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What is the primary function of BGP in inter-domain routing?

To ensure loop-free inter-domain routing

What is internal routing?

The art of getting each router to know how to get to every location in the network

What is the advantage of using internal routing protocols such as OSPF?

They are more robust and complicated

What is the purpose of external routing?

To take external routes into the network and have control over where data goes on the Internet Signup and view all the answers

What is AS_Path tree?

A tree-like structure used to represent the path of AS numbers Signup and view all the answers

What is the purpose of BGP in your network?

To take external routes into the network and have control over where data goes on the Internet Signup and view all the answers

What is the advantage of using BGP?

It's nice to have routing data for parts of the Internet in your routers Signup and view all the answers

Why is internal routing critical?

Because any box inside your network needs to know how to get to any other box inside your network Signup and view all the answers

What is the purpose of the BGP Decision Algorithm?

To select the single best path for reaching a destination Signup and view all the answers

What is the purpose of the Origin Attribute in BGP?

To specify the origin of a route Signup and view all the answers

What is the purpose of the KEEPALIVE message in BGP?

To determine whether peers are reachable Signup and view all the answers

What is the function of the Version field in the OPEN Message format?

To indicate the version of the BGP protocol Signup and view all the answers

What is the purpose of the AS path Attribute in BGP?

To specify the list of ASNs of all ASs a route has traversed Signup and view all the answers

What is the purpose of the Next Hop Attribute in BGP?

To specify where to send data destined for a route Signup and view all the answers

What is the maximum time in seconds that may elapse between the receipt of successive KEEPALIVE or update messages?

The Hold Time Signup and view all the answers

What is the purpose of the Multi-Exit Discriminator Attribute in BGP?

To specify a weight for a route that can be used outside and inside an ASN Signup and view all the answers

What is the BGP Identifier?

A 4-byte field that indicates the sender's ID (Router ID) Signup and view all the answers

What is the purpose of the Local Preference Attribute in BGP?

To specify a weight for a route that can be used inside an ASN Signup and view all the answers

What is the purpose of the NOTIFICATION MESSAGE in BGP?

To detect errors in the routing protocol and close the peer connection Signup and view all the answers

What is the Optional Parameter field in the OPEN Message format?

A triplet that represents the Optional Parameters Signup and view all the answers

What are the possible values of the Origin Attribute in BGP?

IGP, EGP, and Incomplete Signup and view all the answers

How does an AS advertise the networks that it originates in BGP?

Using any of the three methods: redistributing static routes, redistributing dynamic routes, or using the network command Signup and view all the answers

What is the size of the My Autonomous System field in the OPEN Message format?

2-byte field Signup and view all the answers

What is the Loopback address?

The representation of the IP address of a virtual software interface that is considered to be up at all times Signup and view all the answers

What is the main advantage of CIDR over BGP3?

It eliminates the concept of classes Signup and view all the answers

What is the process of combining several different routes into a single route called?

Aggregation Signup and view all the answers

What is required for a router to aggregate an address?

A more specific route of that address in the BGP routing table Signup and view all the answers

What is the purpose of confederations in BGP?

To reduce the IBGP mesh inside the AS Signup and view all the answers

How are mini-ASs connected in a confederation?

They are fully meshed Signup and view all the answers

What is represented by the /16 in the CIDR notation 192.213.0.0/16?

The number of bits in the subnet mask Signup and view all the answers

What is the result of aggregating routes in BGP?

The size of routing tables is minimized Signup and view all the answers

What is the relationship between peer group members and configuration options?

Peer group members can only override options that do not affect outgoing updates Signup and view all the answers

Why would you speak BGP to a single upstream provider?

To have more control in advertising your routes Signup and view all the answers

What is a valid reason to configure BGP as a single-homed network?

To have more control in advertising your routes Signup and view all the answers

What is the goal of multi-homing?

To use both connections in a load-balanced manner Signup and view all the answers

What is 'fail-over' routing?

Routing that switches to another connection when one goes down Signup and view all the answers

Why might an upstream provider not want to configure BGP with a single-homed network?

Because it involves a fair amount of behind-the-scenes work Signup and view all the answers

What might happen if a single-homed network announces its routes more specifically?

The routing tables will be polluted with extra routes Signup and view all the answers

What is a consideration when deciding whether to announce routes more specifically as a single-homed network?

The flap argument and routing-table space Signup and view all the answers

What should you do if you want to configure BGP as a single-homed network?

Filter all incoming routes and announce your networks Signup and view all the answers

What is the main advantage of CIDR over BGP3?

It eliminates the concept of classes (Class A, Class B, and so on) Signup and view all the answers

What is required for a router to aggregate an address?

A more specific route of that address in the BGP routing table Signup and view all the answers

What is the purpose of confederations in BGP?

To reduce the IBGP mesh inside the AS Signup and view all the answers

What is represented by the /16 in the CIDR notation 192.213.0.0/16?

The number of bits in the subnet mask Signup and view all the answers

What is the result of aggregating routes in BGP?

The size of the routing tables is minimized Signup and view all the answers

What is the relationship between peer group members and configuration options?

Peer group members can override configuration options if the options do not affect outgoing updates Signup and view all the answers

What is the process of combining several different routes into a single route called?

Aggregation Signup and view all the answers

How are mini-ASs connected in a confederation?

Each mini-AS is fully meshed, and IBGP is run among its members Signup and view all the answers

What happens if Router A does not redistribute network 170.10.0.0 into an IGP?

Router E will have no way of knowing that network 170.10.0.0 exists and will drop the packets Signup and view all the answers

Why does Router B send traffic to Router E to reach network 170.10.0.0?

Because Router B receives updates about network 170.10.0.0 via IBGP Signup and view all the answers

What will happen if Router B advertises to AS 400 that it can reach 170.10.0.0 before Router E learns about the network via IGP?

Traffic coming from Router D to Router B with a destination of 170.10.0.0 will flow to Router E and be dropped Signup and view all the answers

What is the purpose of redistribution in the context of Figure 60?

To allow IGP to learn about networks advertised by IBGP Signup and view all the answers

In the context of Figure 60, what is the role of Router C?

To send updates about network 170.10.0.0 to Router A Signup and view all the answers

Why does Router E drop packets destined for 170.10.0.0?

Because it has no route to the network Signup and view all the answers

What is the consequence of router B advertising to AS 400 that it can reach 170.10.0.0 before Router E learns about the network via IGP?

Traffic from Router D to Router B will be dropped Signup and view all the answers

What is the relationship between Router A and Router B?

They are IBGP peers Signup and view all the answers

What is the term used to describe a route with a particular starting point and a particular specificity?

Prefix Signup and view all the answers

What is the protocol used to carry routing information between autonomous systems?

BGP Signup and view all the answers

What is the purpose of the initial exchange of all candidate BGP routes between peer routers?

To provide a complete view of the network topology Signup and view all the answers

What is the result of not having strict enough filters on the routes you advertise?

The address space becomes disconnected from the internet Signup and view all the answers

What is the transport protocol used by BGP?

TCP Signup and view all the answers

What is the term used to describe the sequence of AS numbers that indicates the path a route has traversed?

AS Path Signup and view all the answers

What is the purpose of incremental updates in BGP?

To reflect changes in the network topology Signup and view all the answers

What is the relationship between two BGP routers that have formed a transport protocol connection?

They are neighbors or peers Signup and view all the answers

Which protocol does BGP use as its transport protocol?

TCP Signup and view all the answers

What is the purpose of redistributing BGP routing information to Interior Gateway Protocols (IGPs) within an AS?

To allow for internal routing within the AS Signup and view all the answers

What is the term for any two routers that have opened a TCP connection to each other for the purpose of exchanging routing information?

BGP neighbors Signup and view all the answers

What is the purpose of the AS_Path attribute in BGP?

To construct a loop-free map of ASs Signup and view all the answers

What is the result of aggregating routes in BGP?

All of the above Signup and view all the answers

What is the purpose of EBGP in external routing?

To exchange routing information between ASs Signup and view all the answers

What is the term for the process of combining several different routes into a single route?

Route aggregation Signup and view all the answers

What is the purpose of CIDR in IP routing?

To enable supernetting Signup and view all the answers

What is the primary purpose of advertising routes to other networks (Autonomous Systems) in BGP?

To inform other networks about the routes you know how to get to Signup and view all the answers

What is the result of advertising routes that you don't know how to get to in BGP?

Black-holing someone Signup and view all the answers

What is an Autonomous System (AS) in the context of BGP?

A network with its own internal routing scheme Signup and view all the answers

What is the purpose of a Network Operations Center (NOC) in an Autonomous System?

To serve as a point of contact for the network Signup and view all the answers

What is the benefit of advertising routes to other networks in BGP?

It allows other networks to know how to reach destinations within your network Signup and view all the answers

What is the consequence of advertising routes more specifically as a single-homed network?

It may cause other networks to prefer your route Signup and view all the answers

What is the purpose of a border router in an Autonomous System?

To peer and exchange routes with other ASs Signup and view all the answers

What is the importance of knowing how to get to a route in BGP?

It is crucial to ensure proper routing and prevent black-holing Signup and view all the answers

What is the primary function of an Autonomous System (AS)?

To operate under the same administration Signup and view all the answers

What is the benefit of BGP4 over earlier versions of BGP?

It can handle classless routes Signup and view all the answers

What is the purpose of BGP in inter-domain routing?

To provide loop-free interdomain routing between autonomous systems Signup and view all the answers

What is the result of aggregating routes in BGP?

Combining several different routes into a single route Signup and view all the answers

What is the relationship between BGP and internal routing?

BGP is used for interdomain routing between autonomous systems Signup and view all the answers

What is the role of confederations in BGP?

To connect mini-ASs Signup and view all the answers

What is represented by the /16 in the CIDR notation 192.213.0.0/16?

The number of bits in the subnet mask Signup and view all the answers

What is the purpose of BGP neighbors?

To exchange information between autonomous systems Signup and view all the answers

What type of BGP is used to exchange BGP updates within an AS?

IBGP Signup and view all the answers

Why is IBGP more flexible than redistributing routes into IGP?

IBGP provides more efficient ways of controlling the exchange of information within the AS Signup and view all the answers

What type of BGP is used between routers A and B in Figure 9?

EBGP Signup and view all the answers

What is the purpose of keepalive messages in BGP?

To ensure that the connection is up Signup and view all the answers

What is required for IBGP peers to communicate with each other?

IGP running between the peers Signup and view all the answers

What is AS 200 in Figure 9?

A transit AS Signup and view all the answers

What is the main advantage of using IBGP over redistributing routes into IGP?

IBGP provides more efficient ways of controlling the exchange of information within the AS Signup and view all the answers

What is the requirement for BGP speakers within an AS?

They must establish a peer relationship with each other Signup and view all the answers

What is the primary goal of internal routing?

To get each router in your network to know how to get to every location Signup and view all the answers

What is the benefit of using internal routing protocols such as OSPF?

They provide a more robust and complicated way to route data Signup and view all the answers

What is the purpose of BGP in a network with multiple providers?

To allow for more control over where your data goes on the Internet Signup and view all the answers

What is the result of not having a running and happy network?

You can't invite people to send data to your network Signup and view all the answers

What is the advantage of having routing data for parts of the Internet in your routers?

It's nice to have Signup and view all the answers

What is required for a router to know how to get to every location in the network?

A running and happy network Signup and view all the answers

What is the size of the Version field in the OPEN Message format?

1-byte Signup and view all the answers

What is the benefit of using BGP in a network?

It allows for more control over where your data goes on the Internet Signup and view all the answers

What is the purpose of the KEEPALIVE message in BGP?

To determine whether peers are reachable Signup and view all the answers

What is represented by the triplet in the Optional Parameter field?

Authentication information parameter Signup and view all the answers

What is the primary purpose of having internal routing in a network?

To get each router in your network to know how to get to every location Signup and view all the answers

What is the purpose of the NOTIFICATION message in BGP?

To detect errors in the routing protocol Signup and view all the answers

What is the maximum time in seconds that may elapse between the receipt of successive KEEPALIVE or update messages?

The value of the Hold Time field Signup and view all the answers

What is the BGP Identifier?

The highest IP address on the router or the highest loop back address Signup and view all the answers

What is the size of the My Autonomous System field in the OPEN Message format?

2-byte Signup and view all the answers

What is the purpose of the Loopback address?

To represent the highest IP address on the router or the highest loop back address at BGP session startup Signup and view all the answers

What happens when a single-homed network announces its routes more specifically?

Upstream provider may start advertising a more specific route Signup and view all the answers

What is the purpose of an upstream provider redistributing static routes into their IGP?

To advertise routes to the outside world Signup and view all the answers

What happens if a single-homed network is connected to the Internet without speaking BGP to the upstream provider?

The single-homed network's routes are aggregated into the upstream provider's larger block Signup and view all the answers

What is the net difference between using BGP and not using BGP with an upstream provider?

The upstream provider gives the single-homed network all of the routes they have Signup and view all the answers

What happens if an upstream provider withdraws a single-homed network's routes every time the link flips up and down a few times?

The single-homed network is dampened Signup and view all the answers

Why might an upstream provider not want to configure BGP with a single-homed network?

Because the single-homed network might start advertising a more specific route Signup and view all the answers

What is required for a single-homed network to announce its routes more specifically?

A BGP connection with the upstream provider Signup and view all the answers

What happens when a single-homed network has address space 'inside' of the upstream provider's larger 'netblock'?

The single-homed network's routes are aggregated into the upstream provider's larger block Signup and view all the answers

Study Notes

OSPF Configuration and BGP

OSPF configuration involves normal, stub, totally stub, and NSSA types, as well as RIP redistribution.

Internal and External Routing

Internal routing involves configuring routers within a network to reach all destinations, using static routes or internal routing protocols like RIP, RIPv2, OSPF, and IS-IS.
External routing involves peering with other providers or multi-homing to multiple providers, using BGP to take control of where data goes on the Internet.

BGP

BGP ensures loop-free inter-domain routing and maintains routing data for parts of the Internet in routers.
BGP configuration involves OPEN messages, which contain version, autonomous system, hold time, BGP identifier, and optional parameters.

OPEN Message Format

Version: a 1-byte field indicating the BGP protocol version (e.g., BGP3 or BGP4).
My Autonomous System: a 2-byte field indicating the AS number of the BGP router.
Hold Time: the maximum time in seconds between KEEPALIVE or update messages.
BGP Identifier: a 4-byte field indicating the sender's ID (router ID), calculated as the highest IP address on the router or the highest loopback address at BGP session startup.
Optional Parameters: represented by triplets, including authentication information parameters.

BGP Messages

Notification Message: sent when an error is detected, and the peer connection is closed.
KEEPALIVE Message: exchanged periodically to determine peer reachability.

BGP Decision Algorithm

BGP chooses the single best path for reaching a destination based on attributes:
- AS Path Attribute
- Origin Attribute
- Next Hop Attribute
- Weight Attribute
- Local Preference Attribute
- Multi-Exit Discriminator Attribute
- Community Attribute

BGP Attribute Details

ORIGIN: specifies the route origin (IGP, EGP, or Incomplete).
AS_PATH: a list of ASNs traversed by the route.
NEXT_HOP: the IP address where data destined for the route should be sent.
MULTI_EXIT_DISC: a weight designed for routing outside and inside an ASN.
LOCAL_PREF: a weight not designed to be propagated outside an ASN.

CIDR and Aggregate Addresses

BGP4 supports classless interdomain routing (CIDR), improving over BGP3.
CIDR eliminates the concept of classes (A, B, C, etc.), making it easier to aggregate routes.

Confederations

Confederations reduce the IBGP mesh inside an AS by dividing it into multiple mini-ASs.
Each mini-AS is fully meshed, running IBGP among members, with connections to other mini-ASs in the confederation.

BGP and Single-Homed Networks

Single-homed networks may not need BGP, but it can be useful for advertising routes and having more control.

Multi-Homing and Load Balancing

Multi-homing involves using both connections in the same manner and load-balancing them.
The goal is to have roughly half the traffic go through each connection and maintain fail-over routing in case one connection goes down.

BGP Configuration Options

Peer group members can configure to override options that do not affect outgoing updates, but only for incoming updates.

CIDR and Aggregate Addresses

BGP4 supports Classless Interdomain Routing (CIDR), also known as supernetting.
CIDR eliminates the concept of classes (Class A, Class B, etc.) and allows for more flexible IP address representation.
CIDR notation consists of an IP address and a subnet mask (e.g., 192.213.0.0/16).
Aggregation is the process of combining multiple routes into a single route, minimizing routing table size.
A router cannot aggregate an address without a more specific route in the BGP routing table.

Confederations

A confederation is a technique to reduce the IBGP mesh inside an Autonomous System (AS).
Confederations divide the AS into multiple mini-ASs, each fully meshed with IBGP, and assign them to a confederation.
Each mini-AS has a connection to other mini-ASs within the confederation.

How BGP Works

BGP is a path vector protocol used to carry routing information between Autonomous Systems (ASs).
BGP uses TCP as its transport protocol (port 179) for reliability.
Two BGP routers form a transport protocol connection, exchanging messages to open and confirm connection parameters.
Peer routers exchange multiple messages, and in case of disagreement, notification errors are sent, and the peer connection is not established.

IBGP and EBGP

IBGP (Internal BGP) is used among routers within an AS, and EBGP (External BGP) is used between ASs.
IBGP is run among members of a mini-AS in a confederation.
Redistributing BGP routing information to Interior Gateway Protocols (IGPs) allows routers within an AS to learn about routes.

Autonomous Systems

An Autonomous System (AS) is a network with a point of contact (NOC) and one or multiple border routers.
An AS has an internal routing scheme that allows every router to know how to get to every other router and destination within the AS.
Advertising routes to other ASs is like making a promise to carry data to the advertised IP space.
The primary purpose of BGP4 is to advertise routes to other networks (ASs).

Border Gateway Protocol (BGP)

BGP is a protocol that provides loop-free interdomain routing between autonomous systems (AS).
BGP is often used among networks of Internet Service Providers (ISPs).
BGP4 is the current version, which handles aggregation (CIDR) and supernetting, and allows the announcement of "classless routes".

Autonomous System (AS)

An autonomous system is a set of routers that operate under the same administration.
Each AS is identified by an AS number.
BGP constructs a graph of autonomous systems based on the information exchanged between BGP neighbors.

Routing: Internal and External

Internal routing is the process of routing within a network.
Internal routing protocols include RIP, RIPv2, OSPF, and IS-IS.
External routing involves routing between different autonomous systems, often using BGP.

BGP Message Header Format

The BGP message header format includes:
- Version (1-byte field that indicates the version of BGP protocol)
- My Autonomous System (2-byte field that indicates the AS number of the BGP router)
- Hold Time (maximum time in seconds that may elapse between the receipt of successive KEEPALIVE or update messages)
- BGP Identifier (4-byte field that indicates the sender's ID, calculated as the highest IP address on the router or the highest loopback address at BGP session startup)

OPEN Message

The OPEN message is used to establish a BGP connection.
The OPEN message includes the BGP message header format and optional parameters.

Notification Message

A notification message is sent whenever an error is detected, and the peer connection is closed.
Notification messages are used to determine the specific nature of errors that emerge in the routing protocol.

KEEPALIVE Message

KEEPALIVE messages are periodic messages exchanged between peers to determine whether peers are reachable.
BGP peers initially exchange their full BGP routing tables, and thereafter, send incremental updates only.

EBGP and IBGP

EBGP (External BGP) is used for routing between different autonomous systems.
IBGP (Internal BGP) is used for routing within an autonomous system.
EBGP peers are usually directly connected, while IBGP peers do not have to be directly connected.

Confederations and Route Reflectors

Confederations and route reflectors are techniques that alleviate the requirement for a logical full mesh in IBGP.
These techniques allow for more efficient and flexible routing within an autonomous system.

Internal BGP (IBGP)

IBGP is used to exchange BGP updates within an autonomous system.
IBGP provides more flexible and efficient ways of controlling the exchange of information within the AS, and presents a consistent view of the AS to external neighbors.

Internet Connectivity without BGP

Without BGP, a default route can be created towards an upstream provider, and all non-local packets go out the interface specified by the route.
The upstream provider probably puts static routes towards the customer on their side, and redistributes those static routes into their IGP, and then probably redistributes their IGP into BGP.
With BGP, the upstream provider gives the customer all of the routes they have, and listens to the customer's route announcements and then redistributes some or all of those to their peers and customers.

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