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Q391. Which two statements about BGP best-path selection are true? (Choose two.) 

A. The route with the highest local preference is preferred. 

B. The weight attribute is advertised to peers. 

C. The route with the lowest MED is preferred. 

D. A route that originates from iBGP peers is preferred. 

E. A route that originates from a router with a higher BGP router ID is preferred. 

F. The lowest weight advertised is preferred. 

Answer: A,C 


Q392. Refer to the exhibit. 

Which statement is true about the downward bit? 

A. It forces the CE router to use a backup link instead of sending traffic via MPLS VPN. 

B. It informs the PE router that the LSA metric has been recently decreased to 1 and that partial SPF calculation cannot be delayed. 

C. It forces the CE router to install the LSA with the downward bit set into its routing table as a discard route. 

D. It informs the PE router that the LSA was already redistributed into BGP by another PE router and that the LSA must not be redistributed into BGP again. 

Answer:

Explanation: 

From RFC 4577, specifically section 4.2.5.1 

When a type 3 LSA is sent from a PE router to a CE router, the DN bit [OSPF-DN] in the LSA Options field MUST be set. This is used to ensure that if any CE router sends this type 3 LSA to a PE router, the PE router will not redistribute it further. 

When a PE router needs to distribute to a CE router a route that comes from a site outside the latter’s OSPF domain, the PE router presents itself as an ASBR (Autonomous System Border Router), and distributes the route in a type 5 LSA. The DN bit [OSPF-DN] MUST be set in these LSAs to ensure that they will be ignored by any other PE routers that receive them. 


Q393. 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 

Answer: C,D 

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 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 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/Qo S-SRND-Book/VPNQoS.html 


Q394. While troubleshooting an issue for a remote user, you must capture the communication between the user's computer and a server at your location. The traffic passes through a Cisco IOS-XE capable switch. Which statement about obtaining the capture is true? 

A. The Embedded Packet Capture application in the IOS-XE Software can capture the packets, but there is a performance impact. 

B. The Embedded Packet Capture application in the IOS-XE Software can capture the packets without impacting performance. 

C. The Mini Protocol Analyzer embedded in the IOS-XE Software can capture the packets without impacting performance. 

D. The Mini Protocol Analyzer embedded in the IOS-XE Software can be used to capture the packets, but there is a performance impact. 

E. Wireshark can capture packets through a SPAN port, but there is a performance impact. 

Answer:


Q395. Which statement is true about Fast Link Pulses in Ethernet? 

A. They are used during collision detection. 

B. They are used only if the media type is optical. 

C. They are part of UniDirectional Link Detection. 

D. They are used during autonegotiation. 

Answer:

Explanation: 

To make sure that your connection is operating properly, IEEE 802.3 Ethernet employs normal link pulses (NLPs), which are used for verifying link integrity in a 10BaseT system. This signaling gives you the link indication when you attach to the hub and is performed between two directly connected link interfaces (hub-to-station or station-to-station). NLPs are helpful in determining that a link has been established between devices, but they are not a good indicator that your cabling is free of problems. An extension of NLPs is fast link pulses. These do not perform link tests, but instead are employed in the autonegotiation process to advertise a device's capabilities. 

Reference: 

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html 


Q396. Which statement about the VLAN database is true? 

A. Manually deleting the vlan.dat file can cause inconsistency in the database. 

B. Private VLAN information is stored in the database. 

C. VLAN configurations 1 through 4096 are stored in the vlan.dat file. 

D. The VLAN database is used only if the VTP domain name in the VLAN database matches the VTP domain name in the startup-config file. 

Answer:


Q397. When you implement PfR, which IP SLA probe is used to determine the MOS? 

A. jitter 

B. latency 

C. packet loss 

D. throughput 

Answer:


Q398. In the DiffServ model, which class represents the lowest priority with the highest drop probability? 

A. AF11 

B. AF13 

C. AF41 

D. AF43 

Answer:

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 


Q399. Which two mechanisms can be used to eliminate Cisco Express Forwarding polarization? (Choose two.) 

A. alternating cost links 

B. the unique-ID/universal-ID algorithm 

C. Cisco Express Forwarding antipolarization 

D. different hashing inputs at each layer of the network 

Answer: B,D 

Explanation: 

This document describes how Cisco Express Forwarding (CEF) polarization can cause suboptimal use of redundant paths to a destination network. CEF polarization is the effect when a hash algorithm chooses a particular path and the redundant paths remain completely unused. 

How to Avoid CEF Polarization 

. Alternate between default (SIP and DIP) and full (SIP + DIP + Layer4 ports) hashing inputs configuration at each layer of the network. 

. Alternate between an even and odd number of ECMP links at each layer of the network.The CEF load-balancing does not depend on how the protocol routes are inserted in the routing table. Therefore, the OSPF routes exhibit the same behavior as EIGRP. In a hierarchical network where there are several routers that perform load-sharing in a row, they all use same algorithm to load-share. 

The hash algorithm load-balances this way by default: 

1: 1 

2: 7-8 

3: 1-1-1 

4: 1-1-1-2 

5: 1-1-1-1-1 

6: 1-2-2-2-2-2 

7: 1-1-1-1-1-1-1 

8: 1-1-1-2-2-2-2-2 

The number before the colon represents the number of equal-cost paths. The number after the colon represents the proportion of traffic which is forwarded per path. 

This means that: 

For two equal cost paths, load-sharing is 46.666%-53.333%, not 50%-50%. 

For three equal cost paths, load-sharing is 33.33%-33.33%-33.33% (as expected). 

For four equal cost paths, load-sharing is 20%-20%-20%-40% and not 25%-25%-25%-25%. 

This illustrates that, when there is even number of ECMP links, the traffic is not load-balanced. 

.Cisco IOS introduced a concept called unique-ID/universal-ID which helps avoid CEF polarization. This algorithm, called the universal algorithm (the default in current Cisco IOS versions), adds a 32-bit router-specific value to the hash function (called the universal ID - this is a randomly generated value at the time of the switch boot up that can can be manually controlled). This seeds the hash function on each router with a unique ID, which ensures that the same source/destination pair hash into a different value on different routers along the path. This process provides a better network-wide load-sharing and circumvents the polarization issue. This unique -ID concept does not work for an even number of equal-cost paths due to a hardware limitation, but it works perfectly for an odd number of equal-cost paths. In order to overcome this problem, Cisco IOS adds one link to the hardware adjacency table when there is an even number of equal-cost paths in order to make the system believe that there is an odd number of equal-cost links. 

Reference: http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/116376-technote-cef-00.html 


Q400. Refer to the exhibit. 

If a connection failure occurs between R1 and R2, which two actions can you take to allow CR-1 to reach the subnet 192.168.192.0/24 on R2? (Choose two.) 

A. Create a static route on R1 for subnet 192.168.192.0/24 towards R3 and redistribute it into OSPF. 

B. Turn up a BGP session between CR-1 and R1. 

C. Create a static route on R1 for subnet 192.168.192.0/24 towards R3 and redistribute it into BGP. 

D. Turn up an EIGRP session between R1 and R3 with AS 65535. 

E. Create an OSPF virtual link between CR-1 and R2 to bypass R1. 

Answer: A,B