400-101 Guide

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  • Cisco
  • Exam Number/Code 400-101
  • Product Name CCIE Routing and Switching (v5.0)
  • Questions and Answers
  • 911 Q&As
  • Last Updated
  • Jun 18,2018
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Q231. 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. 



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 

Q232. A packet capture log indicates that several router solicitation messages were sent from a local host on the IPv6 segment. What is the expected acknowledgment and its usage? 

A. Router acknowledgment messages will be forwarded upstream, where the DHCP server will allocate addresses to the local host. 

B. Routers on the IPv6 segment will respond with an advertisement that provides an external path from the local subnet, as well as certain data, such as prefix discovery. 

C. Duplicate Address Detection will determine if any other local host is using the same IPv6 address for communication with the IPv6 routers on the segment. 

D. All local host traffic will be redirected to the router with the lowest ICMPv6 signature, which is statically defined by the network administrator. 


Q233. Refer to the exhibit. 

You discover that only 1.5 Mb/s of web traffic can pass during times of congestion on the given network. 

Which two options are possible reasons for this limitation? (Choose two.) 

A. The web traffic class has too little bandwidth reservation. 

B. Video traffic is using too much bandwidth. 

C. The service-policy is on the wrong interface. 

D. The service-policy is going in the wrong direction. 

E. The NAT policy is adding too much overhead. 

Answer: A,B 


In this example, the web traffic will fall into the default class, which is only 15 percent of the 10Mbps Internet connection (1.5Mbps). Meanwhile, video traffic is allowed 50% of the 10 Mbps. 

Q234. Which two statements about VPLS are true? (Choose two.) 

A. Split horizon is used on PE devices to prevent loops. 

B. Spanning tree is extended from CE to CE. 

C. IP is used to switch Ethernet frames between sites. 

D. PE routers dynamically associate to peers. 

E. VPLS extends a Layer 2 broadcast domain. 

Answer: A,E 

Q235. Which statement describes the function of rekey messages? 

A. They prevent unencrypted traffic from passing through a group member before registration. 

B. They refresh IPsec SAs when the key is about to expire. 

C. They trigger a rekey from the server when configuring the rekey ACL. 

D. They authenticate traffic passing through a particular group member. 



Rekey messages are used to refresh IPsec SAs. When the IPsec SAs or the rekey SAs are about to expire, one single rekey message for a particular group is generated on the key server. No new IKE sessions are created for the rekey message distribution. The rekey messages are distributed by the key server over an existing IKE SA. Rekeying can use multicast or unicast messages. 

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_getvpn/configuration/xe-3s/sec-get-vpn-xe-3s-book/sec-get-vpn.html 

Q236. Which command enables L2 QoS support in all VLANs (including the native VLAN)? 

A. switchport priority extend cos 

B. mls qos trust dscp 

C. mls qos rewrite ip dscp 

D. switchport trunk native vlan tag 



You can enter the switchport trunk native vlan tag command to enable the tagging of native VLAN traffic on a per-port basis. When tagging is enabled, all the packets on the native VLAN are tagged and all incoming untagged data packets are dropped, but untagged control packets are accepted. When tagging is enabled, it will allow for L2 QoS support in all VLANs, including the native VLAN. 

Q237. Which statement about UDLD is true? 

A. The udld reset command resets ports that have been error-disabled by both UDLD and Fast UDLD. 

B. Fast UDLD is configured in aggressive mode. 

C. Only bidirectional link failures can be detected in normal mode. 

D. Each switch in a UDLD topology can send and receive packets to and from its neighbors. 


Q238. Which two options about PIM-DM are true? (Choose two.) 

A. PIM-DM initally floods multicast traffic throughout the network. 

B. In a PIM-DM network, routers that have no upstream neighbors prune back unwanted traffic. 

C. PIM-DM supports only shared trees. 

D. PIM-DM uses a pull model to deliver multicast traffic. 

E. PIM-DM cannot be used to build a shared distribution tree. 

Answer: A,E 


Drag and drop the IPv6 multicast feature on the left to its corresponding function on the right. 


Q240. Which three options are best practices for implementing a DMVPN? (Choose three.) 

A. Use IPsec in tunnel mode. 

B. Implement Dead Peer Detection to detect communication loss. 

C. Configure AES for encryption of transported data. 

D. Configure SHA-1 for encryption of transported data. 

E. Deploy IPsec hardware acceleration to minimize router memory overhead. 

F. Configure QoS services only on the head-end router. 

Answer: A,B,C 


Best Practices Summary for Hub-and-Spoke Deployment Model 

This section describes the best practices for a dual DMVPN cloud topology with the hub-and-spoke deployment, supporting IP multicast (IPmc) traffic including routing protocols. 

The following are general best practices: 

. Use IPsec in transport mode 

. Configure Triple DES (3DES) or AES for encryption of transported data (exports of encryption algorithms to certain countries may be prohibited by law). 

Implement Dead Peer Detection (DPD) on the spokes to detect loss of communication between peers. 

. Deploy hardware-acceleration of IPsec to minimize router CPU overhead, to support traffic with low latency and jitter requirements, and for the highest performance for cost. 

. Keep IPsec packet fragmentation to a minimum on the customer network by setting MTU size or using Path MTU Discovery (PMTUD). 

. Use Digital Certificates/Public Key Infrastructure (PKI) for scalable tunnel authentication. 

. Configure a routing protocol (for example, EIGRP, BGP or OSPF) with route summarization help alleviate interface congestion issues and to attempt to keep higher priority traffic from being dropped during times of congestion. 

Reference: http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/DMVPDG/DMV PN_1.html 

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