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April 7, 2017

Down to date Cisco 400-101 - An Overview 241 to 250

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2017 Apr 400-101 test questions

Q241. Refer to the exhibit. 

What does the return code 3 represent in this output? 

A. The mapping of the replying router for the FEC is different. 

B. The packet is label-switched at stack depth. 

C. The return code is reserved. 

D. The upstream index is unknown. 

E. The replying router was the proper egress for the FEC. 



Return Codes The Return Code is set to zero by the sender. The receiver can set it to one of the values listed below. The notation <RSC> refers to the Return Subcode. This field is filled in with the stack-depth for those codes that specify that. For all other codes, the Return Subcode MUST be set to zero. 

Value Meaning 

0 No return code 

1 Malformed echo request received 

2 One or more of the TLVs was not understood 

3 Replying router is an egress for the FEC at stack-depth <RSC> 

4 Replying router has no mapping for the FEC at stack-depth <RSC> 

Reference: https://www.ietf.org/rfc/rfc4379.txt 

Q242. A configuration includes the line ip route permanent. 

Which option is a benefit of configuring this static route as permanent? 

A. It allows the route to be redistributed into the network even if the outgoing interface is down. 

B. It allows the route to be saved in the running configuration of the device. 

C. It places a hidden tag on the route that can be matched on other devices. 

D. It allows the route to have a tracking status even if no tracking object is configured. 


Q243. What is the purpose of Route Target Constraint? 

A. to avoid using route reflectors in MPLS VPN networks 

B. to avoid using multiple route distinguishers per VPN in MPLS VPN networks 

C. to be able to implement VPLS with BGP signaling 

D. to avoid sending unnecessary BGP VPNv4 or VPNv6 updates to the PE router 

E. to avoid BGP having to perform route refreshes 



Some service providers have a very large number of routing updates being sent from RRs to PEs, using considerable resources. A PE does not need routing updates for VRFs that are not on the PE; therefore, the PE determines that many routing updates it receives are “unwanted.” The PE can filter out the unwanted updates using Route Target Constraint. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios/ios_xe/iproute_bgp/configuration/guide/2_xe/irg_x e_book/irg_rt_filter_xe.html. 

Q244. Refer to the exhibit. 

You are bringing a new MPLS router online and have configured only what is shown to bring LDP up. Assume that the peer has been configured in a similar manner. You verify the LDP peer state and see that there are no neighbors. What will the output of show mpls ldp discovery show? 

A. Interfaces: 

Ethernet0/0 (ldp): xmit 

B. Interfaces: 

Ethernet0/0 (ldp): xmit/recv 

LDP Id:; IP addr: 

C. Interfaces: 

Ethernet0/0 (ldp): xmit/recv 

LDP Id:; no route 

D. Interfaces: 

Ethernet0/0 (ldp): xmit/recv 

LDP Id:; no route 


Q245. You are implementing new addressing with EIGRP routing and must use secondary addresses, which are missing from the routing table. Which action is the most efficient solution to the problem? 

A. Disable split-horizon on the interfaces with secondary addresses. 

B. Disable split-horizon inside the EIGRP process on the router with the secondary interface addresses. 

C. Add additional router interfaces and move the secondary addresses to the new interfaces. 

D. Use a different routing protocol and redistribute the routes between EIGRP and the new protocol. 



Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon with EIGRP and RIP. If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by every secondary address. One routing update is sourced per network number unless split horizon is disabled. 



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Q246. Where should the passive-interface command be used? 

A. Under the routing process for interfaces that need to be routed, but prevented from peering 

B. under the routing process for interfaces that need to be routed and allowed to peer 

C. under the interface configuration for interfaces that need to be routed, but prevented from peering 

D. under the interface configuration for interfaces that need to be routed and allowed to peer 

E. under the VTY configuration within global configuration mode 



Passive-interface is a feature you enable on a per interface basis which allows a particular interface to participate in a routing process but prevents that interface from forming neighbor relationships by not sending hello packets and discarding received hello packets. 

Q247. Which algorithm heavily influenced the algorithm used by path-vector protocols? 

A. Bellman-Ford 



D. Spanning-Tree 

E. Adaptive 

F. Deflection 



A path vector protocol is a computer network routing protocol which maintains the path information that gets updated dynamically. Updates which have looped through the network and returned to the same node are easily detected and discarded. This algorithm is sometimes used in Bellman–Ford routing algorithms to avoid "Count to Infinity" problems. 

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

Q248. When VRF-Lite is configured without BGP support,.which statement about the configuration of the route target and route distinguisher is true? 

A. The configuration of the route target and route distinguisher is required. 

B. The configuration of the route target and route distinguisher is not required. 

C. The configuration of the route target is required and the configuration of the route distinguisher is not required. 

D. The configuration of the route target is not required and the configuration of the route distinguisher is required. 


Q249. Refer to the exhibit. 

How many EIGRP routes will appear in the routing table of R2? 

A. 0 

B. 1 

C. 2 

D. 3 



EIGRPv6 on R2 was shut down so there is no EIGRP routes on the routing table of R2. If we turn on EIGRPv6 on R2 (with “no shutdown” command) then we would see the prefix of the loopback interface of R1 in the routing table of R2. 

Note. EIGRPv6 requires the “ipv6 unicast-routing” global command to be turned on first or it will not work. 

Q250. 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. 



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 addresses 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. 



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