significant reason BGP does not account for link speed or link condition by default when choosing a path?

<p>It prioritizes policy decisions over technical metrics. (D)</p> Signup and view all the answers

Which of the following statements about Autonomous Systems (AS) interconnected with BGP is most accurate?

<p>Each AS is controlled by a single administrative entity. (B)</p> Signup and view all the answers

Imagine a scenario where the link between Adelaide and Brisbane fails completely. What will happen to the routing of traffic between Alice Springs and Brisbane?

<p>Traffic will automatically reroute via a different path, if one exists, due to BGP's dynamic nature. (A)</p> Signup and view all the answers

Which of the following best describes the relationship between BGP and Autonomous Systems (AS)?

<p>BGP helps Autonomous Systems exchange routing information with each other. (D)</p> Signup and view all the answers

In the context of BGP, what does the term 'peering' refer to?

<p>The establishment of a BGP session between two routers in different Autonomous Systems. (A)</p> Signup and view all the answers

The range of AS numbers from 0 to 65535 represents 16 bit ASN; however, the range 64,512 to 65,534 are private and can be used within private peering arrangements. What would occur if two companies both used the same private AS number and then tried to peer?

<p>Establishment of peering would be blocked due to the AS number overlap. (D)</p> Signup and view all the answers

Which statement best describes how BGP chooses between multiple paths to the same destination?

<p>BGP uses a complex decision process with multiple attributes, but prefers the 'shortest' AS Path by default. (A)</p> Signup and view all the answers

Which of the following is true regarding traffic flow originating from Alice Springs destined for Brisbane, assuming default BGP configuration and both a direct satellite link and a path via Adelaide are available?

<p>Traffic would use the direct satellite link, as the default BGP configuration would see the shortest AS Path. (D)</p> Signup and view all the answers

What is the purpose of having private Autonomous System Numbers (ASNs)?

<p>To allow internal networks to operate BGP without needing a globally unique ASN. (C)</p> Signup and view all the answers

If an Autonomous System is multi-homed (connected to multiple other ASs), how does BGP influence traffic engineering and path selection outbound?

<p>It can influence outbound traffic using techniques like AS path prepending or local preference. (D)</p> Signup and view all the answers

What happens if a router receives multiple BGP advertisements for the same network prefix?

<p>It applies the BGP best path selection algorithm to choose the best route. (D)</p> Signup and view all the answers

What information that influences routing is contained in the AS Path?

<p>It contains a list of every Autonomous System in the route. (B)</p> Signup and view all the answers

What is a typical result of configuring AS Path Prepending on a BGP router?

<p>Other networks are made less likely to route traffic through that router. (A)</p> Signup and view all the answers

In the context of the Australia network diagram, Brisbane's routing table shows two possible routes to the 10.18.0.0/16 network: a direct route via the satellite link with an AS Path of '202, i' and an indirect route via Adelaide with an AS Path of '201, 202, i'. Assuming default BGP configuration, which route will Brisbane prefer, and why?

<p>The direct route via satellite, as it has a shorter AS Path length. (B)</p> Signup and view all the answers

An enterprise network is connected to two different Internet Service Providers (ISPs) using eBGP. Due to a business agreement, the enterprise wants most of its outbound traffic to use ISP A, but maintain a backup route through ISP B. How can the enterprise achieve this using BGP?

<p>Configure a higher local preference value for routes learned from ISP A. (D)</p> Signup and view all the answers

Given the multi-attribute decision process in BGP, what is an attribute that causes a router to prefer one learned route over another?

<p>The MED or Metric of the learned route on a shared subnet (C)</p> Signup and view all the answers

Flashcards

What is BGP?

A routing protocol used to control how data flows between networks.

What is an Autonomous System (AS)?

Routers controlled by a single entity, representing a network.

What are Autonomous System Numbers (ASN)?

Unique identifiers allocated by IANA to Autonomous Systems.

What is a path-vector protocol?

Exchanges the best path to a destination between peers.

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What is a peering relationship?

A BGP relationship between two different autonomous systems, manually configured.

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What is an AS Path?

The path or way to get from one autonomous system to another.

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What is Internal BGP (iBGP)?

Focuses on routing within an autonomous system.

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What is External BGP (eBGP)?

Focuses on routing between autonomous systems.

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What is AS Path Prepending?

Where an AS advertises the shortest path to other ASes.

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What is TCP?

BGP operates over this reliable, distributed protocol.

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What are internet assigned numbers?

IANA provides these for each autonomous system.

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What is 16 bits?

How many bits are ASN's?

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Study Notes

  • Border Gateway Protocol (BGP) is a routing protocol to control how data flows between points.

Border Gateway Protocol (BGP) Key Points

  • Autonomous System (AS) describes routers controlled by one entity within BGP.
  • Autonomous System Numbers (ASN) identify entities; public ASNs are allocated by IANA.
  • ASN are 16 bits in length and range from 0 to 65,535.
  • IANA directly allocates most public ASN, but ranges like 64,512 to 65,534 are private.
  • Operates over TCP port 179 meaning it's reliable because it includes error correction and flow control.
  • Requires manual configuration of peering relationships.
  • Exchanges the best path to a destination between peers and doesn't exchange every path, only the best path that an AS is aware of.
  • Path-vector protocol; the path is known as the ASPATH or AS Path.
  • BGP focuses on paths and building a network topology map to allow the exchange between different autonomous systems.
  • Doesn't take link speed or condition into account.
  • iBGP focuses on routing inside an autonomous system.
  • eBGP focuses on routing between autonomous systems.
  • Autonomous systems are allocated a number by IANA, the Internet Assigned Numbers Authority.
  • BGP is responsible for choosing the best path for network traffic, and announcing that path to other routers on the internet.

BGP Australia Example

  • Brisbane has an IP address range of 10.16.0.0/16, router IP of 10.16.0.1, and an ASN of 200.
  • Adelaide has a network range of 10.17.0.0/16, router IP of 10.17.0.1, and an ASN of 201.
  • Alice Springs uses network 10.18.0.0/16, router 10.18.0.1, and ASN 202.
  • Brisbane and Adelaide has A1 Gigabit Fiber links.
  • Alice Springs is connected to Brisbane by A5 megabit satellite link with unlimited data cap.
  • Autonomous systems advertise the shortest route they're aware of to any other autonomous systems that they have peering relationships with.
  • AS Path prepares to configure BGP at Alice Springs to make the satellite link look worse than it actually is by adding additional Autonomous systems to the path to make it appear longer.

Route Table Information

  • The route table included the destination.
  • The route table included the next hop so what IP address is needed as the first or next hop to get to that network and 0.0.0.0 means that it's locally connected.
  • The ASPATH shows the path or way to get from one autonomous system to another and the I in this case means that it's the origin, so it's this network.
  • Each AS prepends its number onto the path and peers exchange the best paths they have to a destination.
  • A BGP Autonomous system advertises the shortest path to a destination that it's aware of to all of the other BGP routers that it's paired with.

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