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CS6250 COMPUTER NETWORKS EXAM
1 BRAND NEW OFFICIAL QUESTIONS
WITH ANSWERS.
How does a router process advertisements? - correct answer -
The router consists of a route processor (which is the main
processing unit) and interface cards that receive data packets
which are forwarded via a switching fabric. Let us break down
router processing in a few steps:
1. Initially, the LS update packets which contain LSAs from a
neighboring router reaches the current router's OSPF (which is
the route processor). This is the first trigger for the route
processor. As the LS Updates reach the router, a consistent view
of the topology is being formed and this information is stored in
the link-state database. Entries of LSAs correspond to the
topology which is actually visible from the current router.
2. Using this information from the link-state database, the current
router calculates the shortest path using the shortest path first
(SPF) algorithm. The result of this step is fed to the Forwarding
Information Base (FIB)
3. The information in the FIB is used when a data packet arrives
at an interface card of the router, where the next hop for the
packet is decided and its forwarded to the outgoing interface card.
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What is hot potato routing? - correct answer -Hot potato routing is
a technique/practice of choosing a path within the network, by
choosing the closest egress point(network exit) based on intra
domain path cost (Interior Gateway Protocol/IGP cost).
Hot potato routing simplifies computations for the routers as they
are already aware of the IGP path costs. It makes sure that the
path remains consistent, since the next router in the path will also
choose to send the packet to the same egress point.
Hot potato routing also effectively reduces the network's resource
consumption by getting the traffic out as soon as possible.
● Describe the relationships between ISPs, IXPs, and CDNs. -
correct answer -The Internet is a complex ecosystem. It is built
with a network of networks. These networks include (ISPs)
Internet Service Providers, (IXPs) Internet Exchange Points,
(CDNs) Content Delivery Networks. In 2019, there were apx. 500
IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and
essentially they form the "backbone" network over which smaller
networks can connect
IXPs are interconnection infrastructures, which provide the
physical infrastructure, where multiple networks (eg ISPs and
CDNs) can interconnect and exchange traffic locally.
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CDNs are networks that are created by content providers with the
goal of having greater control of how the content is delivered to
the end-users, and also to reduce connectivity costs. Some
example CDNs include Google and Netflix. These networks have
multiple data centers - and each one of them may be housing
hundreds of servers - that are distributed across the world.
● What is an AS? - correct answer -AS stands for autonomous
systems. ISPs, IXPs and CDNs can all operate as an AS. An AS
is a group of routers including the links among them that operate
under the same administrative authority.
● What kind of relationship does AS have with other parties? -
correct answer -An AS is independent of other ASes. They set
their own policies, make its own traffic engineering decisions and
interconnection strategies and also determine how the traffic
leaves and enters the network.
● What is BGP? - correct answer -BGP stands for Border
Gateway Protocol. The border routers of ASes use BGP to
exchange routing information with each other.
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● How does an AS determine what rules to import/export? -
correct answer -AS business relationships drive an AS' routing
policies and influence which routes an AS needs to import or
export.
There are 3 selective transit routes: Transit customer routes,
Transit Provider Routes, Peer Routes. The main rule is financial
incentive. Transit customer routes > peer routes > transit provider
routes.
Similarly as exporting, ASes are selective about which routes to
import based, primarily, on which neighboring AS advertises them
and what type of business relationship is established. An AS
receives route advertisements from its customers, providers and
peers.
When an AS receives multiple route advertisements towards the
same destination, from multiple ASes, then it needs to rank the
routes before selecting which one to import. The routes that are
preferred first are the customer routes, then the peer routes and
finally the provider routes. The reasoning behind this ranking is
that an AS...
1. wants to ensure that routes towards its customers do not
traverse other ASes unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free"
(under the peering agreement),
3. and finally resorts to import routes learned from providers as
these will add to costs.
CS6250 COMPUTER NETWORKS EXAM
1 BRAND NEW OFFICIAL QUESTIONS
WITH ANSWERS.
How does a router process advertisements? - correct answer -
The router consists of a route processor (which is the main
processing unit) and interface cards that receive data packets
which are forwarded via a switching fabric. Let us break down
router processing in a few steps:
1. Initially, the LS update packets which contain LSAs from a
neighboring router reaches the current router's OSPF (which is
the route processor). This is the first trigger for the route
processor. As the LS Updates reach the router, a consistent view
of the topology is being formed and this information is stored in
the link-state database. Entries of LSAs correspond to the
topology which is actually visible from the current router.
2. Using this information from the link-state database, the current
router calculates the shortest path using the shortest path first
(SPF) algorithm. The result of this step is fed to the Forwarding
Information Base (FIB)
3. The information in the FIB is used when a data packet arrives
at an interface card of the router, where the next hop for the
packet is decided and its forwarded to the outgoing interface card.
, Page | 2
What is hot potato routing? - correct answer -Hot potato routing is
a technique/practice of choosing a path within the network, by
choosing the closest egress point(network exit) based on intra
domain path cost (Interior Gateway Protocol/IGP cost).
Hot potato routing simplifies computations for the routers as they
are already aware of the IGP path costs. It makes sure that the
path remains consistent, since the next router in the path will also
choose to send the packet to the same egress point.
Hot potato routing also effectively reduces the network's resource
consumption by getting the traffic out as soon as possible.
● Describe the relationships between ISPs, IXPs, and CDNs. -
correct answer -The Internet is a complex ecosystem. It is built
with a network of networks. These networks include (ISPs)
Internet Service Providers, (IXPs) Internet Exchange Points,
(CDNs) Content Delivery Networks. In 2019, there were apx. 500
IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and
essentially they form the "backbone" network over which smaller
networks can connect
IXPs are interconnection infrastructures, which provide the
physical infrastructure, where multiple networks (eg ISPs and
CDNs) can interconnect and exchange traffic locally.
, Page | 3
CDNs are networks that are created by content providers with the
goal of having greater control of how the content is delivered to
the end-users, and also to reduce connectivity costs. Some
example CDNs include Google and Netflix. These networks have
multiple data centers - and each one of them may be housing
hundreds of servers - that are distributed across the world.
● What is an AS? - correct answer -AS stands for autonomous
systems. ISPs, IXPs and CDNs can all operate as an AS. An AS
is a group of routers including the links among them that operate
under the same administrative authority.
● What kind of relationship does AS have with other parties? -
correct answer -An AS is independent of other ASes. They set
their own policies, make its own traffic engineering decisions and
interconnection strategies and also determine how the traffic
leaves and enters the network.
● What is BGP? - correct answer -BGP stands for Border
Gateway Protocol. The border routers of ASes use BGP to
exchange routing information with each other.
, Page | 4
● How does an AS determine what rules to import/export? -
correct answer -AS business relationships drive an AS' routing
policies and influence which routes an AS needs to import or
export.
There are 3 selective transit routes: Transit customer routes,
Transit Provider Routes, Peer Routes. The main rule is financial
incentive. Transit customer routes > peer routes > transit provider
routes.
Similarly as exporting, ASes are selective about which routes to
import based, primarily, on which neighboring AS advertises them
and what type of business relationship is established. An AS
receives route advertisements from its customers, providers and
peers.
When an AS receives multiple route advertisements towards the
same destination, from multiple ASes, then it needs to rank the
routes before selecting which one to import. The routes that are
preferred first are the customer routes, then the peer routes and
finally the provider routes. The reasoning behind this ranking is
that an AS...
1. wants to ensure that routes towards its customers do not
traverse other ASes unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free"
(under the peering agreement),
3. and finally resorts to import routes learned from providers as
these will add to costs.
