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February 6, 2018

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Q211. Refer to the exhibit. 

What is the meaning of the asterisk (*) in the output? 

A. PIM neighbor 10.1.5.6 is the RPF neighbor for the group 232.1.1.1 for the shared tree. 

B. PIM neighbor 10.1.5.6 is the one that is seen as the RPF neighbor when performing the command show ip rpf 10.1.4.7. 

C. PIM neighbor 10.1.5.6 is the winner of an assert mechanism. 

D. The RPF neighbor 10.1.5.6 is invalid. 

Answer:

Explanation: 

show ip mroute 

Field 

Descriptions 

Field Description 

RPF neighbor or RPF nbr 

IP address of the upstream router to the source. Tunneling indicates that this router is sending data to the RP encapsulated in register packets. 

The hexadecimal number in parentheses indicates to which RP it is registering. Each bit indicates a different RP if multiple RPs per group are used. If an asterisk (*) appears after the IP address in this field, the RPF neighbor has been learned through an assert. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ipmulti/command/reference/fiprmc_r/1rfmult 3.html 


Q212. Which two options are BGP attributes that are updated when router sends an update to its eBGP peer? (Choose two.) 

A. weight 

B. local preference 

C. AS_path 

D. next-hop 

Answer: C,D 

Explanation: 

AS_Path describes the inter-AS path taken to reach a destination. It gives a list of AS Numbers traversed when reaching to a destination. Every BGP speaker when advertising a route to a peer will include its own AS number in the NLRI. The subsequent BGP speakers who advertise this route will add their own AS number to the AS_Path, the subsequent AS numbers get prepended to the list. The end result is the AS_Path attribute is able to describe all the autonomous systems it has traversed, beginning with the most recent AS and ending with the originating AS. 

NEXT_HOP Attribute specifies the next hop IP address to reach the destination advertised in the NLRI. NEXT_HOP is a well-known mandatory attribute that is included in every eBGP update. 

Reference: http://netcerts.net/bgp-path-attributes-and-the-decision-process/ 


Q213. What is the most secure way to store ISAKMP/IPSec preshared keys in Cisco IOS? 

A. Use the service password-encryption command. 

B. Encrypt the ISAKMP preshared key in secure type 5 format. 

C. Encrypt the ISAKMP preshared key in secure type 7 format. 

D. Encrypt the ISAKMP preshared key in secure type 6 format. 

Answer:

Explanation: 

Using the Encrypted Preshared Key feature, you can securely store plain text passwords in type 6 format in NVRAM using a command-line interface (CLI). Type 6 passwords are encrypted. Although the encrypted passwords can be seen or retrieved, it is difficult to decrypt them to find out the actual password. This is currently the most secure way to store keys. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_ikevpn/configuration/xe-3s/asr1000/sec-ike-for-ipsec-vpns-xe-3s-asr1000-book/sec-encrypt-preshare.html 


Q214. Which three protocols can use enhanced object tracking? (Choose three.) 

A. HSRP 

B. Proxy-ARP 

C. VRRP 

D. GLBP 

E. NTP 

F. DHCP 

Answer: A,C,D 

Explanation: 

The Enhanced Object Tracking feature separates the tracking mechanism from HSRP and creates a separate standalone tracking process that can be used by other processes and HSRP. This feature allows tracking of other objects in addition to the interface line-protocol state. A client process such as HSRP, Virtual Router Redundancy Protocol (VRRP), or Gateway Load Balancing Protocol (GLBP), can register its interest in tracking objects and then be notified when the tracked object changes state. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp/configuration/15-mt/iap-15-mt-book/iap-eot.html 


Q215. DRAG DROP 

Drag and drop the Cisco IOS XE subpackage on the left to the function it performs on the right. 

Answer: 


Q216. Which two Cisco Express Forwarding tables are located in the data plane? (Choose two.) 

A. the forwarding information base 

B. the label forwarding information base 

C. the IP routing table 

D. the label information table 

E. the adjacency table 

Answer: A,B 

Explanation: 

The control plane runs protocols such as OSPF, BGP, STP, LDP. These protocols are needed so that routers and switches know how to forward packets and frames. 

The data plane is where the actual forwarding takes place. The data plane is populated based on the protocols running in the control plane. The Forwarding Information Base (FIB) is used for IP traffic and the Label FIB is used for MPLS. 


Q217. DRAG DROP 

Drag and drop the protocol on the left to the corresponding administrative distance on the right. 

Answer: 


Q218. Which two statements about reverse ARP are true? (Choose two.) 

A. Its servers require static mappings. 

B. It works with AutoInstall to configure new devices. 

C. It provides IP addresses for subnet masks. 

D. It provides IP addresses for default gateways. 

E. It requires less maintenance than DHCP. 

Answer: A,B 


Q219. 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 


Q220. Which two best practices does Cisco recommend to migrate a network from PVST+ to MST? (Choose two.) 

A. Start the migration at the edge nodes and work toward the root bridge. 

B. Before starting the transition, configure one of the edge nodes with a lower priority so that it becomes the root bridge after the transition. 

C. Before starting the transition, ensure that at least two nodes act as the root bridge for all VLANs in the network. 

D. Start the migration at the root bridge and work toward the edge nodes. 

E. Before starting the transition, configure one of the edge nodes with a higher priority so that it becomes the root bridge after the transition. 

F. Before starting the transition, ensure that one node is the root bridge for all VLANs in the network. 

Answer: A,F