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QUESTION 21
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.
Correct Answer: D Explanation
Explanation/Reference:
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.
Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563-143.html
QUESTION 22
Refer to the exhibit.
Video Source S is sending interactive video traffic to Video Receiver R. Router R1 has multiple routing table entries for destination R. Which load-balancing mechanism on R1 can cause out-of-order video traffic to be received by destination R?
A. per-flow load balancing on R1 for destination R
B. per-source-destination pair load balancing on R1 for destination R
C. CEF load balancing on R1 for destination R
D. per-packet load balancing on R1 for destination R
Correct Answer: D Explanation
Explanation/Reference:
Per-packet load balancing guarantees equal load across all links, however potentially the packets may arrive out-of-order at the destination as differential delay may exist within the network.
Reference: http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html
QUESTION 23
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.
Correct Answer: D Explanation
Explanation/Reference:
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
QUESTION 24
Which statement is true regarding the UDP checksum?
A. It is used for congestion control.
B. It cannot be all zeros.
C. It is used by some Internet worms to hide their propagation.
D. It is computed based on the IP pseudo-header.
Correct Answer: D Explanation
Explanation/Reference:
The method used to compute the checksum is defined in RFC 768:
“Checksum is the 16-bit one’s complement of the one’s complement sum of a pseudo header of information from the IP header, the UDP header, and the data,
padded with zero octets at the end (if necessary) to make a multiple of two octets.”
In other words, all 16-bit words are summed using one’s complement arithmetic. Add the 16-bit values up. Each time a carry-out (17th bit) is produced, swing that
bit around and add it back into the least significant bit. The sum is then one’s complemented to yield the value of the UDP checksum field. If the checksum
calculation results in the value zero (all 16 bits 0) it should be sent as the one’s complement (all 1s).
Reference: http://en.wikipedia.org/wiki/User_Datagram_Protocol
QUESTION 25
Which statement describes the purpose of the Payload Type field in the RTP header?
A. It identifies the signaling protocol.
B. It identifies the codec.
C. It identifies the port numbers for RTP.
D. It identifies the port numbers for RTCP.
Correct Answer: B Explanation
Explanation/Reference:
PT, Payload Type. 7 bits: Identifies the format of the RTP payload and determines its interpretation by the application. A profile specifies a default static mapping of payload type codes to payload formats. Additional payload type codes may be defined dynamically through non-RTP means. An RTP sender emits a single RTP payload type at any given time; this field is not intended for multiplexing separate media streams. A full list of codecs and their payload type values can be found at the link below:
Reference: http://www.networksorcery.com/enp/protocol/rtp.htm
QUESTION 26
Which Cisco IOS XE process administers routing and forwarding?
A. Forwarding manager
B. Interface manager
C. Cisco IOS
D. Host manager
Correct Answer: C Explanation
Explanation/Reference:
Some of the processes are listed in the table below:
Reference: http://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/chassis/asrswcfg/Software_Packaging_Architecture.html
QUESTION 27
Which circumstance can cause packet loss due to a microburst?
A. slow convergence
B. a blocked spanning-tree port
C. process switching
D. insufficient buffers
Correct Answer: D Explanation Explanation/Reference:
Micro-bursting is a phenomenon where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network. Symptoms of micro bursts will manifest in the form of ignores and/ or overruns (also shown as accumulated in “input error” counter within show interface output). This is indicative of receive ring and corresponding packet buffer being overwhelmed due to data bursts coming in over extremely short period of time (microseconds).
Reference: http://ccieordie.com/?tag=micro-burst
QUESTION 28
Which two statements about proxy ARP are true? (Choose two.)
A. It is supported on networks without ARP.
B. It allows machines to spoof packets.
C. It must be used on a network with the host on a different subnet.
D. It requires larger ARP tables.
E. It reduces the amount of ARP traffic.
Correct Answer: BD Explanation
Explanation/Reference:
Disadvantages of Proxy ARP
Hosts have no idea of the physical details of their network and assume it to be a flat network in which they can reach any destination simply by sending an ARP
request. But using ARP for everything has disadvantages. These are some of the disadvantages:
It increases the amount of ARP traffic on your segment.
Hosts need larger ARP tables in order to handle IP-to-MAC address mappings.
Security can be undermined. A machine can claim to be another in order to intercept packets, an act called “spoofing.”
It does not work for networks that do not use ARP for address resolution.
It does not generalize to all network topologies. For example, more than one router that connects two physical networks.
Reference: http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13718-5.html
QUESTION 29
Refer to the exhibit.
Routers R1 and R2 are configured as shown, and traffic from R1 fails to reach host 209.165.201.254. Which action can you take to correct the problem?
A. Ensure that R2 has a default route in its routing table.
B. Change the OSPF area type on R1 and R2.
C. Edit the router configurations so that address 209.165.201.254 is a routable address.
D. Remove the default-information originate command from the OSPF configuration of R2. Correct Answer: A
Explanation Explanation/Reference:
Not sure that any of these answers are correct, it appears that this configuration is valid for reaching that one specific host IP. Answer A does have a route to that host so it would not need a default route to get to it. Choice B is incorrect as the area types have nothing to do with this. C is incorrect as that IP address is routable, and D is needed so that R1 will have a default route advertised to it from R2 so that it can reach this destination.
QUESTION 30
Which service is disabled by the no service tcp-small-servers command?
A. the finger service
B. the Telnet service
C. the Maintenance Operation Protocol service
D. the chargen service
Correct Answer: D Explanation
Explanation/Reference:
The TCP small servers are:
Echo: Echoes back whatever you type through the telnet x.x.x.x echo command.
Chargen: Generates a stream of ASCII data. Use the telnet x.x.x.x chargen command.
DiscarD. Throws away whatever you type. Use the telnet x.x.x.x discard command.
DaytimE. Returns system date and time, if it is correct. It is correct if you run Network Time Protocol (NTP), or have set the date and time manually from the
exec level. Use the telnet x.x.x.x daytime command.
Reference: http://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-110/12815-23.html
QUESTION 31
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
Correct Answer: AB Explanation
Explanation/Reference:
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.
QUESTION 32
Which option is the most effective action to avoid packet loss due to microbursts?
A. Implement larger buffers.
B. Install a faster CPU.
C. Install a faster network interface.
D. Configure a larger tx-ring size.
Correct Answer: A Explanation
Explanation/Reference:
You can’t avoid or prevent them as such without modifying the sending host’s application/network stack so it smoothes out the bursts. However, you can manage microbursts by tuning the size of receive buffers / rings to absorb occasional microbursts.
QUESTION 33
Which two statements about packet fragmentation on an IPv6 network are true? (Choose two.)
A. The fragment header is 64 bits long.
B. The identification field is 32 bits long.
C. The fragment header is 32 bits long.
D. The identification field is 64 bits long.
E. The MTU must be a minimum of 1280 bytes.
F. The fragment header is 48 bits long.
Correct Answer: AB Explanation
Explanation/Reference:
The fragment header is shown below, being 64 bits total with a 32 bit identification field: Reference: http://www.openwall.com/presentations/IPv6/img24.html
QUESTION 34
You are backing up a server with a 1 Gbps link and a latency of 2 ms. Which two statements about the backup are true? (Choose two.)
A. The bandwidth delay product is 2 Mb.
B. The default TCP send window size is the limiting factor.
C. The default TCP receive window size is the limiting factor.
D. The bandwidth delay product is 500 Mb.
E. The bandwidth delay product is 50 Mb.
Correct Answer: AC Explanation Explanation/Reference:
1 Gbps is the same as 1000 Mbps, and 1000Mb x .0002 = 2 Mbps. With TCP based data transfers, the receive window is always the limiting factor, as the sender is generally able to send traffic at line rate, but then must wait for the acknowledgements to send more data.
QUESTION 35
Which two pieces of information does RTCP use to inform endpoint devices about the RTP flow? (Choose two.)
A. the transmitted octet
B. the lost packet count
C. session control function provisioning information
D. the CNAME for session participants
E. the authentication method
F. MTU size changes in the path of the flow Correct Answer: AB
Explanation Explanation/Reference:
RTCP transports statistics for a media connection and information such as transmitted octet and packet counts, packet loss, packet delay variation, and round-trip delay time. An application may use this information to control quality of service parameters, perhaps by limiting flow, or using a different codec.
Reference: http://en.wikipedia.org/wiki/RTP_Control_Protocol
QUESTION 36
How many hash buckets does Cisco Express Forwarding use for load balancing?
A. 8
B. 16
C. 24
D. 32
Correct Answer: B Explanation Explanation/Reference:
In order to understand how the load balance takes place, you must first see how the tables relate. The Cisco Express Forwarding table points to 16 hash buckets (load share table), which point to the adjacency table for parallel paths. Each packet to be switched is broken up into the source and destination address pair and checked against the loadshare table.
Reference: http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/18285-loadbal-cef.html
QUESTION 37
Which three features require Cisco Express Forwarding? (Choose three.)
A. NBAR
B. AutoQoS
C. fragmentation
D. MPLS
E. UplinkFast
F. BackboneFast
Correct Answer: ABD Explanation Explanation/Reference:
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
QUESTION 38
Which two options are interface requirements for turbo flooding? (Choose two.)
A. The interface is Ethernet.
B. The interface is configured for ARPA encapsulation.
C. The interface is PPP.
D. The interface is configured for GRE encapsulation.
E. The interface is configured for 802.1Q encapsulation.
Correct Answer: AB Explanation Explanation/Reference:
In the switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARPA encapsulation.
Reference: http://www.cisco.com/c/en/us/td/docs/switches/metro/me3400/software/release/12-2_50_se/configuration/guide/scg/swiprout.html
QUESTION 39
Which three options are sub-subfields of the IPv4 Option Type subfield? (Choose three.)
A. Option Class
B. GET
C. Copied
D. PUSH
E. Option Number
F. TTL
Correct Answer: ACE Explanation Explanation/Reference:
Each IP option has its own subfield format, generally structured as shown below. For most options, all three subfields are used. Option Type, Option Length and Option Data.
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