CS 6250 Quiz 4 Exam Prep (All Quizzes) Latest 2026/2027
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Institute of Technology
2026/2027 | GRAD
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Question:
Describe the relationships between ISPs, IXPs, and CDNs.
Answer
ISPs are organizations that provide a myriad of services related to accessing, using, managing, or participating in the
Internet.
IXPs are interconnection infrastructures that provide the physical infrastructure where multiple networks (e.g., ISPs
and CDNs) can interconnect and exchange traffic locally.
CDNs are networks that content providers create with the goal of having greater control of how the content is delivered
to the end-users while reducing connectivity costs.
Question:
What is an AS?
Answer
An AS is a group of routers (including the links among them) that operate under the same administrative authority. An
ISP, for example, may operate as a single AS, or it may operate through multiple ASes. Each AS implements its own
policies, makes its own traffic engineering decisions and interconnection strategies, and determines how the traffic
leaves and enters its network.
Question:
What kind of relationship does AS have with other parties?
Answer
1. Provider-Customer relationship (or transit): This relationship is based on a financial settlement that determines
how much the customer will pay the provider. The provider forwards the customer's traffic to destinations found in the
provider's routing table (including the opposite direction of the traffic).
2. Peering relationship: In a peering relationship, two ASes share access to a subset of each other's routing tables. The
routes shared between two peers are often restricted to the respective customers of each one. The agreement holds as
long as the traffic exchanged between the two peers is not highly asymmetric.
Question:
What is BGP?
Answer
A protocol that the border routers of the ASes use to exchange routing information with each other.
Question:
How does an AS determine what rules to import/export?
Answer
AS business relationships drive an AS's routing policies and influence which routes an AS needs to import or export.
Exporting - Advertising a route for a destination, to a neighboring AS, means that this route may be selected by that
AS, and traffic will start to flow through. An AS X will advertise routes learned from customers, and routes learned
from providers (but only to X's customers) because those are the only cases where X will likely make money because
more traffic will likely flow through X.
Importing - When an AS receives multiple route advertisements towards the same destination from multiple ASes, it
needs to rank the routes before selecting which one to import. In order of preferred source:
Question:
Customer > Peer > Provider. Why?
Answer
1. An AS wants to ensure that routes toward its customers do not traverse other ASes, unnecessarily generating costs.
2. An AS uses routes learned from peers since these are usually "free" (under the peering agreement).
3. An AS resorts to importing routes learned from providers only when necessary for connectivity since these will add
to costs.
Question:
What were the original design goals of BGP? What was considered later?
Answer
Original goals: - Scalability: As the size of the Internet grows, the same is true for the number of ASes, the number of prefixes in the
routing tables, the network churn, and the BGP traffic exchanged between routers. One of the design goals of BGP is to
manage the complications of this growth while achieving convergence in reasonable timescales and providing loop
free paths. - Express routing policies: BGP has defined route attributes that allow ASes to implement policies (which routes to
import and export) through route filtering and route ranking. Each ASes routing decisions can be kept confidential,
and each AS can implement them independently. - Allow cooperation among ASes: Each AS can still make local decisions (which routes to import and export) while
keeping these decisions confidential from other ASes.
Later consideration: - Security: due to the increase in size and complexity of the Internet, security measures are increasingly desirable.
Question:
What are the basics of BGP?
Answer
A pair of routers, known as BGP peers, exchange routing information over a semi-permanent TCP port connection
called a BGP session. In order to begin a BGP session, a router will send an OPEN message to another router. Then the
sending and receiving routers will send each other announcements from their routing tables.
BGP messages can be 1) UPDATES (advertising new routes or updates to existing routes, or indications that previous
routes are no longer valid) or 2) KEEPALIVE messages to preserve a session.
Question:
What is the difference between iBGP and eBGP?
Answer
iBGP - involves a pair of routers that belong to the same AS
eBGP - invovles a pair of routers in two different ASes
Once a router hears about a route via eBGP, it disseminates that route to other internal (same AS) routers via iBGP.
Question:
What is the difference between iBGP and IGP-like protocols (RIP or OSPF)?
Answer
IGP-like protocols are used to establish paths between the internal routers of an AS based on specific costs within the
AS. In contrast, iBGP is only used to disseminate external routes within the AS.
Question:
How does a router use the BGP decision process to choose which routes to import?
Answer
In the simplest scenario, where there is no policy in place (meaning it does not matter which route will be imported),
the router uses the attribute of the path length to select the route with the fewest number of hops. This simple scenario
rarely occurs in practice.
A router compares a pair of routes by going through the list of attributes, as shown in the figure below. For each
attribute, it selects the route with the attribute value that will help apply the policy. If for a specific attribute, the values
are the same, then it goes to the next attribute.
Example attribute: LocalPref attribute is used to prefer routes learned through a specific AS over other ASes, for
outbound traffic. Assigned by the local AS; higher = more preferred.
Example attribute: MED attribute is used by ASes connected by multiple links to designate which links are preferred
for inbound traffic. Assigned by the neighbor AS; lower = more preferred.
Question:
What are the 2 main challenges with BGP? Why?
Answer
1. Scalability. A large number of updates, overly specific prefixes, etc. can overload router processor/memory capacity.
2. Misconfigurations. Related to the above, misconfigurations can result in excessive route updates, which can lead to
stability and scalability issues.
Question:
What is an IXP?
Answer
IXPs are physical infrastructures that provide the means for ASes to interconnect and directly exchange traffic with
one another.
Question:
What are four reasons for IXP's increased popularity?
Answer
1. IXPs are interconnection hubs handling large traffic volumes
2. An important role in mitigating DDoS attacks
3. "Real-world" infrastructures with a plethora of research opportunities
4. IXPs are active marketplaces and technology innovation hubs
Question:
Which services do IXPs provide?
Answer
1. Public peering
2. Private peering
3. Route servers and Service level agreements
4. Remote peering through resellers
5. Mobile peering
6. DDoS blackholing
7. Free value-added services, like Internet Routing Registry, consumer broadband speed tests, DNS root name servers
Question:
How does a route server work?
Answer
An RS is an IXP entity that makes peering more manageable in the face of a rising number of BGP sessions.
An RS - It collects and shares routing information from its peers or participants of the IXP that connect to the RS. - It executes its own BGP decision process and re-advertises the resulting information (e.g., best route selection) to all
RS's peer routers.
Question:
The Internet topology has been evolving to an increasingly prominent hierarchical structure.
True
False
Answer
False
Increasingly flat, with the rise of IXPs and CDNs
Question:
An Autonomous System operates across multiple administrative domains.
True
False
Answer
False
For two ASes to form a peering agreement, they need to find common ground regarding the internal policies and
traffic engineering approaches that each AS implements.
True
False
Answer
False
A Content Distribution Network (CDN) or an ISP can operate over multiple Autonomous Systems.
True
False
Answer
True
ISP X has the incentive to advertise a route for a customer to a peer AS
True
False
Answer
True
ISPs want to advertise customer routes to as many neighbors as possible so that more traffic flows through the ISP to
the customer
An ISP X has the incentive to advertise the route for its provider's customers to X's peers
True
False
Answer
False
X has no incentive to carry traffic for its provider's routes
ISP X learns multiple routes for the same external destination W. These are advertised from customer C3, peer Y, and
provider P. How would X rank these routes before deciding which one to import?
A. C3 > Y > P
B. Y > C3 > P
C. P > Y > C3
Answer
A.
How does a border router advertise an external route to other routers in the same AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
A.
How does a border router advertise an internal (within the same AS) route to other routers in the same AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
C.
How does a border router advertise an external route to another router in a different AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
B.
Since the BGP path selection process is fixed, an AS has no control over which routes are selected.
True
False
False
e.g. through LocalPref or MED values
An AS can show preference to an external route advertised by one AS over another by assigning a higher LocalPrefer
value to the preferred route.
True
False
True
An AS can show preference for one router for inbound traffic (from neighbors) by assigning that router a lower MED
value.
True
False
True
An AS can use LocalPref to control which routers are used as exit points (for the outgoing traffic), and it can use the
MED attribute to control which routers are used as entry points (for the incoming traffic).
True
False
True
One of the services provided by IXPs is additional security protections such as mitigation of DDoS (Distributed Denial
of Service) attacks.
True
False
True
There are no costs involved for an AS to participate at an IXP.
True
False
False
Since local traffic stays local at IXPs, the IXP infrastructures deal with limited volumes of traffic.
True
False
False
When a large provider or Content Delivery Network joins an IXP, this can act as an incentive for other networks to join
as well.
True
False
True
At an IXP, the members have the choice to peer privately or publicly.
True
False
True
IXPs leading incentive to establish route servers was to charge the participants for using it.
True
False
False
The main incentive for establishing an RS is to make peering more manageable in light of a rising number of BGP
sessions
An IXP route server does not need to run the BGP protocol to facilitate the establishment of multi-lateral peering
sessions.
True
False
False
BGP is core to these peering relationships
For multi-lateral BGP peering sessions at an IXP, the participants have the choice to advertise routes, either directly to
other participants, or to the route server.
True
False
False
I think the idea is that _all_ advertisements pass through the RS server, which applies both import and export filters
to the advertisements.
The Internet topology has been evolving to an increasingly prominent hierarchical structure.
F
An Autonomous System operates across multiple administrative domains.
F
For two ASes to form a peering agreement, they need to find common ground regarding the internal policies and
traffic engineering approaches that each AS implements.
F
A Content Distribution Network (CDN) or an ISP can operate over multiple Autonomous Systems.
T
Consider the figure below that denotes ASes and their relationships.
Note: C1, C2, C3 are customers of ISP-X. ISP-P is a provider of ISP-X.
ISP-X has the incentive to advertise routes for C3 to Y.
T
ISP-X has the incentive to advertise the routes for P's customers to Y and Z.
F
Assume that AS-X learns multiple routes for the same external destination W. These multiple routes are advertised
from C3, from Y and from P. How would AS-X rank these routes before deciding which one to import? - 1) route learned from C3, 2) route learned from Y, 3) route learned from P.
- 1) route learned from Y, 2) route learned from C3, 3) route learned from P. - 1) route learned from P, 2) route learned from Y, 3) route learned from C3.
1) route learned from C3, 2) route learned from Y, 3) route learned from P.
Consider the topology below and the types of BGP sessions the routers form. Select the true statements.
R-A1 R-A3 : iBGP
R-B1 R-A3 : eBGP
R-B3 R-D2 : iBGP
R-D3 R-D2 : eBGP
R-A1 R-A3 : iBGP
R-B1 R-A3 : eBGP
Assume that router R-D2 learns about a route to a destination in AS-A. How would router R-D2 disseminate this route
to R-D3 and R-D1?
Using iBGP.
Using eBGP.
Using IGP.
Using iBGP.
Assume that router R-D2 learns about a route to an internal destination in AS-D. How would router R-D2 disseminate
this route to R-D3 and R-D1?
Using iBGP.
Using eBGP.
Using IGP.
Using IGP.
Assume that router R-B1 learns about a route to AS-C. How would router R-B1 disseminate this route to R-A3?
Using iBGP.
Using eBGP.
Using IGP.
Using eBGP.
Since the BGP path selection process is fixed, an AS has no control over which routes are selected.
True
False
False
Assume that AS-B learns about an external destination both from AS-C and from AS-A. AS-B can show preference to
use the route heard from AS-C by assigning higher LocalPref value to that route.
T
Assume that AS-B advertises the routes to its internal destination to AS-A using the routers R-B1 and R-B4. AS-B can
communicate to AS-A that it prefers R-B1 as an entry point to the network, by assigning lower MED values to these
routes.
T
An AS can use LocalPref to control which routers are used as exit points (for the outgoing traffic), and it can use the
MED attribute to control which routers are used as entry points (for the incoming traffic).
T
One of the services provided by IXPs is additional security protections such as mitigation of DDoS (Distributed Denial
of Service) attacks.
T
There are no costs involved for an AS to participate at an IXP.
T
Since local traffic stays local at IXPs, the IXP infrastructures deal with limited volumes of traffic.
F
When a large provider or Content Delivery Network joins an IXP, this can act as an incentive for other networks to join
as well.
T
At an IXP, the members have the choice to peer privately or publicly.
T
IXPs leading incentive to establish route servers was to charge the participants for using it.
F
An IXP route server does not need to run the BGP protocol to facilitate the establishment of multi-lateral peering
sessions.
F
For multi-lateral BGP peering sessions at an IXP, the participants have the choice to advertise routes, either directly to
other participants, or to the route server.
F
IXP
Interconnection infrastructures which provide physical infrastructure where multiple networks interconnect and
exchange traffic locally
CDNs
Networks created by content providers with the goal of greater control of how it's delivered to end users, reducing
connectivity costs
POPs
1+ routers in a provider network which customer networks can use to connect to provider
Autonomous system
Group of routers including link that operates with same administrative authority
Peering relationship
2 autonomous systems share access to subset of each other's routing tables
What are the design goals of border routing protocol (BGP)?
Scalability, express routing policies, allow cooperation among AS, and security
Express routing policies
BGP has defined route attributes that allow ASes to implement policies such as export/import through route
filtering/ranking
BGP Peers
Exchange routing information over a semi-permanent TCP port called BGP session
2 types of BGP messages
Update and keep alive
BGP update
Contains info about routes that have changed since previous update
BGP announcement
Msgs that advertise new routes, updates existing routes
BGP withdrawal
Msgs that that inform about a previously announced route that is no longer available
BGP Keepalive
Keeps the session going
2 types of BGP Updates
Withdrawal and announcement
BGP Prefix Reachability
Destinations are represented by IP prefixes
eBGP
Sessions b/w border routers of neighboring ASes
iBGP
Sessions between internal routers of the same AS
LocalPref
Attributed used to prefer routes learned through a specific AS over other ASes
MED (multi-exit discirminator)
Used by ASes connected by multiple links to designate which links are preferred for inbound traffic
BGP protocol's 2 limitations are:
Misconfiguration and faults
Peering IXPs
ASes can either peer with one another directly or peer at IXPs which are infrastructures that facilitate peering and
provide more services
Why are IXPs important?
They are interconnection hubs handling high traffic volumes
Important role in DDoS attacks
Real world infrastructures with a plethora of research ops
Active market/tech for innovation hubs
Public peering
2 networks use IXP network infrastructure to establish a connection to exchange traffic based on bilateral relations
and traffic requirements
Private peering
Allow direct traffic exchange between 2 parties, doesn't use IXP public peering infrastructure
Route servers/service level agreement
Many IXPs use service level agreements and free use of IXPs route servers for participants
Remote peering through resellers
Sell IXP ports when they have infrastructure connected to the IXP
DDoS Blackholing
Alleviate effects of DDoS attacks
Multilateral BGP peering session
Route server that facilitates/manages how multiple ASes can talk on control planes simultaneously
Route servers have 2 important route filters:
Import and export filters
Import filters
Ensure that each member of AS only advertises routes that it should advertise
Export filters
Triggered by IXP members themselves to restrict the other set of IXP member ASes that receive their routes
Provider customer/transit relationship
Financial settlement that determines how much a customer will pay provider
ASPath
Attribute for utonomous system number, as an announcement passes through various ASes, identifiers are included in
ASPath
Next Hop
Attribute for router's IP address with interface along the path towards a destination
Flap damping
AS can limit the # of routing changes by explicitly limiting the propagation of unstable routes
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