CCIE Routing and Switching Written Exam v5.1

 

QUESTION 1

Which two options are reasons for TCP starvation? (Choose two.)

 

A.

The use of tail drop

B.

The use of WRED

C.

Mixing TCP and UDP traffic in the same traffic class

D.

The use of TCP congestion control

 

Correct Answer: CD

Explanation:

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 co
ngestion, 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 is assigned to the same service-provider class as (UDP-based) Streaming-Video and the class experiences sustained congestion. Even if WRED or other TCP congestion control mechanisms are 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.

Reference: http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book/VPNQoS.html

 

 

QUESTION 2

Which regular expression will only allow prefixes that originated from AS 65000 and that are learned through AS 65001?

 

A.

^65000_65001$

B.

65000_65001$

C.

^65000_65001

D.

^65001_65000$

 

Correct Answer: D

Explanation:

The following table lists the regular expressions and their meanings:

 

+——————————————————+

| CHAR | USAGE |

+——————————————————|

| ^ | Start of string |

|——|———————————————–|

| $ | End of string |

|——|———————————————–|

| [] | Range of characters |

|——|———————————————–|

| – | Used to specify range ( i.e. [0-9] ) |

|——|———————————————–|

| ( ) | Logical grouping |

|——|———————————————–|

| . | Any single character |

|——|———————————————–|

| * | Zero or more instances |

|——|———————————————–|

| + | One or more instance |

|——|———————————————–|

| ? | Zero or one instance |

|——|———————————————–|

| _ | Comma, open or close brace, open or close |

| | parentheses, start or end of string, or space |

+——————————————————+

Some commonly used regular expressions include:

+————-+—————————+

| Expression | Meaning |

|————-+—————————|

| .* | Anything |

|————-+—————————|

| ^$ | Locally originated routes |

|————-+—————————|

| ^100_ | Learned from AS 100 |

|————-+—————————|

| _100$ | Originated in AS 100 |

|————-+—————————|

| _100_ | Any instance of AS 100 |

|————-+—————————|

| ^[0-9]+$ | Directly connected ASes |

+————-+—————————+

 

Reference: http://blog.ine.com/2008/01/06/understanding-bgp-regular-expressions/

 

 

QUESTION 3

A TCP/IP host is able to transmit small amounts of data (typically less than 1500 bytes), but attempts to transmit larger amounts of data hang and then time out. What is the cause of this problem?

 

A.

A link is flapping between two intermediate devices.

B.

The processor of an intermediate router is averaging 90 percent utilization.

C.

A port on the switch that is connected to the TCP/IP host is duplicating traffic and sending it to a port that has a sniffer attached.

D.

There is a PMTUD failure in the network path.

 

Correct Answer: D

Explanation:

Sometimes, over some IP paths, a TCP/IP node can send small amounts of data (typically less than 1500 bytes) with no difficulty, but transmission attempts with larger amounts of data hang, then time out. Often this is observed as a unidirectional problem in that large data transfers succeed in one direction but fail in the other direction. This problem is likely caused by the TCP MSS value, PMTUD failure, different LAN media types, or defective links.

Reference: http://www.cisco.com/c/en/us/support/docs/additional-legacy-protocols/ms-windows-networking/13709-38.html

 

 

 

 

QUESTION 4

Refer to the exhibit. You have just created a new VRF on PE3. You have enabled debug ip bgp vpnv4 unicast updates on PE1, and you can see the route in the debug, but not in the BGP VPNv4 table. Which two statements are true? (Choose two.)

 

clip_image002

 

A.

VPNv4 is not configured between PE1 and PE3.

B.

address-family ipv4 vrf is not configured on PE3.

C.

After you configure route-target import 999:999 for a VRF on PE3, the route will be accepted.

D.

PE1 will reject the route due to automatic route filtering.

E.

After you configure route-target import 999:999 for a VRF on PE1, the route will be accepted.

 

Correct Answer: DE

Explanation:

The route target extended community for VPLS auto-discovery defines the import and export policies that a VPLS instance uses. The export route target sets an extended community attribute number that is appended to all routes that are exported from the VPLS instance. The import route target value sets a filter that determines the routes that are accepted into the VPLS instance. Any route with a value in its import route target contained in its extended attributes field matching the value in the VPLS instance’s import route target are accepted. Otherwise the route is rejected.

 

 

QUESTION 5

Which mechanism can be used on Layer 2 switches so that only multicast packets with downstream receivers are sent on the multicast router-connected ports?

 

A.

IGMP snooping

B.

Router Guard

C.

PIM snooping

D.

multicast filtering

 

Correct Answer: C

Explanation:

Ideally, the Layer 2 device should forward the multicast transmission only out ports to which receivers are connected and also out any ports that are connected to downstream multicast routers. This configuration requires a Layer 2 device to be able to determine the ports on which multicast routers and receivers for each separate (S,G) or (*,G) multicast group are located. To facilitate intelligent forwarding of multicast traffic on the LAN, Cisco Catalyst switches support two mechanisms:

IGMP snooping – The switch listens in or “snoops” IGMP communications between receivers and multicast routers. This snooping enables the switch to determine which ports are connected to receivers for each multicast group and which ports are connected to multicast routers.

Cisco Group Management Protocol (CGMP) – The switch communicates with multicasts routers, with multicast routers relaying group membership information to switches.

 

Reference: https://www.informit.com/library/content.aspx?b=CCNP_Studies_Switching&seqNum=59

 

 

 

 

 

QUESTION 6

Which congestion-avoidance or congestion-management technique can cause global synchronization?

 

A.

Tail drop

B.

Random early detection

C.

Weighted random early detection

D.

Weighted fair queuing

 

Correct Answer: A

Explanation:

Tail Drop

Tail drop treats all traffic equally and does not differentiate between classes of service. Queues fill during periods of congestion. When the output queue is full and tail drop is in effect, packets are dropped until the congestion is eliminated and the queue is no longer full.

Weighted Random Early Detection

WRED avoids the globalization problems that occur when tail drop is used as the congestion avoidance mechanism on the router. Global synchronization occurs as waves of congestion crest only to be followed by troughs during which the transmission link is not fully utilized. Global synchronization of TCP hosts, for example, can occur because packets are dropped all at once. Global synchronization manifests when multiple TCP hosts reduce their transmission rates in response to packet dropping, then increase their transmission rates once again when the congestion is reduced.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfconav.html#wp1002048

 

 

QUESTION 7

Refer to the exhibit. R2 is mutually redistributing between EIGRP and BGP. Which configuration is necessary to enable R1 to see routes from R3?

 

clip_image004

 

A.

The R3 configuration must include ebgp-multihop to the neighbor statement for R2.

B.

The R2 BGP configuration must include bgp redistribute-internal.

C.

R1 must be configured with next-hop-self for the neighbor going to R2.

D.

The AS numbers configured on R1 and R2 must match.

 

Correct Answer: B

Explanation:

Whenever you redistribute from BGP to something else, BGP will only advertise externally learned routes. To allow the redistribution of iBGP routes into an interior gateway protocol such as EIGRP or OSPF, use the bgp redistribute-internal command in router configuration mode.

 

 

QUESTION 8

Refer to the exhibit. You have configured two routing protocols across this point-to-point link. How many BFD sessions will be established across this link?

 

clip_image005

 

A.

three per interface

B.

one per multicast address

C.

one per routing protocol

D.

one per interface

 

Correct Answer: D

Explanation:

Cisco devices will use one Bidirectional Forwarding Detection (BFD) session for multiple client protocols in the Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T. For example, if a network is running OSPF and EIGRP across the same link to the same peer, only one BFD session will be established, and BFD will share session information with both routing protocols.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html#wp1053749

 

 

QUESTION 9

Which three elements compose a network entity title? (Choose three.)

 

A.

area ID

B.

domain ID

C.

system ID

D.

NSAP selector

E.

MAC address

F.

IP address

 

Correct Answer: ACD

Explanation:

An IS (Intermediate system) is identified by an address known as a network access point (NASAP). The NSAP is divided up into three parts as specified by ISO/AI 10589:

Area address–This field is of variable length, composed of high order octets, and it excludes th
e System ID and N-selector (NSEL) fields. This area address is associated wit a single area within the routing domain.

System ID–This field is 6 octets long and should be set to a unique value with Level 1 and Level

The system IS defines an end system (ES) or an IS in an area. You configure the area address and the system ID with the NET command. You can display the system ID with the show isis topology command.

NSEL–This field is called the N-selector, also referred to as the NSAP, and it specifies the upper- layer protocol. The NSEL is the last byte of the NSAP and identifies a network service user. A network service user is a transport entity or the IS network entity itself. When the N-selector is set to zero, the entire NSAP is called a network entity title (NET).

A NET is an NSAP where the last byte is always the n-selector and is always zero. A NET can be from 8 to 20 bytes in length.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is2.html

 

 

QUESTION 10

Which three statements are true about PPP CHAP authentication? (Choose three.)

 

A.

PPP encapsulation must be enabled globally.

B.

The LCP phase must be complete and in closed state.

C.

The hostname used by a router for CHAP authentication cannot be changed.

D.

PPP encapsulation must be enabled on the interface.

E.

The LCP phase must be complete and in open state.

F.

By default, the router uses its hostname to identify itself to the peer.

 

Correct Answer: DEF

Explanation:

Point-to-Point Protocol (PPP) authentication issues are one of the most common causes for dialup link failures. This document provides some troubleshooting procedures for PPP authentication issues.

Prerequisites

Enable PPP encapsulation

The PPP authentication phase does not begin until the Link Control Protocol (LCP) phase is complete and is in the open state. If debug ppp negotiation does not indicate that LCP is open, troubleshoot this issue before proceeding.

Note: By default, the router uses its hostname to identify itself to the peer. However, this CHAP username can be changed through the ppp chap hostname command.

Reference: http://www.cisco.com/c/en/us/support/docs/wan/point-to-point-protocol-ppp/25647-understanding-ppp-chap.html

 

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