Cisco® CCENT Exam Cram Notes : Datalink Layer, Traffic Priority, Dod And Isi Models, Frame Tagging, And Nat.

17)Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:

1. The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.

2. The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer handles access to shared media, such a Token passing or Ethernet.

18)Debugging output takes priority over other network traffic. Also, the debug all command generates more output than any other debug command, and it can severely affect the router's performance. In practically all cases, it is best to use more specific debug commands.

Default route is used when there is no other known route to a given IP packet's destination. Default route is also known as gateway of last resort.

Distance vector protocol depends only on Hop count to determine the nearest next hop for forwarding a packet. One obvious disadvantage is that, if you have a destination connected through two hops via T1 lines, and if the same destination is also connected through a single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!

19)DOD Model maps to OSI model as below:

Process/ Application maps to OSI's Application, Presentation, Session layers (layers 7,6,5).
Host-to-Host maps to ISO's Transport layer (layer 4).
Internet maps to ISO's Network layer (layer 3).
Network Access maps to ISO's Data Link and Physical Layers (layers 2,1).

During password recovery, the config register and NVRAM are modified. ROM hold the bootstrap code to start up the router and Flash contain the IO image.

Each exclamation mark (!) represents that one UDP segment has been successfully transferred. While erasing, you get 'e' displayed on screen.

EIGRP (as well as IGRP) uses Bandwidth and Delay as default criteria to determine the best path. The description of the terms is given below:

  • Bandwidth: This is the smallest bandwidth between the source and destination.
  • Delay: This is the cumulative interface delay along the path.
  • Reliability: This is the worst case reliability between source and destination based on keep alives.
  • Loading: This is the worst case load on a link between source and destination based on bps.
  • MTU: Smallest MTU in path.

EIGRP uses multicasts to send queries to neighbor routers.

20)ENABLE / DISABLE - Enables or Disables privileged exec mode.

There are five different types of passwords:

  • ENABLE PASSWORD - A global command that restricts access to privileged exec mode. This is a non-encrypted password.
  • ENABLE SECRET - Assigns a one-way encryptographic secret password, available in versions 10.3 and up. This secret password is used instead of the enable password when it exists.
  • Virtual Terminal Password (vty password): The virtual terminal password is used for Telnet sessions into the router. The password can be changed at any time. It can be set up when you configure the router from the console. There can be five distinct passwords corresponding to each vty (vty0 to vty4) or there can be a single password for all vtys.
  • Auxiliary Password: Auxiliary password is used to set password to the auxiliary port. This port is used to access a router through a modem.
  • Console Password: Console password is used to set the console port password.

21)Extended Service Set (ESS) is a set of two or more Basic Service Sets (BSSes) working together to form a single wireless network. Stations are able to move between BSS within a single ESS yet remain "connected" to the fixed network and so continue to receive emails etc. As a Station moves into a new BSS, it will carry out a re-association procedure with the new AP (Access Point).

For accessing a switch over IP network, you need to do some minimum configuration tasks using Console port on the switch. In the given question, the default gateway needs to be configured on the switch for accessing the management interface (VLAN1) from HostA.

For communicating between two hosts directly, you need a cross cable. Also, the subnet mask cannot accommodate the given IP addresses. Therefore, you need to change either the host IP addresses so that they are within a given subnet, or change the subnet address itself so that both the hosts are within the subnet.

For connecting a switch to another switch, a crossover cable is required. For connecting a host to a switch, a straight-through cable is used. For connecting a Cisco Router/Cisco Switch console port, a rollover cable is required.

For IGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information.

For using full duplex Ethernet transmission, a switch is required. A Hub cannot support full duplex transmission. In full duplex mode, there will not be any frame collisions.

22)Frame tagging is a technique, where a unique identifier is tagged to each frame as it is forwarded through the switch fabric.

FTP (File Transfer Protocol) is used for transferring files from and to a client computer to a host.

FTP is a connection oriented protocol. It uses TCP/IP for file transfer. Compare this with UDP protocol used by TFTP.

Given below are the four important forms of NAT (Network Address Translation):

  • Static NAT: It is a one-to-one mapping between an unregistered IP address and a registered IP address.
  • Dynamic NAT: Usually, Dynamic NAT is implemented, where a pool of public IP addresses is shared by an entire private IP subnet. When a private host initiates a connection, a public IP address is selected. The mapping of the computer's non-routable IP address matched to the selected IP address is stored in the NAT Table. As long as the outgoing connection is maintained, the private host can be reached by incoming packets sent to the specified public address. When the binding expires, the address is returned to the pool for reuse.
  • Overloading: A variation of Dynamic NAT, also known as Network Address Port Translation (NAPT) maps multiple unregistered IP addresses to a single registered IP address by multiplexing streams differentiated by the TCP/UDP port number.
  • Overlapping: When Overlapping is employed, the IP addresses used on the internal network are registered IP addresses utilized on another network. To avoid conflict, a NAT Table is built to translate these redundant internal addresses to a unique IP address. Vice versa, when sending packets into the private network, the registered addresses must be translated to an address unique in the network.

Here, you are able to access the computers on the local subnet. However, you are not able to access the computers on remote subnets. Since you are not able to access any of the computers on the remote subnet, you need to ping the default gateway first. The default gateway might have been configured improperly on your computer.

23)I. The number of Hosts required are five. We need to add Hosts of all ones and all zeros to this. This is because all zeros and all ones subnets belong to "this Host" and "all Hosts" broadcasts and can not be used. Therefore, the total number of Hosts addresses to be reserved is 5+2 = 7.

II. We want to implement maximum possible subnets. Therefore, we need to minimize the number of Hosts. This minimum number is 7 here. If we reserve 2 bits, it results in only 2^2=4 Hosts which is less than 7. Therefore, we have to reserve 3 bits for implementing Hosts, resulting in 2^3=8 Host addresses. This is now optimized for maximum number of Subnets (as we have optimized for minimum number of Hosts). This also leaves us with 5 bits in the fourth Octet for implementing subnets.

III. Write the 5 bits available for subnetting in fourth octet in the form 11111000 (Five 1s being subnet bits). The decimal equivalent is 2^7+2^6+2^5+2^4+2^3

= 128 + 64 +32 + 16 + 8 = 248.

IV. Now the subnet mask required is

I. The number of subnets required are four. We need to add subnets of all ones and all zeros to this. This is because all zeros and all ones subnets belong to "this subnet" and "all subnets" broadcasts and can not be used. Therefore, the total number of subnets to be reserved is 4+2 = 6.

II. We want to implement maximum possible Hosts. Therefore, we need to minimize the number of subnets. This minimum number is 6 here. If we reserve 2 bits, it results in only 2^2=4 subnets which is less than 6. Therefore, we have to reserve 3 bits for implementing subnets, resulting in 2^3=8 subnets. This is now optimized for maximum number of Hosts (as we have optimized for minimum number of subnets).

III. Write the 3 bits available for subnetting in fourth octet in the form 11100000 (Five 0s being Host bits). The decimal equivalent is 2^7+2^6+2^5 = 128 + 64 +32 = 224.

II. Now the subnet mask required is

24)IANA (Internet Assigned Numbers Authority) is the organization under the Internet Architecture Board (IAB) of the Internet Society that oversees the allocation of Internet Protocol addresses to Internet service providers (ISPs). ICANN (a non governmental organization) has now assumed responsibility for the tasks formerly performed by IANA. ISPs in turn allot IP addresses to small companies, and businesses.

ICMP (Internet Message Control Protocol) messages are used for basic error reporting between host to host, or host to gateway. It is not used for error reporting between Gateways. ICMP messages are encapsulated using the IP protocol. For example, the command "ping" uses ICMP protocol. In the OSI Reference model, ICMPs are generally considered part of the IP layer.

ICMP (Internet Control Message Protocol) sends an ICMP message if the destination is unreachable. Routers can make alternate route decisions based on ICMP messages, if appropriate.

If no matching MAC entry is found in the MAC table of a switch, the frame will be broadcast to all other ports except the port through which it had arrived.

IGRP update packet is sent every 90 seconds by default. This is 30 Sec for RIP.

IGRP, and OSPF use Autonomous System (AS) numbers. An IGRP AS is a complex network that has diverse bandwidth and delay characteristics.

Important Network Address Translation (NAT) terms of inside local, inside global, outside local, and outside global are explained below:

  • Inside local: A private IP address assigned to a host on the inside network.
  • Inside global: A public IP address that represents one or more inside local IP addresses to the outside world.
  • Outside local: The IP address of an outside host as it appears to the inside network. Not necessarily a legitimate address, it is allocated from an address space routable on the inside.
  • Outside global: The IP address assigned to a host on the outside network. The address is allocated from a globally routable address or network space.

25)In "ad-hoc" or Independent Basic Service Set (IBSS) configuration there is no backbone infrastructure. An IBSS is one BSS or Basic Service Set. Mobiles can talk to each other without the use of an Access Point (AP). In the Extended Service Set (ESS) configuration, there will be two or more Access Points (APs), and users can freely roam between the Access Points without any disconnection or reconfiguration. BSS will use only one Access Point.

In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), a node transmits only after listening to the media, and ensuring that no transmission is taking place. Further, sending nodes detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in efficient use of the media since the bandwidth of transmitting the entire frame is not wasted.

In Cisco routers, the interfaces are "administratively down" by default. You need to enable an interface to use it.

26)In normal course, a DTE device is expected to communicated with a DCE device. In such event, the connections between the DCE and DTE devices is straight. However, if the communication need to happen between two DTEs or two DCEs, the signals need to be rolled. This is achieved by rolling the pins in the cable or in the DB-25 adapter. Typically, a DTE to DTE communication can be achieved by:

  • By connecting DTE to a rolled RJ-45 cable to a straight DB-25 adapter to DTE, OR
  • By connecting a straight RJ-45 cable to a rolled DB-25 adapter to DTE.

Similarly, a DTE to DCE communication can be achieved by:

  • Connecting a DTE to a straight RJ-45 cable to a straight DB-25 adapter to DCE, OR
  • Connecting a rolled RJ-45 cable to a rolled DB-25 adapter to DCE.

Note that rolling the signals twice is same as using straight through cables or adapters without any rolling at all.

In RIP, after route convergence, all routes are distributed to the neighbors by a router.

Internet connectivity is normally achieved using a WAN connection. Therefore, you need a serial port for connecting to the Internet. On the other hand, a switch normally requires an Ethernet port for connectivity.

Inter-Switch Link (ISL) is one of the VLAN trunking protocols used for switched VLAN networks. It uses frame tagging to identify the VLAN. ISL encapsulates the original Ethernet frame, and a VLAN-ID is inserted into the ISL header

Inter-Switch Link and 802.1Q are two VLAN Trunking Protocols used with Fast Ethernet, that Cisco supports. LANE is associated with ATM and 802.10 is associated with FDDI. Also, it is important to note that ISL, 802.1Q, and 802.10 use Frame Tagging to identify the VLANs.

27)IP addresses are written using decimal numbers separated by decimal points. This is called dotted decimal notation of expressing IP addresses.

The different classes of IP addresses is as below:

Class Format Leading Bit Network address Maximum Maximum hostspattern Range networks / nodes

A N.H.H.H 0 0 - 126 127 16,777,214
B N.N.H.H 10 128 - 191 16,384 65,534
C N.N.N.H 110 192 - 223 2,097,152 254

Network address of all zeros means "This network or segment".

Network address of all 1s means " all networks", same as hexadecimal of all Fs.

Network number 127 is reserved for loopback tests.

Host(Node) address of all zeros mean "This Host (Node)".

Host (Node) address of all 1s mean "all Hosts (Nodes) " on the specified network.

IP is the protocol that works at Internet layer. It forwards the packets based on the network address(IP address) in the packets and the routing tables.

IP Packets are transported from source network to the destination network by what is known as routing. Hop-by-hop routing model is used by the Internet for delivery of packets. At each hop, the destination IP address is examined, the best next hop is determined by the routing protocol (such as RIP, OSPF or BGP) and the packet is forwarded by one more hop through this route. The same process takes place at the next hop. During this process, the logical addresses remain same. In an IP network, the logical addresses are IP addresses. The hardware interface addresses, such as MAC address change with each hop.

IP RIP based networks send the complete routing table during update. The default update interval is 30 seconds.

28)ISL, 802.1Q are the VLAN trunking protocols associated with Fast Ethernet. The VLAN trunking protocol defined by 802.10 is associated with FDDI. LANE (LAN Emulation) is associated with ATM.

ISP router is normally configured with static route.

It is important to know the difference between a collision domain and a broadcast domain. When you use Hubs, all the nodes connected to the hub will be in the same collision domain. However, when you use switches and implement VLANs, each VLAN will be in a separate broadcast domain. The packet forwarding between VLANs is achieved through the use of routing.

29)Key word: COPY <source> <destination>

This command copies configuration information to specified location. The following are some frequently used COPY commands:

COPY RUNNING-CONFIGURATION STARTUP-CONFIGURATION (alternatively, you can use an older version of the command, WRITE MEMORY): This command saves the current configuration to NVRAM.

Alternatively, we can issue the command using short form:

COPY RUNNING STARTUP - Copies configuration from RAM to NVRAM

COPY STARTUP RUNNING - This command merges configuration from NVRAM to RAM.

COPY FLASH TFTP - Copies current IOS from router flash memory to TFTP server.

COPY TFTP FLASH - Copies image file from TFTP server to flash. This is used to upgrade the IOS image file to a newer version, or if your IOS image becomes corrupt.

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