BGP and inter-domain routing in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to describe BGP's main function.

Computer Networks

BGP is primarily used to [1] routing information between different autonomous systems.

Drag options to blanks, or click blank then click option'

Ablock

Bignore

Cexchange

Dencrypt

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'block' or 'ignore' because they sound like network actions but are incorrect.

BGP (Border Gateway Protocol) is used to exchange routing information between different autonomous systems (ASes) on the internet.

2fill in blank

medium

Complete the sentence to explain what an autonomous system (AS) is.

Computer Networks

An autonomous system is a collection of IP networks under a common [1] that presents a common routing policy to the internet.

Drag options to blanks, or click blank then click option'

Aprotocol

Badministration

Chardware

Dsoftware

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'protocol' with 'administration'.

An autonomous system (AS) is a group of IP networks managed by a single organization or administration that shares a routing policy.

3fill in blank

hard

Fix the error in the statement about BGP path selection.

Computer Networks

BGP selects the best path based on the [1] attribute, which represents the number of hops to the destination.

Drag options to blanks, or click blank then click option'

AAS_PATH

BNEXT_HOP

CLOCAL_PREF

DMED

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing NEXT_HOP or LOCAL_PREF which serve different purposes.

The AS_PATH attribute in BGP lists the autonomous systems a route has passed through and is used to select the shortest path (fewest AS hops).

4fill in blank

hard

Fill both blanks to complete the BGP route filtering rule.

Computer Networks

A BGP router will [1] routes from an AS if the route's [2] attribute indicates it came from that AS.

Drag options to blanks, or click blank then click option'

Areject

Baccept

CAS_PATH

DNEXT_HOP

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing NEXT_HOP with AS_PATH or choosing accept instead of reject.

BGP routers can reject routes if the AS_PATH attribute shows the route originated from a specific AS, helping prevent routing loops.

5fill in blank

hard

Fill all three blanks to complete the BGP route advertisement example.

Computer Networks

The BGP router advertises a route with the prefix [1] to its neighbor [2] using the [3] attribute to indicate the next hop.

Drag options to blanks, or click blank then click option'

A192.168.1.0/24

BAS 65001

CNEXT_HOP

DAS_PATH

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing AS_PATH with NEXT_HOP or confusing prefix with AS number.

The router advertises the IP prefix 192.168.1.0/24 to neighbor AS 65001 and uses the NEXT_HOP attribute to specify the next hop IP address.

Practice

1. What is the primary purpose of BGP (Border Gateway Protocol) in computer networks?

easy

A. To encrypt data packets for secure communication

B. To exchange routing information between different large networks on the internet

C. To assign IP addresses to devices within a local network

D. To manage wireless connections in home networks

Solution

1. Step 1: Understand BGP's role

   BGP is designed to connect different large networks, called autonomous systems, on the internet.

2. Step 2: Identify BGP's function

   It shares routing information to find the best path for data between these networks.

3. Final Answer:

   To exchange routing information between different large networks on the internet -> Option B

4. Quick Check:

   BGP = Routing between large networks [OK]

Hint: BGP connects big networks by sharing routes [OK]

Common Mistakes:

• Confusing BGP with encryption protocols
• Thinking BGP assigns IP addresses
• Mixing BGP with local network management

2. Which of the following is the correct syntax to define a BGP neighbor in a router configuration?

easy

A. neighbor 192.168.1.1 remote-as 65001

B. set neighbor 192.168.1.1 as 65001

C. neighbor remote-as 65001 192.168.1.1

D. remote-as neighbor 192.168.1.1 65001

Solution

1. Step 1: Recall BGP neighbor syntax

   The standard BGP command to define a neighbor includes the keyword 'neighbor', the IP address, and 'remote-as' followed by the AS number.

2. Step 2: Match syntax to options

   neighbor 192.168.1.1 remote-as 65001 matches this format exactly: 'neighbor 192.168.1.1 remote-as 65001'. Others mix order or keywords incorrectly.

3. Final Answer:

   neighbor 192.168.1.1 remote-as 65001 -> Option A

4. Quick Check:

   neighbor IP remote-as AS_number = correct syntax [OK]

Hint: Remember: neighbor IP then remote-as AS number [OK]

Common Mistakes:

• Swapping the order of IP and AS number
• Using 'set' instead of 'neighbor'
• Placing 'remote-as' before IP address

3. Consider the following simplified BGP routing table entries:

AS Path: 65001 65002 65003
AS Path: 65001 65004
AS Path: 65005 65003

Which AS path will BGP prefer to route traffic?

medium

A. AS Path: 65001 65002 65003

B. AS Path: 65001 65004

C. AS Path: 65005 65003

D. All paths are equally preferred

Solution

1. Step 1: Understand BGP path selection criteria

   BGP prefers the path with the shortest AS path length to reduce the number of hops.

2. Step 2: Compare AS path lengths

   Paths lengths are: 3 for '65001 65002 65003', 2 for '65001 65004', and 2 for '65005 65003'. Among these, the shortest paths are the ones with length 2.

3. Step 3: Consider origin and other attributes

   Since no other attributes are given, BGP will prefer the path with the shortest AS path regardless of starting AS. Both '65001 65004' and '65005 65003' have length 2, but BGP prefers the path with the lowest next-hop IP or other tie-breakers. Without additional info, the shortest path is preferred. So either could be correct, but typically the path starting with the local AS is preferred. However, since local AS is not specified, the shortest path is the main factor.

4. Final Answer:

   AS Path: 65005 65003 -> Option C

5. Quick Check:

   Shortest AS path length = preferred [OK]

Hint: Shortest AS path usually wins in BGP routing [OK]

Common Mistakes:

• Choosing longest AS path
• Ignoring AS path length
• Assuming all paths are equal

4. A network engineer configures BGP neighbors but notices no routes are exchanged. Which of the following is the most likely cause?

medium

A. The subnet mask on the interface is too large

B. The router's hostname is not set

C. The router is missing a default gateway

D. The neighbor IP address is incorrect or unreachable

Solution

1. Step 1: Identify common BGP neighbor issues

   If no routes are exchanged, a common cause is that the neighbor IP is wrong or unreachable, preventing session establishment.

2. Step 2: Evaluate other options

   Hostname settings do not affect BGP neighbor sessions. Missing default gateway or subnet mask issues may affect general connectivity but are less direct causes than neighbor IP errors.

3. Final Answer:

   The neighbor IP address is incorrect or unreachable -> Option D

4. Quick Check:

   Neighbor IP correct and reachable = routes exchanged [OK]

Hint: Check neighbor IP and reachability first [OK]

Common Mistakes:

• Ignoring neighbor IP correctness
• Blaming unrelated settings like hostname
• Overlooking basic connectivity issues

5. An ISP uses BGP to connect with multiple other ISPs. They want to prefer routes learned from ISP A over ISP B, even if ISP B's path is shorter. Which BGP feature should they use to achieve this?

hard

A. AS path prepending to make ISP B's path appear longer

B. Changing the router's hostname to ISP A

C. Using a static route instead of BGP

D. Disabling BGP on ISP B's connection

Solution

1. Step 1: Understand BGP path preference controls

   BGP chooses routes based on path attributes like AS path length. To prefer one ISP's route, you can make the other ISP's path look longer.

2. Step 2: Apply AS path prepending

   AS path prepending adds extra AS numbers to the path advertised to ISP B, making it appear longer and less preferred.

3. Step 3: Evaluate other options

   Changing hostname or using static routes won't affect BGP path selection properly. Disabling BGP on ISP B is extreme and not practical.

4. Final Answer:

   AS path prepending to make ISP B's path appear longer -> Option A

5. Quick Check:

   AS path prepending = prefer one ISP over another [OK]

Hint: Use AS path prepending to prefer routes [OK]

Common Mistakes:

• Thinking hostname affects routing preference
• Using static routes instead of BGP
• Disabling connections instead of adjusting path attributes