Routing

Question 1

Routers

Answer 1

Used to forward traffic between subnets, between an internal and external
network, or between two external networks
o Each subnet or external network is going to be its own broadcast domain
o Multilayer switches also perform routing functions
▪ Switch
● Layer 2 Switch
▪ Multilayer Switch
● Router

Question 2

Routing Fundamentals

Answer 2

o Traffic is routed to flow between subnets
o Each subnet is its own broadcast domain
o Routers are the layer 3 devices that separate broadcast domains, but multilayer
switches can also separate broadcast domains

Question 3

Routing Decisions

Answer 3

Layer 3 to Layer 2 Mapping
● Routers use ARP caches to map an IP address to a given MAC
address
▪ Make packet-forwarding decisions based on its internal routing tables

Question 4

Routing Tables

Answer 4

▪ Table kept by the router to help determine which route entry is the best
fit for the network
▪ A route entry with the longest prefix is the most specific network
▪ 10.1.1.0/24 more specific than 10.0.0.0/8

Question 5

Sources of Routing Information

Answer 5

▪ Directly Connected Routes
● Learned by physical connection between routers
▪ Static Routes
● Manually configured by an administrator
● Default static route (0.0.0.0/0) is a special case
o “If I don’t know where, then send out default static route.”
▪ Dynamic Routing Protocols
● Learned by exchanging information between routers

Question 6

Dynamic Routing Protocols

Answer 6

▪ More than one route can exist for a network
▪ Different protocols consider different criteria when deciding which route
to give preference
▪ Based on number of hops (hop count in RIP), link bandwidths (OSPF), or
other criteria

Question 7

Preventing Routing Loops

Answer 7

▪ Split horizon
● Prevents a route learned on one interface from being advertised
back out of that same interface
▪ Poison reverse
● Causes a route received on one interface to be advertised back
out of that same interface with a metric considered to be infinite

Question 8

Routing Protocols

Answer 8

o Internal and Exterior Routing Protocols
▪ Interior Gateway Protocols (IGP)
● Operate within an autonomous system
▪ Exterior Gateway Protocols (EGP)
● Operated between autonomous systems

Question 9

o Router Advertisement Method

Answer 9

▪ Characteristic of a routing protocol
▪ How does it receive, advertise, and store routing information?
● Distance vector
● Link state
▪ Not every routing protocol fits neatly into one of these two categories
(hybrids exist)

Question 10

Distance Vector

Answer 10

▪ Sends full copy of routing table
to its directly connected
neighbors at regular intervals
▪ Slow convergence time
● Time it takes for all
routers to update their
routing tables in
response to a topology
change
▪ Holding-down timers speeds up convergence
● Prevents updates for a specific period of time
▪ Uses hop count as a metric

Question 11

Link State

Answer 11

▪ Requires all routers to know about the paths that all other routers can
reach in the network
▪ Information is flooded throughout the link-state domain (OSPF or IS-IS) to
ensure routers have synchronized information
▪ Faster convergence time and uses cost or other factors as a metric
▪ Each router constructs its own relative shortest-path tree with itself as
the root for all known routes in the network

Question 12

Routing Information Protocol (RIP)

Answer 12

▪ Interior Gateway Protocol
▪ Distance-vector protocol using hop count
▪ Maximum hops of 15, 16 is infinite
▪ Oldest dynamic routing protocol, provides updates every 30 seconds
▪ Easy to configure and runs over UDP

Question 13

Open Shortest Path First (OSPF)

Answer 13

▪ Interior Gateway Protocol
▪ Link-state protocol using cost
▪ Cost is based on link speed between routers

Question 14

Intermediate System to Intermediate System (IS-IS)

Answer 14

▪ Interior Gateway Protocol
▪ Link-state protocol using cost
▪ Cost is based on link speed between two routers
▪ Functions like OSPF protocol, but not as popular or widely utilized

Question 15

Enhanced Interior Gateway Routing Protocol (EIGRP)

Answer 15

▪ Interior Gateway Protocol
▪ Advanced distance-vector protocol using bandwidth and delay making it
a hybrid of distance-vector and link-state
▪ Proprietary Cisco protocol that is popular in Cisco-only networks

Question 16

Border Gateway Protocol (BGP)

Answer 16

▪ External Gateway Protocol
▪ Path vector using the number of autonomous system hops instead of
router hops
▪ Widespread utilization, this protocol runs the backbone of the Internet
▪ Does not converge quickly, though, when the topology changes

Question 17

Route Believability

Answer 17

▪ If a network is using more than one routing protocol, how does it choose
which routing protocol to make decisions from?
▪ Some routing protocols are considered more believable than others, so
routers use an index of believability called administrative distance (AD)
▪ If a route has a lower the administrative distance (AD), the route is more
believable

Question 18

Address Translation

Answer 18

▪ Network Address Translation (NAT) is used to conserve the limited supply
of IPv4 addresses
▪ NAT translates private IP addresses to public IP addresses for routing over
public networks
▪ Port Address Translation (PAT) is a variation of address translation that
utilizes port numbers instead of IP addresses for translation

Question 19

Types of Address Translation

Answer 19

Dynamic NAT (DNAT)
● IP addresses automatically assigned from a pool
● One-to-one translations
▪ Static NAT (SNAT)
● IP addresses manually assigned
● One-to-one translations
▪ Port Address Translation (PAT)
● Multiple private IP addresses share one public IP
● Many-to-one translation
● Common in small networks

Question 20

Names of NAT IP Addresses

Answer 20

▪ Inside local
● Private IP address referencing an inside device
▪ Inside global
● Public IP address referencing an inside device
▪ Outside local
● Private IP address referencing an outside device
▪ Outside global
● Public IP address referencing an outside device

Question 21

Multicast Routing
o What is multicast touring?

Answer 21

Multicast sender sends traffic to a Class D IP Address, known as a
multicast group
▪ Goal
● Send the traffic only to the devices that want it
▪ Two primary protocols
● Internet Group Management Protocol (IGMP)
● Protocol Independent Multicast (PIM)

Question 22

o Internet Group Management Protocol (IGMP)

Answer 22

▪ Used by clients and routers to let routers known which interfaces have
multicast receivers
▪ Used by clients to join a multicast group
▪ Versions
● IGMPv1
o Client requests to join the group and is asked every 60
seconds if it wants to remain in the group
● IGMPv2
o Client can send a leave message to exit multicast group
● IGMPv3
o Client can request multicast from only specific server
o Called source-specific multicast (SSM)
o Allows multiple video streams to single multicast

Question 23

Protocol Independent Multicast (PIM)

Answer 23

▪ Routes multicast traffic between multicast-enabled routers
▪ Multicast routing protocol forms a multicast distribution tree
● PIM Dense Mode (PIM-DM)
o Uses periodic flood and prune behavior to form optimal
distribution tree
o Causes a negative performance impact on the network
o Rarely used in modern networks
● PIM Sparse Mode (PIM-SM)
o Initially uses a shared distribution tree, which may be
suboptimal, but…

o Eventually creates an optimal distribution tree through
shortest
path tree (SPT) switchover

Question 24

PIM Dense Mode: Flooding

Answer 24

o Uses source distribution tree (SDT) to
form an optimal path between source
router and last-hop router
o Before the optimal path is formed, entire
network is initially flooded and consumes
unnecessary bandwidth

Question 25

PIM Dense Mode: Pruning

Answer 25

If a router receives multicast traffic in the
initial flood and the traffic is not needed,
then the router sends a prune message
asking to be removed from the source
distribution tree

Question 26

PIM Dense Mode: After Pruning

Answer 26

o After sending prune messages, the
resulting source distribution tree has an
optimal path between source router and
last-hop router
o Flood and prune repeat every 3 minutes
which can cause excessive performance
impacts on the network

Question 27

PIM Sparse Mode: Shared Distribution Tree

Answer 27

An optimal path between the source and
last-hop routers is not initially created
o Instead, a multicast source sends traffic
directly to a rendezvous point (RP)
o All last-hop routers send join messages to
the RP