CCIE Routing and Switching Written Exam v5.1

 

QUESTION 151

Which statement describes the function of the tracking object created by the track 10 ip route 192.168.99.0/24 reachability command?

 

A.

It tracks the reachability of route 192.168.99.0/24.

B.

It tracks the line protocol status of the interface on which route 192.168.99.0/24 is received.

C.

It tracks exactly 10 occurrences of route 192.168.99.0/24.

D.

It tracks the summary route 192.168.99.0/24 and all routes contained within.

 

Correct Answer: A

Explanation:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

track object-number {interface interface-id{line-protocol | ip routing} | ip route ip- address/prefix-length {metric threshold | reachability} | list {boolean {and | or}} | {threshold { weight | percentage}}}

(Optional) Create a tracking list to track the configured state and enter tracking configuration mode.

 

clip_image002The object-number range is from 1 to 500.

clip_image002[1]Enter interface interface-id to select an interface to track.

clip_image002[2]Enter line-protocol to track the interface line protocol state or enter ip routing to track the interface IP routing state.

clip_image002[3]Enter ip route ip-address/prefix-length to track the state of an IP route.

clip_image002[4]Enter metric threshold to track the threshold metric or enter reachability to track if the route is reachable.

clip_image002[5]The default up threshold is 254 and the default down threshold is 255.

clip_image002[6]Enter list to track objects grouped in a list. Configure the list as described on the previous pages.

 

Note

Repeat this step for each interface to be tracked.

 

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12- 2_52_se/configuration/guide/3560scg/sweot.html

 

 

QUESTION 152

Which statement is true regarding UDLD and STP timers?

 

A.

The UDLD message timer should be two times the STP forward delay to prevent loops.

B.

UDLD and STP are unrelated features, and there is no relation between the timers.

C.

The timers need to be synced by using the spanning-tree udld-sync command.

D.

The timers should be set in such a way that UDLD is detected before the STP forward delay expires.

 

Correct Answer: D

Explanation:

UDLD is designed to be a helper for STP. Therefore, UDLD should be able to detect an unidirectional link before STP would unblock the port due to missed BPDUs. Thus, when you configure UDLD timers, make sure your values are set so that unidirectional link is detected before “STP MaxAge + 2xForwardDelay” expires.

Reference: http://blog.ine.com/tag/stp/

 

 

QUESTION 153

What is a cause for unicast flooding?

 

A.

Unicast flooding occurs when multicast traffic arrives on a Layer 2 switch that has directly connected multicast receivers.

B.

When PIM snooping is not enabled, unicast flooding occurs on the switch that interconnects the PIM-enabled routers.

C.

A man-in-the-middle attack can cause the ARP cache of an end host to have the wrong MAC address. Instead of having the MAC address of the default gateway, it has a MAC address of the man-in-the-middle. This causes all traffic to be unicast flooded through the man-in-the-middle, which can then sniff all packets.

D.

Forwarding table overflow prevents new MAC addresses from being learned, and packets destined to those MAC addresses are flooded until space becomes available in the forwarding table.

 

Correct Answer: D

Explanation:

Causes of Flooding

The very cause of flooding is that destination MAC address of the packet is not in the L2 forwarding table of the switch. In this case the packet will be flooded out of all forwarding ports in its VLAN (except the port it was received on). Below case studies display most common reasons for destination MAC address not being known to the switch.

Cause 1: Asymmetric Routing

Large amounts of flooded traffic might saturate low-bandwidth links causing network performance issues or complete connectivity outage to devices connected across such low-bandwidth links Cause 2: Spanning-Tree Protocol Topology Changes

Another common issue caused by flooding is Spanning-Tree Protocol (STP) Topology Change Notification (TCN). TCN is designed to correct forwarding tables after the forwarding topology has changed. This is necessary to avoid a connectivity outage, as after a topology change some destinations previously accessible via particular ports might become accessible via different ports. TCN operates by shortening the forwarding table aging time, such that if the address is not relearned, it will age out and flooding will occur

Cause 3: Forwarding Table Overflow

Another possible cause of flooding can be overflow of the switch forwarding table. In this case, new addresses cannot be learned and packets destined to such addresses are flooded until some space becomes available in the forwarding table. New a
ddresses will then be learned. This is possible but rare, since most modern switches have large enough forwarding tables to accommodate MAC addresses for most designs.

Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563-143.html

 

 

QUESTION 154

Refer to the exhibit. Notice that debug ip bgp updates has been enabled. What can you conclude from the debug output?

 

clip_image004

 

A.

This is the result of the clear ip bgp 10.1.3.4 in command.

B.

This is the result of the clear ip bgp 10.1.3.4 out command.

C.

BGP neighbor 10.1.3.4 performed a graceful restart.

D.

BGP neighbor 10.1.3.4 established a new BGP session.

 

Correct Answer: A

Explanation:

If you enter the clear ip bgp out command for a BGP peer, that router resends its BGP prefixes to that peer. This does not cause a change in the best path on the receiving BGP peer. Hence, there is no change in the Table Version on that peer.

When you run the debug ip bgp updates on the receiving router, you see:

BGP(0): 10.1.3.4 rcvd UPDATE w/ attr: nexthop 10.1.3.4, origin i, metric 0, merged path 4, AS_PATHBGP(0): 10.1.3.4 rcvd 10.100.1.1/32…duplicate ignored

The received update is recognized as a duplicate, so it is ignored and no best path change occurs.

Reference: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/116511-technote-tableversion-00.html

 

 

 

 

 

 

 

 

 

 

QUESTION 155

DRAG DROP

clip_image006

 

Correct Answer:

clip_image008

 

 

QUESTION 156

What is the most efficient way to confirm whether microbursts of traffic are occurring?

 

A.

Monitor the output traffic rate using the show interface command.

B.

Monitor the output traffic rate using the show controllers command.

C.

Check the CPU utilization of the router.

D.

Sniff the traffic and plo
t the packet rate over time.

 

Correct Answer: D

Explanation:

Micro-bursting is a phenomenon where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network. In order to troubleshoot microbursts, you need a packet sniffer that can capture traffic over a long period of time and allow you to analyze it in the form of a graph which displays the saturation points (packet rate during microbursts versus total available bandwidth). You can eventually trace it to the source causing the bursts (e.g. stock trading applications).

Reference: Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 989-994). Kindle Edition.

 

 

QUESTION 157

What is Nagle’s algorithm used for?

 

A.

To increase the latency

B.

To calculate the best path in distance vector routing protocols

C.

To calculate the best path in link state routing protocols

D.

To resolve issues caused by poorly implemented TCP flow control.

 

Correct Answer: D

Explanation:

Silly window syndrome is a problem in computer networking caused by poorly implemented TCP flow control. A serious problem can arise in the sliding window operation when the sending application program creates data slowly, the receiving application program consumes data slowly, or both. If a server with this problem is unable to process all incoming data, it requests that its clients reduce the amount of data they send at a time (the window setting on a TCP packet). If the server continues to be unable to process all incoming data, the window becomes smaller and smaller, sometimes to the point that the data transmitted is smaller than the packet header, making data transmission extremely inefficient. The name of this problem is due to the window size shrinking to a “silly” value. When there is no synchronization between the sender and receiver regarding capacity of the flow of data or the size of the packet, the window syndrome problem is created. When the silly window syndrome is created by the sender, Nagle’s algorithm is used. Nagle’s solution requires that the sender send the first segment even if it is a small one, then that it wait until an ACK is received or a maximum sized segment (MSS) is accumulated.

Reference: http://en.wikipedia.org/wiki/Silly_window_syndrome

 

 

QUESTION 158

Which two statements about private VLANs are true? (Choose two.)

 

A.

Only one isolated VLAN can be mapped to a primary VLAN.

B.

Only one community VLAN can be mapped to a primary VLAN.

C.

Multiple isolated VLANs can be mapped to a primary VLAN.

D.

Multiple community VLANs can be mapped to a primary VLAN.

 

Correct Answer: AD

Explanation:

An isolated VLAN is a secondary VLAN that carries unidirectional traffic upstream from the hosts toward the promiscuous ports. You can configure only one isolated VLAN in a PVLAN domain. An isolated VLAN can have several isolated ports. The traffic from each isolated port also remains completely separate. Only one isolated VLAN can be mapped under a given primary VLAN.

A community VLAN is a secondary VLAN that carries upstream traffic from the community ports to the promiscuous port and to other host ports in the same community. You can configure multiple community VLANs in a PVLAN domain. The ports within one community can communicate, but these ports cannot communicate with ports in any other community or isolated VLAN in the private VLAN.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus6000/sw/layer2/6x/b_6k_Layer2_Config_6x/b_6k_Layer2_Config_602N12_chapter_011.html

 

 

QUESTION 159

In the DiffServ model, which class represents the lowest priority with the lowest drop probability?

 

A.

AF11

B.

AF13

C.

AF41

D.

AF43

 

Correct Answer: A

Explanation:

Assured Forwarding (AF) Behavior Group

 

Class 1

Class 2

Class 3

Class 4

Low Drop

AF11 (DSCP 10)

AF21 (DSCP 18)

AF31 (DSCP 26)

AF41 (DSCP 34)

Med Drop

AF12 (DSCP 12)

AF22 (DSCP 20)

AF32 (DSCP 28)

AF42 (DSCP 36)

High Drop

AF13 (DSCP 14)

AF23 (DSCP 22)

AF33 (DSCP 30)

AF43 (DSCP 38)

 

Reference: http://en.wikipedia.org/wiki/Differentiated_services

 

 

QUESTION 160

Which two mechanisms provide Cisco IOS XE Software with control plane and data plane separation? (Choose two.)

 

A.

Forwarding and Feature Manager

B.

Forwarding Engine Driver

C.

Forwarding Performance Management

D.

Forwarding Information Base

 

Correct Answer: AB

Explanation:

Control Plane and Data Plane Separation

IOS XE introduces an opportunity to enable teams to now build drivers for new Data Plane ASICs outside the IOS instance and have them program to a set of standard APIs which in turn enforces Control Plane and Data Plane processing separation.

IOS XE accomplishes Control Plane / Data Plane separation through the introduction of the Forwarding and Feature Manager (FFM) and its standard interface to the Forwarding Engine Driver (FED). FFM provides a set of APIs to Control Plane processes. In turn, the FFM programs the Data Plane via the FED and maintains forwarding state for the system. The FED is the instantiation of the hardware driver for the Data Plane and is provided by the platform.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/ios-xe-3sg/QA_C67-622903.html

 

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