10 Tips For Abreast of the times 300-101 examcollection

Answer: A 

Explanation: 

We came across a recent issue where a user setup a router for NetFlow export but was unable to see the

OUT traffic for the interfaces in NetFlow Analyzer. Every NetFlow configuration aspect was checked and

nothing incorrect was found. That is when we noticed the `no ip cef' command on the router. CEF was

enabled at the global level and within seconds, NetFlow Analyzer started showing OUT traffic for the

interfaces. This is why this topic is about Cisco Express Forwarding.

What is switching?

A Router must make decisions about where to forward the packets passing through. This decision-making

process is called "switching". Switching is what a router does when it makes the following decisions:

1.Whether to forward or not forward the packets after checking that the destination for the packet is

reachable.

2.If the destination is reachable, what is the next hop of the router and which interface will the router use to

get to that destination.

What is CEF?

CEF is one of the available switching options for Cisco routers. Based on the routing table, CEF creates its

own table, called the Forwarding Information Base (FIB). The FIB is organized differently than the routing

table and CEF uses the FIB to decide which interface to send traffic from. CEF offers the following

benefits:

1.Better performance than fast-switching (the default) and takes less CPU to perform the same task.

2.When enabled, allows for advanced features like NBAR

3.Overall, CEF can switch traffic faster than route-caching using fast-switching

How to enable CEF?

CEF is disabled by default on all routers except the 7xxx series routers. Enabling and Disabling CEF is

easy. To enable CEF, go into global configuration mode and

enter the CEF command.

Router# config t

Router(config)# ip cef

Router(config)#

To disable CEF, simply use the `no' form of the command, ie. `no ip cef`.

Why CEF Needed when enabling NetFlow ?

CEF is a prerequisite to enable NetFlow on the router interfaces. CEF decides through which interface

traffic is exiting the router. Any NetFlow analyzer product will calculate the OUT traffic for an interface

based on the Destination Interface value present in the NetFlow packets exported from the router. If the

CEF is disabled on the router, the NetFlow packets exported from the router will have "Destination

interface" as "null" and this leads NetFlow Analyzer to show no OUT traffic for the interfaces. Without

enabling the CEF on the router, the NetFlow packets did not mark the destination interfaces and so

NetFlow Analyzer was not able to show the OUT traffic for the interfaces. Reference: https://

blogs.manageengine.com/network-2/netflowanalyzer/2010/05/19/need-for-cef- in-netflow-data-export.html

Answer: A,C,D 

Explanation: 

Answer: B,C,D 

Explanation: 

TCP Selective Acknowledgment

The TCP Selective Acknowledgment feature improves performance if multiple packets are lost from one

TCP window of data.

Prior to this feature, because of limited information available from cumulative acknowledgments, a TCP

sender could learn about only one lost packet per-round-trip

time. An aggressive sender could choose to resend packets early, but such re-sent segments might have

already been successfully received.

The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host

returns selective acknowledgment packets to the sender,

informing the sender of data that has been received. In other words, the receiver can acknowledge packets

received out of order. The sender can then resend only

missing data segments (instead of everything since the first missing packet).

Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would

receive acknowledgment of only packets 1, 2, and 3. Packets

4 through 8 would need to be re-sent. With selective acknowledgment, TCP receives acknowledgment of

packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 must be

re-sent.

TCP selective acknowledgment is used only when multiple packets are dropped within one TCP window.

There is no performance impact when the feature is

enabled but not used. Use the ip tcp selective-ack command in global configuration mode to enable TCP

selective acknowledgment.

Refer to RFC 2021 for more details about TCP selective acknowledgment.

TCP Time Stamp

The TCP time-stamp option provides improved TCP round-trip time measurements. Because the time

stamps are always sent and echoed in both directions and the time-stamp value in the header is always

changing, TCP header compression will not compress the outgoing packet. To allow TCP header

compression over a serial link, the TCP time-stamp option is disabled. Use the ip tcp timestamp command

to enable the TCP time-stamp option.

TCP Explicit Congestion Notification

The TCP Explicit Congestion Notification (ECN) feature allows an intermediate router to notify end hosts of

impending network congestion. It also provides enhanced support for TCP sessions associated with

applications, such as Telnet, web browsing, and transfer of audio and video data that are sensitive to delay

or packet loss. The benefit of this feature is the reduction of delay and packet loss in data transmissions.

Use the ip tcp ecn command in global configuration mode to enable TCP ECN.

TCP Keepalive Timer

The TCP Keepalive Timer feature provides a mechanism to identify dead connections. When a TCP

connection on a routing device is idle for too long, the device sends a TCP keepalive packet to the peer

with only the Acknowledgment (ACK) flag turned on. If a response packet (a TCP ACK packet) is not

received after the device sends a specific number of probes, the connection is considered dead and the

device initiating the probes frees resources used by the TCP connection. Reference: http://www.cisco.com/

c/en/us/td/docs/ios-xml/ios/ipapp/configuration/xe-3s/asr1000/iap-xe-3s-asr1000-book/iap-tcp.html#GUID-22A82C5F-631F-4390-9838-F2E48FFEEA01

Answer: E 

Explanation: 

Answer: A 

Explanation: 

Enabling Flow-Label Marking in Packets that Originate from the Device This feature allows the device to

track destinations to which the device has sent packets that

are 1280 bytes or larger.

SUMMARY STEPS

1.enable

2.configure terminal

3.ipv6 flowset

4.exit

5.clear ipv6 mtu

DETAILED STEPS

Command or Action Purpose

Step 1 enable Enables privileged EXEC mode.

Enter your password if prompted.

Example:

Device> enable

Step 2 configure terminal Enters global configuration mode.

Example:

Device# configure

terminal

Step 3 ipv6 flowset Configures flow-label marking in 1280-byte or larger packets sent by the device.

Example:

Device# configure

terminal

Step 3 ipv6 flowset Configures flow-label marking in 1280-byte or larger packets sent by the device.

Example:

Device(config)# ipv6

flowset

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6_basic/configuration/15- mt/ip6b-15-mtbook/ip6-mtu-path-disc.html

Answer: A,C 

Explanation: 

Answer: A 

Explanation: 

TCP global synchronization in computer networks can happen to TCP/IP flows during periods of

congestion because each sender will reduce their transmission rate at the same time when packet loss

occurs. Routers on the Internet normally have packet queues, to allow them to hold packets when the

network is busy, rather than discarding them. Because routers have limited resources, the size of these

queues is also limited. The simplest technique to limit queue size is known as tail drop. The queue is

allowed to fill to its maximum size, and then any new packets are simply discarded, until there is space in

the queue again. This causes problems when used on TCP/IP routers handling multiple TCP streams,

especially when bursty traffic is present. While the network is stable, the queue is constantly full, and there

are no problems except that the full queue results in high latency. However, the introduction of a sudden

burst of traffic may cause large numbers of established, steady streams to lose packets simultaneously.

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

Answer: C 

Explanation: 

RFC 6296 describes a stateless IPv6-to-IPv6 Network Prefix Translation (NPTv6) function,

designed to provide address independence to the edge network. It is transport-agnostic with respect to

transports that do not checksum the IP header, such as SCTP, and to transports that use the TCP/UDP/

DCCP (Datagram Congestion Control Protocol) pseudo-header and checksum NPTv6 provides a simple

and compelling solution to meet the address-independence requirement in IPv6. The addressindependence

benefit stems directly from the translation function of the network prefix translator. To avoid

as many of the issues associated with NAPT44 as possible, NPTv6 is defined to include a two-way,

checksum-neutral, algorithmic translation function, and nothing else. Reference: http://tools.ietf.org/html/

rfc6296

Answer: D 

Explanation: 

PPPoE is composed of two main phases:

Active Discovery Phase--In this phase, the PPPoE client locates a PPPoE server, called an access concentrator. During this phase, a Session ID is assigned and the PPPoE layer is established.

PPP Session Phase--In this phase, PPP options are negotiated and authentication is performed. Once the

link setup is completed, PPPoE functions as a Layer 2 encapsulation method, allowing data to be transferred over the PPP link within PPPoE headers.

Reference: 

http://www.cisco.com/c/en/us/td/docs/security/asa/asa92/configuration/vpn/asa-vpn- cli/vpnpppoe.html

Answer: A 

Explanation: