BGP Basics and Network Management Quiz

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Questions and Answers

What is the primary purpose of BGP in internet routing?

To connect multiple routers within a single AS

To directly route packets to their specific destination addresses

To obtain prefix reachability information and determine the best routes (correct)

To provide detailed error messaging for routing failures

Which components define the entries in a BGP router's forwarding table?

Protocols and protocols versions

IP addresses and port numbers

Gateway routers and internal routers

Prefixes and interface numbers (correct)

What distinguishes a gateway router from an internal router in BGP?

Internal routers have lower performance compared to gateway routers.

Gateway routers connect directly to other ASs, while internal routers connect only within the AS. (correct)

Gateway routers connect to hosts within the AS, while internal routers connect to external ASs.

Gateway routers only use BGP, while internal routers can use any routing protocol.

How does BGP determine the best route to a prefix?

Through a local BGP route-selection procedure considering policies and reachability Signup and view all the answers

What type of protocol is BGP categorized as?

A decentralized asynchronous protocol Signup and view all the answers

What kind of information does an AS advertise using BGP?

Prefix reachability information Signup and view all the answers

What is a characteristic of BGP routing tables?

They often contain over half a million routes Signup and view all the answers

What is the primary function of the source quench ICMP message?

To inform the source to decrease its transmission rate due to congestion Signup and view all the answers

Which of the following statements accurately describes how traceroute operates?

It sends UDP datagrams with unlikely UDP port numbers, incrementing TTL for each subsequent message. Signup and view all the answers

What are the main components involved in network management?

Managing server, managed device, data, and network management agent Signup and view all the answers

Which aspect of network management does not fall within the described considerations?

Software deployment and version control Signup and view all the answers

In traceroute operations, when does the source obtain the round-trip time for a datagram?

When the source receives an ICMP message indicating TTL expiration Signup and view all the answers

What process occurs when the link between switches s1 and s2 goes down?

Switch s1 informs the SDN controller using an OpenFlow message. Signup and view all the answers

Which role does the SDN controller play when it receives a notification of a link state change?

It informs the link-state manager. Signup and view all the answers

In what scenario does the link-state routing application become active?

When link state changes are detected. Signup and view all the answers

What is the primary function of ICMP in networking?

To communicate network-layer information, particularly for error reporting. Signup and view all the answers

What ICMP message does the ping program send to the specified host?

ICMP type 8 code 0 Signup and view all the answers

How does the Internet Control Message Protocol (ICMP) relate to IP datagrams?

ICMP messages are carried inside IP datagrams. Signup and view all the answers

What is one of the aims of network functions virtualization (NFV)?

To replace dedicated hardware with commodity servers and services. Signup and view all the answers

Which of the following is NOT a step taken by the flow table manager after link-state changes?

Notify the SDN controller of completed updates. Signup and view all the answers

What does Dijkstra’s algorithm specifically do in the context of SDN?

It computes new least-cost paths upon receiving notifications of link state changes. Signup and view all the answers

What identifies an ICMP message, allowing it to function correctly?

The type and code field along with the header. Signup and view all the answers

What does the term 'eBGP' refer to?

A BGP session between routers in different autonomous systems. Signup and view all the answers

Which statement about iBGP connections is correct?

They can exist without needing a direct physical link. Signup and view all the answers

What is included in a BGP message when advertising a prefix?

The AS-PATH and NEXT-HOP attributes. Signup and view all the answers

How do routers typically exchange routing information in BGP?

Over semi-permanent TCP connections using port 179. Signup and view all the answers

What happens after gateway router 3a sends the eBGP message 'AS3 x'?

Gateway router 2c forwards the iBGP message 'AS3 x' to all routers in AS2. Signup and view all the answers

Why might a router need to choose among different paths to reach a destination?

To determine the best path based on routing metrics and attributes. Signup and view all the answers

What information does AS2 learn as it sends the message 'AS2 AS3 x' to AS1?

The existence of x and the path taken through both ASs. Signup and view all the answers

What is the role of the NEXT-HOP attribute in BGP?

Defines the immediate router to which packets should be sent next. Signup and view all the answers

When a router receives reachability information for multiple paths, what is one primary criterion it uses to determine the best path?

The path with the least number of AS hops. Signup and view all the answers

Which attribute is primarily used for the initial route selection in BGP?

Local preference value Signup and view all the answers

What is used to break ties among routes with the same local preference value?

Shortest AS-PATH Signup and view all the answers

What disadvantage does IP-anycast present when used with CDNs?

Inconsistent TCP packet routing Signup and view all the answers

How does BGP handle multiple route advertisements for the same IP address?

It treats them as independent paths to a single location Signup and view all the answers

What does the term 'hot potato routing' refer to in BGP?

Choosing the closest NEXT-HOP router Signup and view all the answers

Which mechanism is primarily used to direct DNS queries to the nearest root DNS server?

Anycast routing Signup and view all the answers

What happens when a BGP router receives multiple routes from different ASs?

It evaluates local preference followed by AS-PATH length Signup and view all the answers

What is the primary reason CDNs choose not to implement IP-anycast?

Potential for inconsistent packet delivery Signup and view all the answers

Which of the following is not a criterion used in the BGP route-selection algorithm?

Next-hop bandwidth Signup and view all the answers

In the event of a routing tie, which BGP attribute is utilized last in the route selection process?

BGP identifier Signup and view all the answers

Study Notes

Computer Networks: The Network Layer

This presentation discusses the network layer and its control plane.
The outline covers various topics such as introductions, routing algorithms, intra-AS routing (OSPF), inter-AS routing (BGP), SDN control planes, ICMP protocol, and network management protocols.

Routing Among the ISPs: BGP

BGP (Border Gateway Protocol) is a decentralized, asynchronous protocol used for inter-AS routing.
BGP routes packets to CIDRized prefixes (representing subnets or collections of subnets), not specific destination addresses.
A router's forwarding table has entries in the format (x, I), where x is a prefix (e.g., 138.16.68/22) and I is an interface number.

The Role of BGP

BGP allows routers to obtain prefix reachability information from neighboring autonomous systems (ASs).
Each subnet advertises its existence to the rest of the internet within each associated autonomous system.
BGP utilizes a route-selection procedure to determine the "best" routes, considering both reachability and policy.
BGP routing tables can contain over half a million routes.

Advertising BGP Route Information

Each router in an autonomous system (AS) is either a gateway router or an internal router.
A gateway router connects to one or more routers in other ASs and directly interacts with other Autonomous Systems.
An internal router connects only with hosts and routers within its own AS.
ASes exchange routing information via routers.

Advertising BGP Route Information Example

In the displayed network example, AS3 advertises subnet x to AS2.
AS2 advertises the path AS2-AS3-x to AS1.
Each AS simultaneously learns about the existence of a path and the existence of the path itself to destination x.
Routers, not autonomous systems, are the agents acting upon routing information.

Advertising BGP Route Information (TCP Connections)

BGP routing information exchange happens over semi-permanent TCP connections with port 179.
External BGP (eBGP) connections span across different autonomous systems.
Internal BGP (iBGP) connections exist between routers within the same autonomous system.
Typically, there is one eBGP connection per direct link between gateway routers in different ASes.
There can be multiple iBGP connections between routers within an AS that don't always correspond to physical links.

Determining the Best Routes

Routers choose the best route among multiple available paths based on attributes like AS-PATH and NEXT-HOP. The AS-PATH attribute records the sequence of ASes in the route, and the NEXT-HOP attribute identifies the IP address of the next router along the chosen path.
This prioritizes paths with shorter AS-PATHs, but also considers policy and local preference.

Hot Potato Routing

The hot-potato routing algorithm prioritizes sending network traffic out of an AS as quickly as possible, without considering costs or intermediate network segments outside the specific AS.
Two routers in the same AS can use different paths to reach the same destination prefix.

IP-Anycast

IP-anycast allows replicating content across multiple servers in different countries.
This allows users to access the nearest server efficiently.
BGP is used to create paths to multiple copies of the server.

Routing Policy

Routing policies are used to determine routing within and between autonomous systems.
Policies favor traffic going to a direct customer through the shortest path, minimizing cost and routing overhead.
The policy decides which route from multiple possible ones is to be used for traffic with a common destination prefix.

Access ISP Policy

All the traffic entering or leaving an access network, must be destined to or from the access network where the traffic originated.
- Selective route advertisement policy is used to only advertise the paths for given access network, without unnecessary routing complexity.

Backbone/Provider ISP Policy

ISPs commonly use the rule that traffic routed across their backbone network must have a destination or source within a customer network of that ISP.
Peerings between ISPs are negotiated privately and have confidential agreements.

Why Different Routing Protocols?

Policy dictates routing decisions between Autonomous Systems (ASs).
Scaling capability to manage large networks is a key aspect when selecting routing protocols.
Performance of the chosen routes is a lesser concern between ASs, but it is important inside an AS where routing prioritizes high throughput and minimized end-to-end latency.

Obtaining Internet Presence

Obtaining internet connectivity requires contracting with and connecting to an ISP.
The ISP provides an IP address range.
The customer (e.g., an organization) assigns these IP addresses within the range to devices (e.g., Web Servers, mail servers, DNS servers, and gateway router).
The customer then contracts with an internet registrar to obtain a relevant domain name which in turn is registered to be associated with the provided IP addresses of its services.

ICMP: The Internet Control Message Protocol

ICMP is used by systems (hosts or routers) to exchange network-layer information, with a most frequent use-case as an error reporting tool.
ICMP messages are transported within IP datagrams.
Format of an ICMP message, includes headers fields for Type and Code that describe the type and specific nuance of the message.

The Internet Control Message Protocol Example Messages

Ping: the program sends ICMP message type 8, code 0.
Source quench: ICMP message to force a host that overloaded a router to reduce its transmission rate. This is seldom (if ever) used.
Traceroute: the program sends a series of UDP packets with incrementing Time-To-Live (TTL) values to determine paths, or routes, needed to reach a target.

Network Management

Managing network components, including people and software tools in addition to hardware components such as routers, and switches.
Objectives of network management include deploying, integrating, and coordinating network elements, as well as monitoring, evaluation, analyzing the performance of the operational network, identifying and responding to faults, and implementing changes to meet performance expectations defined by Service Level Agreements (SLAs).

Components of Network Management

A managing server located in a central network operations center (NOC), to oversee, and execute, all operations pertaining to networked devices within the purview of that NOC.
Managed devices include all network-connected components: hosts, routers, switches, and other hardware components.
Network management agents are software processes running on the managed devices to communicate with the managing server.

Ways to Manage the Network

Command line interface (CLI): a method to manage networks which is prone to errors, and difficult to automate or scale to larger networks.
SNMP/MIB: a technique using Simple Network Management Protocol (SNMP) that can query and set values of network data via Management Information Base (MIB) objects.
NETCONF/YANG: data modeling and communication language and protocol used to configure and manage devices remotely and uniformly.

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Description

Test your knowledge on Border Gateway Protocol (BGP) and its role in internet routing. This quiz covers key concepts, components of BGP, and aspects of network management. Challenge yourself with questions about routing tables, ICMP messages, and traceroute operations.