Topic 1, Network Principles

QUESTION NO: 1

Refer to the exhibit.

Based on this FIB table, which statement is correct?

A. There is no default gateway.
B. The IP address of the router on FastEthernet is 209.168.201.1.
C. The gateway of last resort is 192.168.201.1.
D. The router will listen for all multicast traffic.

 

 

Answer: C
Explanation:

The 0.0.0.0/0 route is the default route and is listed as the first CEF entry. Here we see the next hop for this default route lists 192.168.201.1 as the default router (gateway of last resort).

 

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QUESTION NO: 2

Refer to the exhibit.

A network administrator checks this adjacency table on a router. What is a possible cause for the incomplete marking?

A. incomplete ARP information
B. incorrect ACL
C. dynamic routing protocol failure
D. serial link congestion

 

 

Answer: A
Explanation:

To display information about the Cisco Express Forwarding adjacency table or the hardware Layer 3-switching adjacency table, use the show adjacency command.
Reasons for Incomplete Adjacencies
There are two known reasons for an incomplete adjacency:
No ARP Entry
When CEF cannot locate a valid adjacency for a destination prefix, it punts the packets to the CPU for ARP resolution and, in turn, for completion of the adjacency.

 

 

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QUESTION NO: 3

A network engineer notices that transmission rates of senders of TCP traffic sharply increase and decrease simultaneously during periods of congestion. Which condition causes this?

A. global synchronization
B. tail drop
C. random early detection
D. queue management algorithm

 

 

Answer: A

Explanation:

TCP global synchronization in computer networks can happen to TCP/IP flows during periods of congestion because each sender will reduce their transmission rate at the same time when packet loss occurs.
Routers on the Internet normally have packet queues, to allow them to hold packets when the network is busy, rather than discarding them. Because routers have limited resources, the size of these queues is also limited. The simplest technique to limit queue size is known as tail drop. The queue is allowed to fill to its maximum size, and then any new packets are simply discarded, until there is space in the queue again. This causes problems when used on TCP/IP routers handling multiple TCP streams, especially when bursty traffic is present. While the network is stable, the queue is constantly full, and there are no problems except that the full queue results in high latency. However, the introduction of a sudden burst of traffic may cause large numbers of established, steady streams to lose packets simultaneously.

 

 

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QUESTION NO: 4

Which three problems result from application mixing of UDP and TCP streams within a network with no QoS? (Choose three.)

A. starvation
B. jitter
C. latency
D. windowing
E. lower throughput

 

Answer: A,C,E
Explanation:

It is a general best practice not to mix TCP-based traffic with UDP-based traffic (especially streaming video) within a single service provider class due to the behaviors of these protocols during periods of congestion. Specifically, TCP transmitters will throttle-back flows when drops have been detected. Although some UDP applications have application-level windowing, flow control, and retransmission capabilities, most UDP transmitters are completely oblivious to drops and thus never lower transmission rates due to dropping. When TCP flows are combined with UDP flows in a single service provider class and the class experiences congestion, then TCP flows will continually lower their rates, potentially giving up their bandwidth to drop-oblivious UDP flows. This effect is called TCP-starvation/UDP-dominance. This can increase latency and lower the overall throughput.
TCP-starvation/UDP-dominance likely occurs if (TCP-based) mission-critical data is assigned to the same service provider class as (UDP-based) streaming video and the class experiences sustained congestion. Even if WRED is enabled on the service provider class, the same behavior would be observed, as WRED (for the most part) only affects TCP-based flows. Granted, it is not always possible to separate TCP-based flows from UDP-based flows, but it is beneficial to be aware of this behavior when making such application-mixing decisions.

 

 

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QUESTION NO: 5

Which method allows IPv4 and IPv6 to work together without requiring both to be used for a single connection during the migration process?

A. dual-stack method
B. 6to4 tunneling
C. GRE tunneling
D. NAT-PT

 

 

Answer: A
Explanation:

Dual stack means that devices are able to run IPv4 and IPv6 in parallel. It allows hosts to simultaneously reach IPv4 and IPv6 content, so it offers a very flexible coexistence strategy. For sessions that support IPv6, IPv6 is used on a dual stack endpoint. If both endpoints support IPv4 only, then IPv4 is used.
Benefits:
• Native dual stack does not require any tunneling mechanisms on internal networks
• Both IPv4 and IPv6 run independent of each other
• Dual stack supports gradual migration of endpoints, networks, and applications.

 

 

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QUESTION NO: 6

Which statement about the use of tunneling to migrate to IPv6 is true?

A. Tunneling is less secure than dual stack or translation.
B. Tunneling is more difficult to configure than dual stack or translation.
C. Tunneling does not enable users of the new protocol to communicate with users of the old protocol without dual-stack hosts.
D. Tunneling destinations are manually determined by the IPv4 address in the low-order 32 bits of IPv4-compatible IPv6 addresses.

 

Answer: C
Explanation:

Using the tunneling option, organizations build an overlay network that tunnels one protocol over the other by encapsulating IPv6 packets within IPv4 packets and IPv4 packets within IPv6 packets. The advantage of this approach is that the new protocol can work without disturbing the old protocol, thus providing connectivity between users of the new protocol.
Tunneling has two disadvantages, as discussed in RFC 6144:
• Users of the new architecture cannot use the services of the underlying infrastructure.
• Tunneling does not enable users of the new protocol to communicate with users of the old protocol without dual-stack hosts, which negates interoperability.

 

 

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QUESTION NO: 7

A network administrator executes the command clear ip route. Which two tables does this command clear and rebuild? (Choose two.)

A. IP routing
B. FIB
C. ARP cache
D. MAC address table
E. Cisco Express Forwarding table
F. topology table

 

Answer: A,B
Explanation:

To clear one or more entries in the IP routing table, use the following commands in any mode:
Command
Purpose
clear ip route {* | {route |
prefix/length}[next-hop interface]}
[vrf vrf-name]

Example:

switch(config)# clear ip route
10.2.2.2
Clears one or more routes from both the unicast RIB and all the module FIBs. The route options are as follows:
•
*—All routes.
•
route—An individual IP route.
•
prefix/length—Any IP prefix.
•
next-hop—The next-hop address
•
interface—The interface to reach the next-hop address.
The vrf-name can be any case-sensitive, alphanumeric string up to 32 characters.

 

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QUESTION NO: 8

Which switching method is used when entries are present in the output of the command show ip cache?

A. fast switching
B. process switching
C. Cisco Express Forwarding switching
D. cut-through packet switching

 

 

Answer: A
Explanation:

Fast switching allows higher throughput by switching a packet using a cache created by the initial packet sent to a particular destination. Destination addresses are stored in the high-speed cache to expedite forwarding. Routers offer better packet-transfer performance when fast switching is enabled. Fast switching is enabled by default on all interfaces that support fast switching. To display the routing table cache used to fast switch IP traffic, use the “show ip cache” EXEC command.

 

 

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QUESTION NO: 9

Which two actions must you perform to enable and use window scaling on a router? (Choose two.)

A. Execute the command ip tcp window-size 65536.
B. Set window scaling to be used on the remote host.
C. Execute the command ip tcp queuemax.
D. Set TCP options to "enabled" on the remote host.
E. Execute the command ip tcp adjust-mss.

 

Answer: A,B
Explanation:

The TCP Window Scaling feature adds support for the Window Scaling option in RFC 1323, TCP Extensions for High Performance . A larger window size is recommended to improve TCP performance in network paths with large bandwidth-delay product characteristics that are called Long Fat Networks (LFNs). The TCP Window Scaling enhancement provides that support. The window scaling extension in Cisco IOS software expands the definition of the TCP window to 32 bits and then uses a scale factor to carry this 32-bit value in the 16-bit window field of the TCP header. The window size can increase to a scale factor of 14. Typical applications use a scale factor of 3 when deployed in LFNs.
The TCP Window Scaling feature complies with RFC 1323. The larger scalable window size will allow TCP to perform better over LFNs. Use the ip tcp window-size command in global configuration mode to configure the TCP window size. In order for this to work, the remote host must also support this feature and its window size must be increased.

 

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QUESTION NO: 10

Which three TCP enhancements can be used with TCP selective acknowledgments? (Choose three.)

A. header compression
B. explicit congestion notification
C. keepalive
D. time stamps
E. TCP path discovery
F. MTU window

 

 

Answer: B,C,D
Explanation:

TCP Selective Acknowledgment
The TCP Selective Acknowledgment feature improves performance if multiple packets are lost from one TCP window of data. Prior to this feature, because of limited information available from cumulative acknowledgments, a TCP sender could learn about only one lost packet per-round-trip time. An aggressive sender could choose to resend packets early, but such re-sent segments might have already been successfully received. The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host returns selective acknowledgment packets to the sender, informing the sender of data that has been received. In other words, the receiver can acknowledge packets received out of order. The sender can then resend only missing data segments (instead of everything since the first
missing packet).
Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would receive acknowledgment of only packets 1, 2, and 3. Packets 4 through 8 would need to be re-sent. With selective acknowledgment, TCP receives acknowledgment of packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 must be re-sent. TCP selective acknowledgment is used only when multiple packets are dropped within one TCP window. There is no performance impact when the feature is enabled but not used. Use the ip tcp selective-ack command in global configuration mode to enable TCP selective acknowledgment. Refer to RFC 2018 for more details about TCP selective acknowledgment.
TCP Time Stamp
The TCP time-stamp option provides improved TCP round-trip time measurements. Because the time stamps are always sent and echoed in both directions and the time-stamp value in the header is always changing, TCP header compression will not compress the outgoing packet. To allow TCP header compression over a serial link, the TCP time-stamp option is disabled. Use the ip tcp timestamp command to enable the TCP time-stamp option.
TCP Explicit Congestion Notification
The TCP Explicit Congestion Notification (ECN) feature allows an intermediate router to notify end hosts of impending network congestion. It also provides enhanced support for TCP sessions associated with applications, such as Telnet, web browsing, and transfer of audio and video data that are sensitive to delay or packet loss. The benefit of this feature is the reduction of delay and packet loss in data transmissions. Use the ip tcp ecn command in global configuration mode to enable TCP ECN.
TCP Keepalive Timer
The TCP Keepalive Timer feature provides a mechanism to identify dead connections. When a TCP connection on a routing device is idle for too long, the device sends a TCP keepalive packet to the peer with only the Acknowledgment (ACK) flag turned on. If a response packet (a TCP ACK packet) is not received after the device sends a specific number of probes, the connection is considered dead and the device initiating the probes frees resources used by the TCP connection.

 

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QUESTION NO: 11

A network administrator uses IP SLA to measure UDP performance and notices that packets on one router have a higher one-way delay compared to the opposite direction. Which UDP characteristic does this scenario describe?

A. latency
B. starvation
C. connectionless communication
D. nonsequencing unordered packets
E. jitter

 

 

Answer: A
Explanation:

Cisco IOS IP SLAs provides a proactive notification feature with an SNMP trap. Each measurement operation can monitor against a pre-set performance threshold. Cisco IOS IP SLAs generates an SNMP trap to alert management applications if this threshold is crossed. Several SNMP traps are available: round trip time, average jitter, one-way latency, jitter, packet loss, MOS, and connectivity tests.
Here is a partial sample output from the IP SLA statistics that can be seen:
router#show ip sla statistics 1
Round Trip Time (RTT) for Index 55
Latest RTT: 1 ms
Latest operation start time: *23:43:31.845 UTC Thu Feb 3 2005
Latest operation return code: OK
RTT Values:
Number Of RTT: 10 RTT Min/Avg/Max: 1/1/1 milliseconds
Latency one-way time:
Number of Latency one-way Samples: 0
Source to Destination Latency one way Min/Avg/Max: 0/0/0 milliseconds
Destination to Source Latency one way Min/Avg/Max: 0/0/0 milliseconds

 

 

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QUESTION NO: 12

Under which condition does UDP dominance occur?

A. when TCP traffic is in the same class as UDP
B. when UDP flows are assigned a lower priority queue
C. when WRED is enabled
D. when ACLs are in place to block TCP traffic

 

 

Answer: A
Explanation:

Mixing TCP with UDP
It is a general best practice to not mix TCP-based traffic with UDP-based traffic (especially Streaming-Video) within a single service-provider class because of the behaviors of these protocols during periods of congestion. Specifically, TCP transmitters throttle back flows when drops are detected. Although some UDP applications have application-level windowing, flow control, and retransmission capabilities, most UDP transmitters are completely oblivious to drops and, thus, never lower transmission rates because of dropping. When TCP flows are combined with UDP flows within a single service-provider class and the class experiences congestion, TCP flows continually lower their transmission rates, potentially giving up their bandwidth to UDP flows that are oblivious to drops. This effect is called TCP starvation/UDP dominance.
TCP starvation/UDP dominance likely occurs if (TCP-based) Mission-Critical Data isassigned to the same service provider class as (UDP-based) Streaming-Video and the class experiences sustained congestion. Even if WRED is enabled on the service-provider class, the same behavior would be observed because WRED (for the most part) manages congestion only on TCP-based flows.

 

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Topic 2, Layer 2 Technologies

QUESTION NO: 13

Prior to enabling PPPoE in a virtual private dialup network group, which task must be completed?

A. Disable CDP on the interface.
B. Execute the vpdn enable command.
C. Execute the no switchport command.
D. Enable QoS FIFO for PPPoE support.

 

 

Answer: B

Explanation:

Enabling PPPoE in a VPDN Group
Perform this task to enable PPPoE in a virtual private dial-up network (VPDN) group.
Restrictions
This task applies only to releases prior to Cisco IOS Release 12.2(13)T.
SUMMARY STEPS
1.
enable
2.
configure terminal
3.
vpdn enable
4.
vpdn-group name
5.
request-dialin
6.
protocol pppoe
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
•
Enter your password if prompted.
Step 2
configure terminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3
Step 4
vpdn-group name
Example:
Router(config)# vpdn-group group1
Associates a VPDN group with a customer or VPDN profile.
Step 5
request-dialin
Example:
Router(config-vpdn)# request-dialin
Creates a request-dialin VPDN subgroup.
Step 6
protocol pppoe
Example:
Router(config-vpdn-req-in)# protocol pppoe
Enables the VPDN subgroup to establish PPPoE

 

 

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QUESTION NO: 14

A network engineer has been asked to ensure that the PPPoE connection is established and authenticated using an encrypted password. Which technology, in combination with PPPoE, can be used for authentication in this manner?

A. PAP
B. dot1x
C. IPsec
D. CHAP
E. ESP

 

Answer: D
Explanation:

With PPPoE, the two authentication options are PAP and CHAP. When CHAP is enabled on an interface and a remote device attempts to connect to it, the access server sends a CHAP packet to the remote device. The CHAP packet requests or "challenges" the remote device to respond. The challenge packet consists of an ID, a random number, and the host name of the local router. When the remote device receives the challenge packet, it concatenates the ID, the remote device's password, and the random number, and then encrypts all of it using the remote device's password. The remote device sends the results back to the access server, along with the name
associated with the password used in the encryption process.
When the access server receives the response, it uses the name it received to retrieve a password stored in its user database. The retrieved password should be the same password the remote device used in its encryption process. The access server then encrypts the concatenated information with the newly retrieved password—if the result matches the result sent in the response packet, authentication succeeds. The benefit of using CHAP authentication is that the remote device's password is never transmitted in clear text (encrypted). This prevents other devices from stealing it and gaining illegal access to the ISP's network.

 

 

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QUESTION NO: 15

A corporate policy requires PPPoE to be enabled and to maintain a connection with the ISP, even if no interesting traffic exists. Which feature can be used to accomplish this task?

A. TCP Adjust
B. Dialer Persistent
C. PPPoE Groups
D. half-bridging
E. Peer Neighbor Route

 

 

Answer: B
Explanation:

A new interface configuration command, dialer persistent, allows a dial-on-demand routing (DDR) dialer profile connection to be brought up without being triggered by interesting traffic. When configured, the dialer persistent command starts a timer when the dialer interface starts up and starts the connection when the timer expires. If interesting traffic arrives before the timer expires, the connection is still brought up and set as persistent. The command provides a default timer interval, or you can set a custom timer interval. To configure a dialer interface as persistent, use the following commands beginning in global configuration mode:
Command
Purpose
Step 1
Router(config)# interface dialer number
Creates a dialer interface and enters interface configuration mode.
Step 2
Router(config-if)# ip address address mask
Specifies the IP address and mask of the dialer interface as a node in the destination network to
be called.
Step 3
Router(config-if)# encapsulation type
Specifies the encapsulation type.
Step 4
Router(config-if)# dialer string dial-string class class-name
Specifies the remote destination to call and the map class that defines characteristics for calls to this destination.
Step 5
Router(config-if)# dialer pool number
Specifies the dialing pool to use for calls to this destination.
Step 6
Router(config-if)# dialer-group group-number
Assigns the dialer interface to a dialer group.
Step 7
Router(config-if)# dialer-list dialer-group protocol protocol-name {permit | deny | list access-listnumber}
Specifies an access list by list number or by protocol and list number to define the interesting packets that can trigger a call.
Step 8
Router(config-if)# dialer remote-name user-name
(Optional) Specifies the authentication name of the remote router on the destination subnetwork for a dialer interface.
Step 9
Router(config-if)# dialer persistent [delay [initial] seconds | max-attempts number]
Forces a dialer interface to be connected at all times, even in the absence of interesting traffic.

 

 

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QUESTION NO: 16

Which PPP authentication method sends authentication information in cleartext?

A. MS CHAP
B. CDPCP
C. CHAP
D. PAP

 

Answer: D
Explanation:

PAP authentication involves a two-way handshake where the username and password are sent across the link in clear text; hence, PAP authentication does not provide any protection against playback and line sniffing.
CHAP authentication, on the other hand, periodically verifies the identity of the remote node using a three-way handshake. After the PPP link is established, the host sends a "challenge" message to the remote node. The remote node responds with a value calculated using a one-way hash function. The host checks the response against its own calculation of the expected hash value. If the values match, the authentication is acknowledged; otherwise, the connection is terminated.

 

 

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QUESTION NO: 17

Which protocol uses dynamic address mapping to request the next-hop protocol address for a specific connection?

A. Frame Relay inverse ARP
B. static DLCI mapping
C. Frame Relay broadcast queue
D. dynamic DLCI mapping

 

Answer: A
Explanation:

Dynamic address mapping uses Frame Relay Inverse ARP to request the next-hop protocol address for a specific connection, given its known DLCI. Responses to Inverse ARP requests are entered in an address-to-DLCI mapping table on the router or access server; the table is then used to supply the next-hop protocol address or the DLCI for outgoing traffic.

 

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QUESTION NO: 18

Which statement is true about the PPP Session Phase of PPPoE?

A. PPP options are negotiated and authentication is not performed. Once the link setup is completed, PPPoE functions as a Layer 3 encapsulation method that allows data to be transferred over the PPP link within PPPoE headers.
B. PPP options are not negotiated and authentication is performed. Once the link setup is completed, PPPoE functions as a Layer 4 encapsulation method that allows data to be transferred over the PPP link within PPPoE headers.
C. PPP options are automatically enabled and authorization is performed. Once the link setup is completed, PPPoE functions as a Layer 2 encapsulation method that allows data to be encrypted over the PPP link within PPPoE headers.
D. PPP options are negotiated and authentication is performed. Once the link setup is completed, PPPoE functions as a Layer 2 encapsulation method that allows data to be transferred over the PPP link within PPPoE headers.

 

 

Answer: D

 

 

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QUESTION NO: 19

PPPoE is composed of which two phases?

A. Active Authentication Phase and PPP Session Phase
B. Passive Discovery Phase and PPP Session Phase
C. Active Authorization Phase and PPP Session Phase
D. Active Discovery Phase and PPP Session Phase

 

Answer: D
Explanation:

 

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QUESTION NO: 20

Refer to the exhibit.

Which one statement is true?

A. Traffic from the 172.16.0.0/16 network will be blocked by the ACL.
B. The 10.0.0.0/8 network will not be advertised by Router B because the network statement for the 10.0.0.0/8 network is missing from Router B.
C. The 10.0.0.0/8 network will not be in the routing table on Router B.
D. Users on the 10.0.0.0/8 network can successfully ping users on the 192.168.5.0/24 network, but users on the 192.168.5.0/24 cannot successfully ping users on the 10.0.0.0/8 network.
E. Router B will not advertise the 10.0.0.0/8 network because it is blocked by the ACL.

 

 

Answer: E
Explanation:

You can filter what individual routes are sent (out) or received (in) to any interface within your EIGRP configuration.
One example is noted above. If you filter outbound, the next neighbor(s) will not know about anything except the 172.16.0.0/16 route and therefore won't send it to anyone else downstream. If you filter inbound, YOU won't know about the route and therefore won't send it to anyone else downstream.