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Q131. 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. 

Answer:

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 sends the first segment even if it is a small one, then that it waits until an ACK is received or a maximum sized segment (MSS) is accumulated. 

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


Q132. Refer to the exhibit. 

Which two statements about this configuration are true? (Choose two.) 

A. It allows 172.16.0.0/16 to be distributed into EIGRP. 

B. It allows a default route to be distributed into EIGRP. 

C. It allows 172.16.0.0/16 and larger subnets to be distributed into EIGRP. 

D. It prevents 172.16.0.0/16 from being distributed into EIGRP. 

E. It prevents a default route from being distributed into EIGRP. 

F. It creates summary routes and injects them into EIGRP. 

Answer: A,B 

Explanation: 

In this example, the prefix list is configured to only allow the two specific routes of 172.16.0.0/16 and the default route. Any other routes will be filtered. 


Q133. Which three features require Cisco Express Forwarding? (Choose three.) 

A. NBAR 

B. AutoQoS 

C. fragmentation 

D. MPLS 

E. UplinkFast 

F. BackboneFast 

Answer: A,B,D 

Explanation: 

QoS Features That Require CEF 

These class-based QoS features are supported only on routers that run CEF. 

.Network Based Application Recognition (NBAR) provides intelligent network classification. For more information, refer to Network Based Application Recognition. 

. The AutoQoS -VoIP feature simplifies and speeds up the implementation and provisioning of QoS for VoIP traffic. This feature is enabled with the help of the auto qos voip command. CEF must be enabled at the interface or ATM PVC before the auto qos command can be used. For more information about this feature and its prerequisites, refer to AutoQoS -VoIP. 

From MPLS Fundamentals - Luc De Ghein 

Why Is CEF Needed in MPLS Networks? 

Concerning MPLS, CEF is special for a certain reason; otherwise, this book would not explicitly cover it. Labeled packets that enter the router are switched according to the label forwarding information base (LFIB) on the router. IP packets that enter the router are switched according to the CEF table on the router. Regardless of whether the packet is switched according to the LFIB or the CEF table, the outgoing packet can be a labeled packet or an IP packet 

Reference: http://www.cisco.com/c/en/us/support/docs/asynchronous-transfer-mode-atm/ip-to-atm-class-of-service/4800-cefreq.html 


Q134. Which two statements about IPv4 and IPv6 networks are true? (Choose two.) 

A. In IPv6, hosts perform fragmentation. 

B. IPv6 uses a UDP checksum to verify packet integrity. 

C. In IPv6, routers perform fragmentation. 

D. In IPv4, fragmentation is performed by the source of the packet. 

E. IPv4 uses an optional checksum at the transport layer. 

F. IPv6 uses a required checksum at the network layer. 

Answer: A,B 


Q135. DRAG DROP 

Answer: 


Q136. Refer to the exhibit. 

Which two commands are required on R3 in order for MPLS to function? (Choose two.) 

A. mpls ip 

B. ip cef 

C. mpls label protocol tdp 

D. mpls ip propagate-ttl 

Answer: A,B 


Q137. Which statement about the overload bit in IS-IS is true? 

A. The IS-IS adjacencies on the links for which the overload bit is set are brought down. 

B. Routers running SPF ignore LSPs with the overload bit set and hence avoid blackholing traffic. 

C. A router setting the overload bit becomes unreachable to all other routers in the IS-IS area. 

D. The overload bit in IS-IS is used only for external prefixes. 

Answer:

Explanation: 

The OL bit is used to prevent unintentional blackholing of packets in BGP transit networks. Due to the nature of these protocols, IS-IS and OSPF converge must faster than BGP. Thus there is a possibility that while the IGP has converged, IBGP is still learning the routes. In that case if other IBGP routers start sending traffic towards this IBGP router that has not yet completely converged it will start dropping traffic. This is because it isnt yet aware of the complete BGP routes. OL bit comes handy in such situations. When a new IBGP neighbor is added or a router restarts, the IS-IS OL bit is set. Since directly connected (including loopbacks) addresses on an “overloaded” router are considered by other routers, IBGP can be bought up and can begin exchanging routes. Other routers will not use this router for transit traffic and will route the packets out through an alternate path. Once BGP has converged, the OL bit is cleared and this router can begin forwarding transit traffic. 

Reference: https://routingfreak.wordpress.com/category/ospf-vs-is-is/ 


Q138. Which three statements describe the characteristics of a VPLS architecture? (Choose three.) 

A. It forwards Ethernet frames. 

B. It maps MAC address destinations to IP next hops. 

C. It supports MAC address aging. 

D. It replicates broadcast and multicast frames to multiple ports. 

E. It conveys MAC address reachability information in a separate control protocol. 

F. It can suppress the flooding of traffic. 

Answer: A,C,D 

Explanation: 

As a VPLS forwards Ethernet frames at Layer 2, the operation of VPLS is exactly the same as that found within IEEE 802.1 bridges in that VPLS will self learn source MAC address to port associations, and frames are forwarded based upon the destination MAC address. Like other 802.1 bridges, MAC address aging is supported. 

Reference: http://www.cisco.com/en/US/products/hw/routers/ps368/products_white_paper09186a0080 1f6084.shtml 


Q139. Refer to the exhibit. 

Which statement is true? 

A. IS-IS has been enabled on R4 for IPv6, single-topology. 

B. IS-IS has been enabled on R4 for IPv6, multitopology. 

C. IS-IS has been enabled on R4 for IPv6, single-topology and multitopology. 

D. R4 advertises IPv6 prefixes, but it does not forward IPv6 traffic, because the protocol has not been enabled under router IS-IS. 

Answer:

Explanation: 

When working with IPv6 prefixes in IS-IS, you can configure IS-IS to be in a single topology for both IPv4 and IPv6 or to run different topologies for IPv4 and IPv6. By default, IS-IS works in single-topology mode when activating IPv4 and IPv6. This means that the IS-IS topology will be built based on IS Reachability TLVs. When the base topology is built, then IPv4 prefixes (IP Reachability TLV) and IPv6 prefixes (IPv6 Reachability TLV) are added to each node as leaves, without checking if there is IPv6 connectivity between nodes. 

Reference: https://blog.initialdraft.com/archives/3381/ 


Q140. 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 


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