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2017 Apr 400-101 exam topics

Q121. Which timer expiration can lead to an EIGRP route becoming stuck in active? 

A. hello 

B. active 

C. query 

D. hold 

Answer:

Explanation: 

As noted above, when a route goes into the active state, the router queries its neighbors to find a path to the pertinent network. At this point, the router starts a three minute active timer by which time it must receive replies from all queried neighbors. If a neighbor has feasible successors for the route, it will recalculate its own local distance to the network and report this back. However, if a neighbor does not have a feasible successor, it also goes into active state. In some cases, multiple routers along multiple query paths will go into active state as routers continue to query for the desired route. In most cases, this process will yield responses from all queried routers and the sought after route will transition back into the passive state within the three minute SIA query timer. In the case that none of the queried routers can provide a feasible successor, the route is cleared. In some cases, a response is not received between two neighbor routers because of link failures, congestion or some other adverse condition in either the network or on the queried router, and the three minute active timer expires on the router originating the query. When this happens, the querying router that did not receive a response logs a “DUAL-3-SIA” or “stuck-in-active” error for the route and then drops and restarts its adjacency with the non-responding router 

Reference: http://www.packetdesign.com/resources/technical-briefs/diagnosing-eigrp-stuck-active 


Q122. Which Carrier Ethernet service supports the multiplexing of multiple point-to-point EVCs across as a single UNI? 

A. EPL 

B. EVPL 

C. EMS 

D. ERMS 

Answer:

Explanation: 

Ethernet Relay Service (ERS or EVPL) 

An Ethernet Virtual Circuit (EVC) is used to logically connect endpoints, but multiple EVCs 

could exist per single UNI. Each EVC is distinguished by 802.1q VLAN tag identification. 

The ERS network acts as if the Ethernet frames have crossed a switched network, and certain control traffic is not carried between ends of the EVC. ERS is analogous to Frame Relay where the CE-VLAN tag plays the role of a Data-Link Connection Identifier (DLCI). The MEF term for this service is EVPL. 

Reference: http://www.cisco.com/c/en/us/td/docs/net_mgmt/ip_solution_center/5-1/carrier_ethernet/user/guide/l2vpn51book/concepts.html 


Q123. Refer to the exhibit. 

All switches have default bridge priorities, and originate BPDUs with MAC addresses as indicated. The numbers shown are STP link metrics. 

After STP converges, you discover that traffic from switch SWG toward switch SWD takes a less optimal path. What can you do to optimize the STP tree in this switched network? 

A. Change the priority of switch SWA to a lower value than the default value. 

B. Change the priority of switch SWB to a higher value than the default value. 

C. Change the priority of switch SWG to a higher value than the default value. 

D. Change the priority of switch SWD to a lower value than the default value. 

Answer:

Explanation: 

In this topology, we see that all port paths and priorities are the same, so the lowest MAC address will be used to determine the best STP path. From SWG, SWE will be chosen as the next switch in the path because it has a lower MAC address than SWF. From SWE, traffic will go to SWC because it has a lower MAC address, and then to SWD, instead of going from SWE directly to SWD. If we lower the priority of SWD (lower means better with STP) then traffic will be sent directly to SWD. 


Q124. Which additional feature must be enabled on a switch to allow PIM snooping to function correctly? 

A. IGMP snooping 

B. port security 

C. storm control 

D. dynamic ARP inspection 

Answer:


Q125. Which two statements about MAC Authentication Bypass are true? (Choose two.) 

A. Traffic from an endpoint is authorized to pass after MAB authenticates the MAC address of the endpoint. 

B. During the learning stage, the switch examines multiple packets from the endpoint to determine the MAC address of the endpoint. 

C. After the switch learns the MAC address of the endpoint, it uses TACACS+ to authenticate it. 

D. After learning a source MAC address, it sends the host a RADIUS Account-Request message to validate the address. 

E. The MAC address of a device serves as its user name and password to authenticate with a RADIUS server. 

Answer: A,E 


Far out 400-101 exams:

Q126. Refer to the exhibit. 

If EIGRP is configured between two routers as shown in this output, which statement about their EIGRP relationship is true? 

A. The routers will establish an EIGRP relationship successfully. 

B. The routers are using different authentication key-strings. 

C. The reliability metric is enabled. 

D. The delay metric is disabled. 

Answer:

Explanation: 

The 5 K values used in EIGRP are: 

K1 = Bandwidth modifier 

K2 = Load modifier 

K3 = Delay modifier 

K4 = Reliability modifier 

K5 = Additional Reliability modifier 

However, by default, only K1 and K3 are used (bandwidth and delay). In this output we see that K1, K3, and K4 (Reliability) are all set. 


Q127. What is the function of an EIGRP sequence TLV packet? 

A. to acknowledge a set of sequence numbers during the startup update process 

B. to list the peers that should listen to the next multicast packet during the reliable multicast process 

C. to list the peers that should not listen to the next multicast packet during the reliable multicast process 

D. to define the initial sequence number when bringing up a new peer 

Answer:

Explanation: 

EIGRP sends updates and other information between routers using multicast packets to 224.0.0.10. For example in the topology below, R1 made a change in the topology and it needs to send updates to R2 & R3. It sends multicast packets to EIGRP multicast address 224.0.0.10. Both R2 & R3 can receive the updates and acknowledge back to R1 using unicast. Simple, right? But what if R1 sends out updates, only R2 replies but R3 never does? In the case a router sends out a multicast packet that must be reliable delivered (like in this case), an EIGRP process will wait until the RTO (retransmission timeout) period has passed before beginning a recovery action. This period is calculated from the SRTT (smooth round-trip time). After R1 sends out updates it will wait for this period to expire. Then it makes a list of all the neighbors from which it did not receive an Acknowledgement (ACK). Next it sends out a packet telling these routers stop listening to multicast until they are been notified that it is safe again. Finally the router will begin sending unicast packets with the information to the routers that didn’t answer, continuing until they are caught up. In our example the process will be like this: 

1. R1 sends out updates to 224.0.0.10 

2. R2 responds but R3 does not 

3. R1 waits for the RTO period to expire 

4. R1 then sends out an unreliable-multicast packet, called a sequence TLV (Type-Length-Value) packet, which tells R3 not to listen to multicast packets any more 

5. R1 continues sending any other muticast traffic it has and delivering all traffic, using unicast to R3, until it acknowledges all the packets 

6. Once R3 has caught up, R1 will send another sequence TLV, telling R3 to begin listening to multicast again. The sequence TLV packet contains a list of the nodes that should not listen to multicast packets while the recovery takes place. But notice that the TLV packet in step 6 does not contain any nodes in the list. 

Note. In the case R3 still does not reply in step 4, R1 will attempt to retransmit the unicast 16 times or continue to retransmit until the hold time for the neighbor in question expires. After this time, R1 will declare a retransmission limit exceeded error and will reset the neighbor. 

(Reference: EIGRP for IP: Basic Operation and Configuration) 


Q128. DRAG DROP 

Drag and drop the MPLS term on the left to the function it performs on the right. 

Answer: 


Q129. Refer to the exhibit. 

Which statement about configuring the switch to manage traffic is true? 

A. The switchport priority extend cos command on interface FastEthernet0/0 prevents traffic to and from the PC from taking advantage of the high-priority data queue that is assigned to the IP phone. 

B. The switchport priority extend cos command on interface FastEthernet0/0 enables traffic to and from the PC to use the high priority data queue that is assigned to the IP phone. 

C. When the switch is configured to trust the CoS label of incoming traffic, the trusted boundary feature is disabled automatically. 

D. The mls qos cos override command on interface FastEthernet0/0 configures the port to trust the CoS label of traffic to and from the PC. 

Answer:

Explanation: 

In some situations, you can prevent a PC connected to the Cisco IP Phone from taking advantage of a high-priority data queue. You can use the switchport priority extend cos interface configuration command to configure the telephone through the switch CLI to override the priority of the traffic received from the PC. 

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_22_ea2/configuration/guide/2950scg/swqos.html 


Q130. Which regular expression will match prefixes that originated from AS200? 

A. ^$ 

B. ^200_ 

C. _200$ D. ^200) 

E. _200_ 

Answer:

Explanation: 

Example on how to deny all prefixes originated in Autonomous System 200 

router bgp 100 

neighbor 10.1.1.1 remote-as 65535 

neighbor 10.1.1.1 route-map map1 in 

route-map map1 permit 10 

match as-path 1 

ip as-path access-list 5 deny _200$ 

ip as-path access-list 5 permit .* 

Reference: http://www.cisco.com/c/en/us/td/docs/ios/termserv/configuration/guide/12_4t/tsv_12_4t_bo ok/tsv_reg_express.html