Chapter 6 Reading Organizer

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CCNA Discovery 4.0


Semester 3


Luse

Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

1

Chapter 6

Reading Organizer


After completion of this chapter, you should be able to:




Describe and plan a network using OSPF



Design and configure a network using single
-
area OSPF



Work with multi
-
protocol environments


6.1


1. OSPF is a classless interior

gateway protocol (IGP). It divides the network into different sections, which are
referred to as areas. Explain the advantages to this.


This division allows for greater scalability. Working with multiple areas allows the network administrator
to select
ively enable route summarization and to isolate routing issues within a single area.


2. Explain how Link
-
state routing protocols, such as OSPF, differ from distance vector protocols.


Link
-
state routing protocols, such as OSPF, do not send frequent per
iodic updates of the entire routing
table. Instead, after the network converges, a link
-
state protocol sends an update only when a change in
the topology occurs, such as a link going down. In addition, OSPF performs a full update every 30
minutes.




3. C
ompare distance vector protocols and, link
-
state routing protocols and list the differences:


a.

Requires more complex network planning and configuration



b.

Requires increased router resources



c.

Requires more memory for storing multiple tables




d. Requi
res more CPU and processing power for the complex routing calculations







4.


CCNA Discovery 4.0


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5. Explain in detail what OSPF bases its cost metric on for an individual link.


OSPF bases the cost metric for an individual link on its bandwidth or speed. The metric
for a particular
destination network is the sum of all link costs in the path. If there are multiple paths to the network,
the path with the lowest overall cost is the preferred path and is placed in the routing table.




6. When does convergence occur on

routers in a network running OSPF?


a.

Receive information about every destination on the network




b.

Process this information with the SPF algorithm





c. Update their routing tables










CCNA Discovery 4.0


Semester 3


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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7. OSPF routers establish and maintain neighbor relationships
, or adjacencies, with other connected OSPF
routers. Explain what adjacency is and how it works.


Adjacency is an advanced form of neighborship between routers that are willing to exchange routing
information. When routers initiate an adjacency with neig
hbors, an exchange of link
-
state updates
begins. Routers reach a FULL state of adjacency when they have synchronized views on their link
-
state
database.




8. When do routers reach a FULL state of adjacency?


Routers reach a FULL state of adjacency when t
hey have synchronized views on their link
-
state
database.






9. The router goes through several state changes before becoming fully adjacent with its neighbor. List and
explain each state.


a.

Init
-

The router received an initial hello packet from its nei
ghbor. When a router receives a hello
packet from a neighbor, it lists the sending router ID in its own hello packet as an acknowledgment.





b.

2
-
Way
-

Bi
-
directional communication is established in that each router has seen the hello packet
from each other.

This state is attained when the router receiving the hello packet sees its own
Router ID within the neighbor field of the hello packet. At this state, a router decides whether to
become fully adjacent with this neighbor.





c.

Exstart
-

The routers establis
h a master
-
slave relationship and choose the initial sequence number
for adjacency formation. Between two routers, the router with the higher router ID becomes the
master and starts the exchange.





d.

Exchange
-

OSPF routers exchange database descriptor (DB
D) packets that contain link
-
state
advertisement (LSA) headers only. The DBD describes the contents of the entire link
-
state database.
Each DBD packet has a sequence number which can be incremented only by the master.




CCNA Discovery 4.0


Semester 3


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

4

e.

Loading
-

Based on the information p
rovided by the DBDs, routers send link
-
state request packets for
more specific information. The neighbor provides the requested link
-
state information in link
-
state
update packets.





f.

Full
-

All the router and network LSAs are exchanged and the router data
bases are fully synchronized.






10. The OSPF Hello protocol is used to initially establish and maintain adjacencies. The hello protocol sends very
small hello packets to directly connected OSPF routers. What address used to send these updates?



The he
llo protocol sends very small hello packets to directly connected OSPF routers on the multicast
address of 224.0.0.5.




11. What is the purpose of the DR and BDR in an OSPF network?



T
o

reduce the number of updates sent, unnecessary traffic flow, and pro
cessing overhead on all routers.
This is accomplished by requiring all routers to accept updates from the DR only.


.


12. How does OSPF accomplish this? (question 11)


T
he router with information about the link sends the information to the DR, using the

multicast address
224.0.0.6. The DR is responsible for distributing the change to all other OSPF routers, using multicast
224.0.0.5. In addition to reducing the number of updates sent across the network, this process also
ensures that all routers receive
the same information at the same time from a single source.








13. What address does the DR receive updates on about link problems on the network?


When a link fails, the router with information about the link sends the information to the DR, using t
he
multicast address 224.0.0.6.




CCNA Discovery 4.0


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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14. What address does the DR send updates out on?



The DR is responsible for distributing the change to all other OSPF routers, using multicast 224.0.0.5.



15. How is the DR and BDR elected?


Within a local network, t
he router with the highest router ID is elected the DR. The second highest is
elected as the BDR.


16. The router ID is an IP address that is determined by what factors:


a. The value configured with the router
-
id command




b. If no value is set with th
e router
-
id command, the highest configured IP address on any loopback
interface




c. If no loopback interface is configured, the highest IP address on any active physical interface




16. In some cases, an administrator may want specific routers to be t
he DR and BDR. These might be routers
with more processing power or lighter traffic load. An administrator can force the DR and BDR election by
configuring a priority using the interface configuration command. What command would be used?



ip ospf priorit
y number

By default, OSPF routers have a priority value of 1. If the priority value is changed on a router, the
highest priority setting will win the election for DR, regardless of highest router ID. The highest value
that can be set for router priority is

255. A value of 0 signifies that the router is ineligible to be DR or
BDR.





17. List the three Link types identified by OSPF:


a.

Broadcast Networks


b.

Point
-
to
-
point


c.

Non
-
Broadcast Multi
-
Access (NBMA) Networks



CCNA Discovery 4.0


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Introducing Routing and Switching in the Enterprise


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18. Of the three types of OSPF Links which o
ne requires a DR election?


On
broadcast multi
-
access networks
, such as Ethernet, the number of neighbor relationships can
become large, and therefore a DR election is required.








19.




(See attached File)





20. What is the default OSPF network Ar
ea number?


All OSPF networks begin with Area 0, also called the backbone area. As the network is expanded, other
areas can be created that are adjacent to Area 0. These other areas can be assigned any number, up to
65,535.






CCNA Discovery 4.0


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Introducing Routing and Switching in the Enterprise


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21.



1. Hierarchical net
work
-

All OSPF areas that make up an enterprise network

2. Area 51
-

non
-
backbone area

3. ABR
-

Router between Area 0 and another AS

4. ASBR
-

Using multiple OSPF area

5. Area 0
-

backbone area

6. SPF algorithm
-

formula that helps determine the best pat
h

7. AS
-

Router between Area 0 and another OSPF area

6.2


22. What commands do you use to set up OSPF on your router?




router(config)#router ospf <process
-
id>


Router(config
-
router)#network <network
-
address> <wildcard
-
mask> area <area
-
id>









23. Ex
plain how to determine the wildcard mask?


When used for network summarization, or supernetting, the wildcard mask is the inverse of the subnet
mask. To determine the wildcard mask for a network or subnet, simply subtract the decimal subnet
mask for the in
terface from the all 255s mask (255.255.255.255).




CCNA Discovery 4.0


Semester 3


Luse

Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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24. Determine the subnet mask and wildcard mask required to advertise the specified network a
dd
resses in
OSPF.





Subnet Mask: 255.0.0.0





Subnet Mask:

255.255.254.0


Wildcard Mask: 0.255.255.255




Wildcard Mask: 0.0.1.255





Subnet Mask:

255.255.240.0






Subnet Mask: 255.255.255.192


Wildcard Mask: 0.0.15.255






Wildcard Mask: 0.0.0.63







Subnet Mask: 255.255.255.252




Subnet Mask:

255.255.0.0


Wildcard Mask: 0.0.0.3





Wildcard M
ask: 0.0.255.255





Complete Lab Activity 6.2.1.4


25. Explain in detail a potential security problem with OSPF.


The default configuration of OSPF exchanges information between neighbors in plain text. This poses
potential security threats to a network.

A hacker on a network could use packet
-
sniffing software to
capture and read OSPF updates and determine network information.




CCNA Discovery 4.0


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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26. Explain the solution that is available for this problem?


To eliminate this potential security problem, configure OSPF aut
hentication between routers. When
authentication is enabled in an area, routers will only share information if the authentication
information matches. With simple password authentication, configure each router with a password,
called a key. This method pro
vides only a basic level of security because the key passes between routers
in plain text form. It is just as easy to view the key as it is the plain text.






Complete Lab Activity 6.2.2.2


27. List and explain the four methods routers use to determine t
he DR in the order of priority.



a.

Interface Priority
-

the interface priority is set with the priority command






b.

Router ID
-
The router ID is set with OSPF router
-
id configuration command






c.

Highest Loopback
-

The loopback interface with the highest IP

address is used as the router ID by
default. OSPF favors loopback interfaces since they are logical interfaces and not physical interfaces.
Logical interfaces are always up






d.

Highest Physical Interface Address
-

The router uses the highest active IP add
ress from one of its
interfaces as the router ID. This option poses a problem if interfaces go down or are reconfigured








Complete Lab Activity 6.2.3.2




CCNA Discovery 4.0


Semester 3


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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28. Another parameter related to the OSPF cost metric is the reference bandwidth. What does this

calculate?


Another parameter related to the OSPF cost metric is the reference bandwidth, which is used to
calculate interface cost, also referred to as the link cost. The bandwidth value calculation of each
interface uses the equation 100,000,000/bandwid
th. 100,000,000, or 10^8, is known as the reference
bandwidth.





Complete Lab Activity 6.2.3.5


29. If the router ID of the neighboring router is not displayed, or if it does not show a state of FULL, what does
this mean? Explain.



If the router ID of

the neighboring router is not displayed, or if it does not show a state of FULL, the two
routers have not formed an OSPF adjacency. If a router is a DROther, adjacency occurs if the state is
FULL or 2WAY. If this is a multi
-
access Ethernet network, DR and

BDR labels display after FULL/ in the
State column.






30. Two routers may not form an OSPF adjacency if:


a.

The subnet masks do not match, causing the routers to be on separate networks


b.

OSPF hello or dead timers do not match


c.

OSPF network types do not
match


d.

There is a missing or incorrect OSPF network command














31. Answer the following questions based on the following show ip route command output.



CCNA Discovery 4.0


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Introducing Routing and Switching in the Enterprise


IST 203 Summer 2009

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a.

How many networks were learned by OSPF?

3


b.

What is the administrative distance for OSPF ro
utes?

110


c.

How many subnets are there for the 172.16.0.0 network?

2 subnets


d.

What is the metric for the path to the 192.168.10.4 network?

Via 192.168.10.10 Serial 0/0/1


Complete Lab Activity 6.2.4.4

6.3


32. List the commands required to configure a route
r to distribute a default route into the OSPF network?


R1(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0/0

R1(config)#router ospf 1

R1(config
-
router)#default
-
information originate








Complete Lab Activity 6.3.1.3



33. What is the supernet addresses an
d CIDR if you summarize the following addresses?



210.50.0.0/24

210.50.1.0/24

210.50.2.0/24

210.50.3.0/24

Answer: ______210.50.4.0/22___ 255.255.255.252
______________
___________



CCNA Discovery 4.0


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Complete Lab Activity 6.3.2.2



34. List the advantages of using OSPF.



a.

Uses bandwidth as a metric


b. Converges quickly using triggered updates


c. Limits routing loops through consistent view of network topology


d. Routing decisions based on latest information


e. Minimizes link
-
state database
-

fewer SPF calculation
s


f. Converges faster


g. Maps the topology of an area on each router


h. Supports CIDR and VLSM


i. Designed hierarchically using areas





34. List the disadvantages of using OSPF.



a. Requires more memory and processor power


b. Requires more compl
ex and expensive implementation


c. Requires an administrator who understands the protocol


d. Floods the network initially with LSAs noticeably degrading network performance







35. What are the default administrative distances for the following protoc
ols?



Static


1

Internal EIGRP

90


OSPF


110


RIP


120