思科认证考试（ＣＣＮＡ）考试模拟题４０—５９及答案(2)
添加时间: 2007-9-7 8:13:55 作者: Cisco认证考试 阅读次数:84 来源: http://www.d9soft.com
Answer
f) Data Link Layer 2
48. [OxNx] Frames are converted to Bits at which layer?
a) Application Layer 7
b) Presentation Layer 6
c) Session Layer 5
d) Transport Layer 4
e) Network Layer 3
f) Data Link Layer 2
g) Physical Layer 1
Answer
g) Physical Layer 1
49. [OxNx] Which of the following could be a feature of a connection-oriented service?
a) Flow Control
b) "Three-Way" hand shake
c) Error checking
d) Windowing
e) All of the above
Answer
e) All of the above
50. [OxNx] With a connection-oriented service a host will ACK (Acknowledge) with: (Answer all that apply)
a) The last (SEQ) sequence number received from the remote host
b) The next expected (SEQ) sequence number from the remote host
c) The last ACK number from the remote host
d) The current SEQ number from the remote host "plus" 1
e) None of the above
Answer
b) The next expected (SEQ) sequence number from the remote host
d) The current SEQ number from the remote host "plus" 1.
Following are three captures that clearly show the ACK and SEQ numbers of a three-way handshake.
First Part
Second Part
Third Part
51. [OxNx] Simple Network Management Protocol uses what numeric value, in decimal, of the destination port field of the User Datagram Protocol (UDP) header?
a) 20
b) 21
c) 23
d) 69
e) 161
Answer
e) 161
52. [OxNx] The first segment of the "Three-Way Handshake" will most likely carry an ACK of (Choose the best answer):
a) 1
b) The SEQ number of the remote host "plus" 1
c) 0
d) There is no ACK.
e) A randomly generated number
Answer
c) 0
Below is an example of the first part of a Three-Way Handshake.
In this first part of the handshake, take note of the Sequence and Acknowledgement numbers. Since this is the first part of the handshake the ACK is 0. The host initiating this connection-oriented conversation can not acknowledge a number that has not been received. This is the key factor in creating an access list that provides the ability to telnet out, but not allow the outside world to telnet in. The key word "established" means permit only those packets where the ACK is non-zero. (Refer to Question 4 of the June Issue.)
53. [OxNx] Which one of the following layers "best" describes the ability of the upper layers to gain independence over LAN media access?
a) Application Layer 7
b) Presentation Layer 6
c) Session Layer 5
d) Transport Layer 4
e) Network Layer 3
f) Data Link Layer 2
g) Physical Layer 1
h) MAC sublayer
i) LLC sublayer
Answer
i) LLC sublayer
54. [OxNx] The Media Access Control (MAC) address is made up of the following two parts.
a) Vendor Code
b) Network Address
c) Host Address
d) Serial Number
e) No such address
Answer
a) Vendor Code
d) Serial Number
The MAC address consists of 48 bits. The first 24 bits are the Vendor code or the Organizational Unique Identifier (OUI) number. The IEEE assigns these addresses to each vendor. It is the responsibility of each vendor to make sure that each card produced has a unique serial number. A vendor may have more than one Vendor Code.
Has anyone ever caught a vendor in a screw up? When would a user ever know if there has been a screw up by a vendor?
Some Examples of OUI numbers:
CISCO 00 00 0c xx xx xx
Novell 00 00 1b xx xx xx
3COM 02 60 8c xx xx xx
Take note of the organizational code below:
55. [OxNx] Which one of the following describes the process of a TCP/IP host learning a remote host MAC address?
a) ARP (Address Resolution Protocol)
b) SLARP (Serial Line Address Resolution Protocol)
c) RARP (Reverse Address Resolution Protocol)
d) IARP (Inverse Address Resolution Protocol)
e) MOP (Maintenance Operation Protocol)
Answer
a) ARP (Address Resolution Protocol)
Below is an ARP request: Take note of the destination MAC address!
Followed by an ARP reply:
Notice that the reply includes the source MAC address.
56. [OxNx] What is the major difference between the Ethernet version 2 frame and the 802.3 frame?
a) 802.3 frame uses a two-byte Length field where Ethernet describes type.
b) 802.3 frame uses a two-byte field for sequence numbers where Ethernet describes Type.
c) Ethernet uses a 802.2 header to describe type.
d) Ethernet uses a SNAP header.
e) There is no major difference.
f) None of the above.
Answer
a) 802.3 frame uses a two-byte Length field where Ethernet describes type.
Bob Metcalf usually gets named as the man who invented ethernet, he was working for XEROX at the time. In this original ethernet frame from the 1970s there was a two-byte field describing type. The IEEE took the two byte field and used it to describe length. IEEE uses SSAP, DSAP and sometimes SNAP to describe type.
57. [OxNx] What is the purpose of the 802.3 Subnet Access Protocol (SNAP) format?
a) To make ethernet perform faster
b) To identify the length of the frame
c) To identify proprietary protocols
d) To allow windowing
e) To allow flow control
Answer
c) To identify proprietary protocols
The following is a capture of a proprietary protocol.
Can you identify the vendor? (That s easy.) Can you identify the protocol? (That s a little harder.)
58. [OxNx] When a Destination Service Access Point (DSAP) and Source Service Access Point (SSAP) are set to AA:
a) All stations with AA contained in their MAC addresses would receive this frame.
b) All stations would receive this frame, because it acts like a broadcast.
c) The encapsulated protocol contained in the frame must be IP.
d) The encapsulated protocol contained in the frame is proprietary.
e) None of the above.
Answer
d) The encapsulated protocol contained in the frame is proprietary.
Refer to the following capture:
The above capture is Cisco s proprietary Cisco Discovery Protocol (CDP).
59. [RINx] What Cisco feature of RIP will reduce convergence time?
a) Cisco uses full horizons as opposed to split horizons
b) The hold down timer is not activated
c) Event triggered updates are used
d) Infinity is 10 instead of 16
e) There is no Cisco feature to enhance the performance of IP RIP
Answer
c) Event triggered updates are used
Cisco uses triggered updates with IP RIP in addition to the periodic updates. Refer to the following outline that organizes the major routing protocols. Details of these routing protocols are will be addressed in later editions.
I. Interior Routing Protocols
A. Distance Vector
RIP
Cisco’s IGRP
Novell’s IPX RIP
Appletalk RTMP
B. Link State
OSPF
IS-IS
NLSP
C. Hybrid
Cisco’s EIGRP
II. Exterior Routing Protocols
A. BGP4
B. EOP
To distinguish between Distance Vector and Link State I ask students to answer the following three questions:
What does the routing protocol talk ?
When does the routing protocol talk ?
Who does the routing protocol talk ?
Distance Vector talks the entire routing table to directly connected neighbors (less split horizon) periodically!
Link State talks just the state of the link to all neighbors when the state of the link changes. I like to use the following question in class to differentiate between distance vector and link state routing protocols.
What protocol are you using if your mother calls you every night to let you know that everyone in the family is alive ?
Suppose you make an agreement with your mother to call you just when there is a death in the family ?
Suppose Mom dies?
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