Ccna - Chapter 1

Front 1

CollisionDomain

Back 1

Collision Domain is where data packets collide when beingsent over a shared medium. This is a problem with Hubs. On switches each portis its own collision domain.

Front 2

BroadcastDomain

Back 2

Is a logical division of a computer network in which allnodes can reach one another. All devices attached to a switch compose abroadcast domain and are by default part of the same broadcast domain. This means you may have a root switch thathas other switches attached to it with hosts attached to them. However, such asetup will create a lot of congestion due to the ARP requests or broadcast storm.

Front 3

Broadcast storms:

Back 3

A broadcaststorm is the situation in which messages broadcast on a network cause multiplehosts to respond simultaneously by broadcasting their own messages.

Front 4

Segmentation

Back 4

Breakinga network to smaller networks. Eachsegment can then be attached to a switch thus making each segment its owncollision domain.

Front 5

Switch

Back 5

Deals MAC addy. A switch can segment a network. However, it cannot create separate broadcastdomains. It operates at Layer 2. It seeks for MAC addresses for each of thehost attached to the switch in order to establish a communication betweenhosts. The switch holds a table with the MAC address of all attached hosts andit matches it with the IP addresses of each host. Switches, "switch"frames from one port to another with in the switched network it self.

Front 6

Router

Back 6

Deals with packets (IP addy).It creates separate broadcast andcollision domains. They can connect two or more networks. They seek othernetworks and they forward the packets to the appropriate router via the use ofrouting protocols. They operate at Layer 3 by using Logical IP addressing.Routers also use packet filtering. In addition routers do not allow broadcasts to reach other networks. Routerscan use access lists, created by an administrator, to control security based onthe types of packets allowed to enter or exit an interface. Routers can providelayer 2 bridging functions if needed and can simultaneously route through thesame interface. Layer 3 devices— in this case, routers— provide connectionsbetween virtual LANs. In addition, with routing you also have Badnwidth controland QOS.

Front 7

packet filtering

Back 7

Refers to protocols in which messages are divided into packets beforethey are sent. Each packet is then transmitted individually and can even followdifferent routes to its destination. Once all the packets forming a messagearrive at the destination, they are recompiled into the original message.cast

Front 8

routing protocols

Back 8

A routing protocol specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network

Front 9

Routing Table:

Back 9

A routing table is used to match the info in the packet header with thedestination route and forwards it. The routing table consists from entries forconnected networks and remote networks. The connected networks are directlyconnected to the router’s interface. Remote networks are not directly connectedto the router but the routing table of the routers will enable the routers toforward the packet to a specific remote network. The decision on what path the packet willtake depends on routing protocols being used. One route entry can refer to a larger general network or to a specificsubnet on that same network. If the subnet is not part of the routing tablethen the packet will be forwarded to the general network and from there it isassumed that there will be a route that will know where to forward the packetin order to reach the intended subnet. Arouting table contains the following information 9

Front 10

Network Address

Back 10

The routing addresses from different routing protocols (IP,IPV6, etc).

Front 11

Interface

Back 11

The exit interface a packet will take when destined for a specific network.

Front 12

Metric

Back 12

The distance to the remote network. Each routing protocol uses adifferent method to calculate distance from one router to another

Front 13

How do routers learn about routes?

Back 13

Static Routing: Each router is statically configured with the routing information. This is very reliable but it adds a lot of administration cost.

Dynamic Routing: Routers learn automatically about changes on the network through the use of routing protocols (RIP-EIGRP-OSPF). Routers share dynamically information about their routing. '<1

Front 14

What can cause a LAN traffic congestion

Back 14

- Too many hosts in a Collision or BroadcastDomain

- Broadcast storms·

- Too much multicast traffic·

- Low bandwidth

- Existence of Hubs

- Lots of ARP broadcasts which can lead to abroadcast storm.

Front 15

Type of data on each OSI layer

Back 15

- Transport layer --> Segments

- Network layer --> packets

- Data Link layer --> frames

- Physical --> bits

Front 16

PDU Vs SDU

Back 16

Both termsare used to describe the process of encapsulation/decapsulation. PDU isthe encapsulation of data at the current layer in order to be passed at thenext layer. When the PDU is received by the next layer, that same PDU istreated like a SDU (data), which in turn will be turned into a PDU (encapsulation)to be passed along to the next lower layer. The above describes the process ofa top to down direction at the OSI model.

Front 17

OSI - Layers

Back 17

Application

Presentation

Session

Transport

Network

Data Link

Physical

Front 18

OSI - Application

Back 18

File, print, message, database, and application services. TheApplication layer works as the interface between actual application programs.

Front 19

OSI - Presentation

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Data encryption, compression, and translation services. It presents datato the Application layer and is responsible for data translation and codeformattingnguag

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OSI - Session

Back 20

Dialogcontrol. The Session layer is responsible for setting up, managing, anddismantling sessions between Presentation layer entities and keeping user dataseparatedFragme

Front 21

OSI - Transport

Back 21

End-to-endconnection. The Transport layer ensures the reliable arrival of messages andprovides error checking mechanisms and data flow controls. The Transport layeris responsible for providing mechanisms for multiplexing upper-layerapplications, establishing sessions, and tearing down virtual circuits.The Transport layerprovides services for both "connection-mode" transmissions andfor "connectionless-mode" transmissions.

Front 22

Connection - mode - TCP handShake

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A connection oriented service needs to meet thefollowing requirements.

1) Handsake

2) Sequences

3) AKCs

4) Flow control.

Front 23

Segments

Back 23

Contain the encapsulated data from the application layer. The size ofthe segment is being determined by the smallest MTU value encountered acrossthe network. The MTU (A maximum transmission unit) is the physicallimitation of what a router is willing to handle at any point. Each segment has a sequence numberwhich is being used to re-assemble the data when it reaches the destination.For each segment being sent an ACK is being sent back. If the ACK is not sentthen the segment is being resent.b7

Front 24

MTU

Back 24

Is determined by the data link layer and the requirements are passed to the network layer. Each router determines the MTU size. Initially the host will do an MTU discovery. If the size of the packet is larger than the MTU allowed by a router, the router will request for a smaller size. This request is being sent over to the host and the host adjust the size accordingly. If there is a router that demands a smaller MTU, the router then will fragment the packets before sending them over to the next router. The fragments are being assembled at the host. The transport layer is not aware of any of this.

Front 25

Flow control

Back 25

o Flow control prevents a sending host on one sideof the connection from overflowing the buffers in the receiving host. The receiving end can seta "Not Ready" signal to the sender is order to pause the transmissionof segments if it needs to. Flow control specifies theamount of segments can be sent before an ACK is expected by the sender. Thereceiver is always the one who specifies the window size. The sender cannotsent any more segments before the ACK has been received (by the sender). If theACK is not received then the sender will put the data it wants to send into abuffer. Reliable data transportemploys a connection-oriented communications session between systems, and theprotocols involved ensure that the following will be achieved:§ The segments delivered are acknowledged back tothe sender upon their reception. Any segments not acknowledged areretransmitted. § Segments are sequenced back into their properorder upon arrival at their destination.§ Amanageable data flow is maintained in order to avoid congestion, overloading,or worse, data loss.

Front 26

Windowing

Back 26

Thequantity of data segments, measured in bytes, that the transmitting machine isallowed to send without receiving an acknowledgment is called a window. Windowsare used to control the amount of outstanding, un-acked data segments. The sizeof the window controls how much information is transferred from one end to theother before an acknowledgement is required. While some protocols quantifyinformation depending on the number of packets, TCP/ IP measures it by countingthe number of bytesthat ڋ;9

Front 27

Acknowledgments

Back 27

Thesender documents each segment measured in bytes, then sends and waits for thisacknowledgment before sending the next segment. Also important is that when itsends a segment, the transmitting machine starts a timer and will retransmit ifit expires before it gets an acknowledgment back from the receiving end.uired.Ë

Front 28

SessionMultiplexing

Back 28

ithappens when a client connects to a server with multiple browser sessions open.The client data from each browser session must be separate when the serverapplication receives it.

Front 29

Connectionless - mode - UDP

Back 29

Under UDP mode datagrams are being used. In this mode there is no error checking. Thedata is being requested and the remote host sends the data with out concerningit self if the client machine received the data in a proper format/sequence nordoes it care if it received the data at all for that matter. Datagrams arereceived by the client and are assembled in the order they were received andnot by the use of sequence numbers.protᜋ7

Front 30

Datagrams

Back 30

UDP uses datagrams which are smaller than Segments because they do nothave sequence numbers. As mentioned above, error checking is not performed andit is of no concern.

Front 31

OSI - Network

Back 31

Routing Deals with packets. Itforwards/moves packets through paths created by logical paths within thenetwork. Routers operate at this layer. Routers may know about directlyconnected networks or remote networks. There are a variety of routing protocolsthat allow routers to be aware of the logical topology of the network. All thisis done by using IPs/Network Masks. There are two types of packets going through routers. peŋ*9

Front 32

Data Packets

Back 32

contains the actual data. These exist in the OSI Network level

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Route Update Packets

Back 33

contain information/updates in order to updateneighborhood routers for the state of other routers/networks. These exist in the OSI Network level

Front 34

Network Addresses

Back 34

A router must maintain a routing table for individual routing protocolsbecause each routed protocol keeps track of a network with a differentaddressing scheme. These exist in the OSI Network level

Front 35

Metric

Back 35

The distance to the remote network. Different routing protocols usedifferent ways of computing this distance. Some protocols use hop count, orbandwidth, delay of the line, tick count, in order to determine the best path. Part of the OSI Network level

Front 36

OSI - Data Link

Back 36

Framing Deals with frames.This is where switches operate. IP is not used here but the physical addressMAC of each network card. The ARP tableholds information about which IP address on the local network (the hostsattached to the switch) are paired with which MAC address. The data link alsohandles network topology, and flowcontrol

Front 37

Ethernet Frames

Back 37

Is part of Data Link. It consists from:

o Preamble: Used for clock sync

o Start Frame Delimeter: Indicates the preamble's end and that the MACaddy will follow.

o FCS: It is anerror detection operation. Sender generates a value by running CRC against thedata contained in the frame. That value is placed in the FCS and the fame istransmitted. The receiver will run the same CRC against the frame's data andcompare it to the answer provided by the incoming CRC. If there is a miss matchthe frame is considered corrupt but there is no special notification sent backto the sender. It is an error detectionand not correction function.