10 cau hoi can biet

8/6/2019 10 Cau Hoi Can Biet

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Cu hi 1: Proxy ARP v Default Gateway

Trong chng trnh CCNA Academy, chng ta vn thng nhm ln v Proxy ARPv Default Gateway. Hm nay trong khun kh bi vit ny, ti xin c trnh by vcch thc hot ng ca Proxy ARP v Default Gateway.

1. Proxy ARP

_ Trng hp 1: Gi s trong mng LAN ca chng ta c mt host A vi a ch192.168.1.2/24, host ny mun lin h ti mt host B c a ch l 192.168.1.3/24,trong khi bng ARP ca host A cha c a ch MAC ca host B. Host A u tin sso snh IP ch vi network ca mnh, nu trng th n s broadcast ARP tmMAC ca host B trong LAN. Nu host B c trong LAN ca host A, n s tr li chohost A a ch MAC ca mnh v host A s cp nht vo bng ARP.

_ Trng hp 2: Gi s trong mng LAN ca chng ta c mt host A vi a ch192.168.1.2/24, host ny mun lin h ti mt host B c a ch l 192.168.2.3/24,trong khi bng ARP ca host A cha c a ch MAC ca host B. Host A u tin sso snh IP ch vi network ca mnh, tuy nhin host B li thuc v mt networkkhc, theo qui nh ca TCP/IP, host A phi b gi tin nu khng c default gateway,v trong trng hp ny d nhin chng ta khng thit lp default gateway cho host A.C 2 hnh x c th xy ra:a. Host A b gi tin.b. Nu host A chy h iu hnh Windows XP, n s khng tun th theo quy nhca TCP/IP, n vn s broadcast ARP tm a ch MAC ca host B. Lc ny, gi strong mng LAN ca chng ta c mt Router X bt chc nng Proxy ARP v routerny bit ng i n network ca host B, Router X s tr li v cho host A vi thngtin: i n host B, host A cn i n a ch MAC l MAC ca interface giao tip

LAN ca Router X v a ch IP la IP ca host B. Nh vy, host A b "la", host Acp nht a ch MAC host B vo bng ARP trong khi a ch MAC ny li l a chMAC ca Router X.

2.Default Gateway

_ Trng hp 1: Ging trng hp 1 ca Proxy ARP_ Trng hp 2: Gi s trong mng LAN ca chng ta c mt host A vi a ch192.168.1.2/24, host ny mun lin h ti mt host B c a ch l 192.168.2.3/24,trong khi bng ARP ca host A cha c a ch MAC ca host B. Host A u tin sso snh IP ch vi network ca mnh, tuy nhin host B li nm mt network khc,lc ny host A s chuyn gi tin ca mnh n thng Default Gateway vi MAC ch

l Default Gateway v IP ch l Host B.

Nh vy, i vi trng hp ARP Proxy, host A b "nh la" v a ch MAC cahost B. Cn i vi Default Gateway, host A nhn bit host B nm network khcnn nhn thy r con ng i n host B l i qua Default Gateway.

S c cu hi t ra, khi no nn s dng Default Gateway v khi no nn s dngProxy ARP. C rt nhiu trng hp s dng 2 phng thc ny, ti xin c numt v d: Gi s trong LAN ca chng ta c 3 router kt ni thnh 1 LAN trung tm,mi router c thm mt LAN na sau lng. Nu chng ta s dng Default Gatewaycho cc host LAN trung tm l mt trong s 3 router trn th s c bt li xy ra,

chng ta khng bit chc chn i n mng ch l cc LAN pha sau cc Router


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th Default Gateway no l tt nht. Trong trng hp ny th s dng PRoxy ARP sc li hn, v Router no gn mng ch nht s tr li nhanh hn cc Router khc.

Cu hi 2:Mng Frame Relay qun l lu lng mng th no?

phc v qun l lu lng (flow control) trn mng, Frame Relay (FR) a ra mts khi nim sau:

- CIR (Committed Information Rate): lu lng m mng FR m bo s truyn y cho khch hng. n v tnh l byte/giy.- DE (Discard Eligible): frame l ng c vin b b, khng truyn. L tn hiu cho

bit gi tin mang n s b b nu cc tng i FR b nghn mch v c truyn nungc li.- EIR (Excess Information Rate): lu lng m mng FR c gng s truyn cho khchhng, nhng khng m bo. n v tnh l byte/giy.- Bc (Commited Burst): lu lng c mmng FR m bo s truyn. n v l

byte.- Be (Excess Burst): lu lng c mng FR c gng truyn, nhng khng m bochc chn. n v l byte.- Tc (Commited Time): khang thi gian m FR Switch s s dng xem xt tnhtrng s dng mng ca khch hng, hay c th ni l tnh tan cng n vi khchhng. Ta c mi lin h Bc = Tc * CIR; Be = EIR * Tc.

Cch thc hat ng:- FR switch co mt b m (counter) t cng giao tip vi thit b ca khch hng.- Counter ny bt u l 0.

- Counter ny m tng s byte m FR switch nhn c t thit b ca khch hng.- Mi khi ht mt Tc th counter = counter Bc hoc = 0 nu s ny m.

Mi khi c mt frame t thit b khch n th:- counter = counter + frame size- nu counter > Bc + Be th frame b drop- cn khng nu counter > Bc th frame b nh du DE- cn li l bnh thng.

Cu hi 3:Access network

There are various methods of managing access to a network. If all network stationstried to talk at once, the messages would become unintelligible, and nocommunication could occur. Therefore a method of being sure that stations coordinatethe sending of messages must be achieved. There are several methods listed belowwhich have various advantages and disadvantages.

Contention

- Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) - Used by

Ethernet

- Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)


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Token Passing

- A token is passed from one computer to another, which provides transmission

permission.

Demand Priority

- Describes a method where intelligent hubs control data transmission. A

computer will send a demand signal to the hub indicating that it wants to

transmit. The hub will respond with an acknowledgement that will allow the

computer to transmit. The hub will allow computers to transmit in turn. An

example of a demand priority network is 100VG-AnyLAN (IEEE 802.12). It uses

a star-bus topology.

Polling

- A central controller, also called the primary device will poll computers, calledsecondary devices, to find out if they have data to transmit. Of so the central

controller will allow them to transmit for a limited time, then the next device is

polled.

- Token passing performs better when the network has a lot of traffic, while

Ethernet which uses CSMA/CD is generally faster but loses performance when

the network has a lot of traffic. CSMA/CD is basically a method that allows

network stations to transmit any time they want. They, however, sense the

network line and detect if another station has transmitted at the same time they

did. This is called a collision. If a collision happened, the stations involved will

retransmit at a later, randomly set time in hopes of avoiding another collision.

Cu hi 4:What are differences between routed and routingprotocols?

Routing Protocols

A routing protocol is the communication used between routers. A routing protocolallows routers to share information about networks and their proximity to each other.Routers use this information to build and maintain routing tables.

Examples of routing Protocols are as follows:* Routing Information Protocol (RIP)* Interior Gateway Routing Protocol (IGRP)* Enhanced Interior Gateway Routing Protocol (EIGRP)* Open Shortest Path First (OSPF)

Routed Protocols

A routed protocol is used to direct user traffic. A routed protocol provides enough

information in its network layer address to allow a packet to be forwarded from onehost to another based on the addressing scheme.


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Examples of routed Protocols are as follows:* Internet Protocol (IP)* Internetwork Packet Exchange (IPX

Cu hi 5:What are differences between Network Broadcastingand Multicasting ?

Network Repeater

A repeater connects two segments of your network cable. It retimes and regeneratesthe signals to proper amplitudes and sends them to the other segments. When talkingabout, ethernet topology, you are probably talking about using a hub as a repeater.Repeaters require a small amount of time to regenerate the signal. This can cause a

propagation delay which can affect network communication when there are several

repeaters in a row. Many network architectures limit the number of repeaters that canbe used in a row. Repeaters work only at the physical layer of the OSI network model.

Bridge

A bridge reads the outermost section of data on the data packet, to tell where themessage is going. It reduces the traffic on other network segments, since it does notsend all packets. Bridges can be programmed to reject packets from particularnetworks. Bridging occurs at the data link layer of the OSI model, which means the

bridge cannot read IP addresses, but only the outermost hardware address of thepacket. In our case the bridge can read the ethernet data which gives the hardware

address of the destination address, not the IP address. Bridges forward all broadcastmessages. Only a special bridge called a translation bridge will allow two networks ofdifferent architectures to be connected. Bridges do not normally allow connection ofnetworks with different architectures. The hardware address is also called the MAC(media access control) address. To determine the network segment a MAC address

belongs to, bridges use one of:

* Transparent Bridging - They build a table of addresses (bridging table) as theyreceive packets. If the address is not in the bridging table, the packet is forwarded toall segments other than the one it came from. This type of bridge is used on ethernetnetworks.

Source route bridging - The source computer provides path information inside thepacket. This is used on Token Ring networks.

Network Router

A router is used to route data packets between two networks. It reads the informationin each packet to tell where it is going. If it is destined for an immediate network ithas access to, it will strip the outer packet, readdress the packet to the proper ethernetaddress, and transmit it on that network. If it is destined for another network and must

be sent to another router, it will re-package the outer packet to be received by the nextrouter and send it to the next router. The section on routing explains the theory behind


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this and how routing tables are used to help determine packet destinations. Routingoccurs at the network layer of the OSI model. They can connect networks withdifferent architectures such as Token Ring and Ethernet. Although they can transforminformation at the data link level, routers cannot transform information from one dataformat such as TCP/IP to another such as IPX/SPX. Routers do not send broadcast

packets or corrupted packets. If the routing table does not indicate the proper addressof a packet, the packet is discarded.

Cu hi 6:What are advantages and disadvantages of link-staterouting?

Advantages of link-state routing

Link-state protocols use cost metrics to choose paths through the network. The costmetric reflects the capacity of the links on those paths.

Link-state protocols use triggered updates and LSA floods to immediately reportchanges in the network topology to all routers in the network. This leads to fastconvergence times. Each router has a complete and synchronized picture of the network. Therefore, it isvery difficult for routing loops to occur. Routers use the latest information to make the best routing decisions. The link-state database sizes can be minimized with careful network design. Thisleads to smaller Dijkstra calculations and faster convergence. Every router, at the very least, maps the topology of its own area of the network.This attribute helps to troubleshoot problems that can occur. Link-state protocols support CIDR and VLSM.

Disadvantages of link-state routing

They require more memory and processor power than distance vector protocols. Thismakes it expensive to use for organizations with small budgets and legacy hardware. They require strict hierarchical network design, so that a network can be broken intosmaller areas to reduce the size of the topology tables. They require an administrator who understands the protocols well. They flood the network with LSAs during the initial discovery process. This processcan significantly decrease the capability of the network to transport data. It cannoticeably degrade the network performance

Cu hi 7:What are intranets and extranets

One common configuration of a LAN is an intranet. Intranet Web servers differ frompublic Web servers in that the public must have the proper permissions and passwordsto access the intranet of an organization. Intranets are designed to permit users whohave access privileges to the internal LAN of the organization. Within an intranet,Web servers are installed in the network. Browser technology is used as the commonfront end to access information on servers such as financial, graphical, or text-baseddata.


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Extranets refer to applications and services that are Intranet based, and use extended,secure access to external users or enterprises. This access is usually accomplishedthrough passwords, user IDs, and other application-level security. An extranet is theextension of two or more intranet strategies with a secure interaction between

participant enterprises and their respective intranets

Cu hi 8:What are network protocols?

Protocol suites are collections of protocols that enable network communicationbetween hosts. A protocol is a formal description of a set of rules and conventions thatgovern a particular aspect of how devices on a network communicate. Protocolsdetermine the format, timing, sequencing, and error control in data communication.Without protocols, the computer cannot make or rebuild the stream of incoming bitsfrom another computer into the original format.

Protocols control all aspects of data communication, which include the following:

How the physical network is built How computers connect to the network How the data is formatted for transmission How that data is sent How to deal with errors

These network rules are created and maintained by many different organizations andcommittees. Included in these groups are the Institute of Electrical and ElectronicEngineers (IEEE), American National Standards Institute (ANSI),

Telecommunications Industry Association (TIA), Electronic Industries Alliance (EIA)and the International Telecommunications Union (ITU), formerly known as theComit Consultatif International Tlphonique et Tlgraphique (CCITT).

Cu hi 9:What are the requirements for an internet connection?

The Internet is the largest data network on earth. The Internet consists of many largeand small networks that are interconnected. Individual computers are the sources anddestinations of information through the Internet. Connection to the Internet can be

broken down into the physical connection, the logical connection, and applications.

A physical connection is made by connecting an adapter card, such as a modem or aNIC, from a PC to a network. The physical connection is used to transfer signalsbetween PCs within the local-area network (LAN) and to remote devices on theInternet.

The logical connection uses standards called protocols. A protocol is a formaldescription of a set of rules and conventions that govern how devices on a networkcommunicate. Connections to the Internet may use multiple protocols. TheTransmission Control Protocol/Internet Protocol (TCP/IP) suite is the primary set of

protocols used on the Internet. The TCP/IP suite works together to transmit and

receive data, or information.


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The last part of the connection are the applications, or software programs, thatinterpret and display data in an understandable form. Applications work with

protocols to send and receive data across the Internet. A Web browser displays HTMLas a Web page. Examples of Web browsers include Internet Explorer and Netscape.File Transfer Protocol (FTP) is used to download files and programs from the

Internet. Web browsers also use proprietary plug-in applications to display specialdata types such as movies or flash animations.

This is an introductory view of the Internet, and it may seem to be a simplisticprocess. As the topic is explored in greater depth, students will learn that datatransmission across the Internet is a complicated task.

Cu hi 10:How does ARP and RARP work ?

ARP - Address Resolution Protocol

When user type data in any applications, Click Send Button and the followinghappened:

1. The application will pass Users data in a buffer, to a socket.2. The data was put inside a TCP data packet with a TCP header added to the data.This header contained a source and destination port number along with some otherinformation and a checksum.3. The TCP packet was be placed inside an IP data packet with a source anddestination IP address along with some other data for network management.4. The IP data packet was placed inside an ethernet data packet. This data packet

includes the destination and source address of the network interface cards (NIC) onthe two computers. The address here is the hardware address of the respective cardsand is called the MAC address.5. The ethernet packet was transmitted over the network line.6. With a direct connection between the two computers, the network interface card onthe intended machine, recognized its address and grabbed the data.7. The IP data packet was extracted from the ethernet data packet.8. The TCP data packet was extracted from the IP data packet.9. The data was extracted from the TCP packet and the program displayed theretrieved data (text) in the text display window for the intended recipient to read.

In step 4 above, the IP data was going to be placed inside an ethernet data packet, butthe computer constructing the packet does not have the ethernet address of therecipient's computer. The computer that is sending the data, in order to create theethernet part of the packet, must get the ethernet hardware (MAC) address of thecomputer with the intended IP address. This must be accomplished before the ethernet

packet can be constructed. The ethernet device driver software on the receivingcomputer is not programmed to look at IP addresses encased in the ethernet packet. Ifit did, the protocols could not be independent and changes to one would affect theother. This is where address resolution protocol (ARP) is used. Senders computersends a network broadcast asking the computer that has the recipient's IP address tosend it's ethernet address. This is done by broadcasting. The ethernet destination is set

with all bits on so all ethernet cards on the network will receive the data packet. TheARP message consists of an ethernet header and ARP packet.


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The ethernet header contains: A 6 byte ethernet destination address. A 6 byte ethernet source address. A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 forRARP

The encapsulated ARP data packet contains the following: Type of hardware address (2 bytes). 1=ethernet. Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IPaddress. Byte size of the hardware address (1 byte). 6 Byte size of the protocol address (1 byte). 4 Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARPreply. The sender's ethernet address (6 bytes) The sender's IP address (4 bytes)

The recipient's ethernet address (6 bytes) The recipient's IP address (4 bytes)

When the ARP reply is sent, the recipient's ethernet address is left blank.

In order to increase the efficiency of the network and not tie up bandwidth doing ARPbroadcasting, each computer keeps a table of IP addresses and matching ethernetaddresses in memory. This is called ARP cache. Before sending a broadcast, thesending computer will check to see if the information is in it's ARP cache. If it is itwill complete the ethernet data packet without an ARP broadcast. Each entrynormally lasts 20 minutes after it is created. RFC 1122 specifies that it should be

possible to configure the ARP cache timeout value on the host. To examine the cacheon a Windows, UNIX, or Linux computer type 'arp -a'.

If the receiving host is on another network, the sending computer will go through itsroute table and determine the correct router (A router should be between two or morenetworks) to send to, and it will substitute the ethernet address of the router in theethernet message. The encased IP address will still have the intended IP address.When the router gets the message, it looks at the IP data to tell where to send the datanext. If the recipient is on a network the router is connected to, it will do the ARPresolution either using it's ARP buffer cache or broadcasting.

Reverse Address Resolution Protocol (RARP)

As mentioned earlier, reverse address resolution protocol (RARP) is used for disklesscomputers to determine their IP address using the network. The RARP messageformat is very similar to the ARP format. When the booting computer sends the

broadcast ARP request, it places its own hardware address in both the sending andreceiving fields in the encapsulated ARP data packet. The RARP server will fill in thecorrect sending and receiving IP addresses in its response to the message. This waythe booting computer will know its IP address when it gets the message from theRARP server

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