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Cisco® CCNA Exam Cram Notes : Ios Passwords, Router Memory Components, Default Administrative Distances, And Packet Forwarding.

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41. 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.

41. Internal memory components of a Cisco router:

1. ROM (Read Only Memory); Memory containing micro-code for basic functions to start and maintain the router. ROM is not typically used after the IOS is loaded.

2. RAM/DRAM : stores the running configuration, routing tables, and packet buffers. Some routers, such as the 2500 series, run IOS from Flash, not RAM.

3. NVRAM (Non-Volatile Ram): Memory that does not lose information when power is lost. Stores the systemís configuration file and the configuration register. NVRAM uses a battery to maintain the data when power is turned off.

4.Flash Memory: Stores the compressed IOS (IOS stands for Cisco Internetwork Operating System) image. Flash memory is either EEPROM or PCMCIA card. Flash memory enables you to copy multiple versions of IOS software. This allows you to load a new level of the operating system in every router in your network and then, to upgrade the whole network to that version at an appropriate time.

42. While a packet travels through an Internetwork, it usually involves multiple hops. Note that the logical address (IP address) of source (that created the packet) and destination (final intended destination) remain constant, the hardware (Interface) addresses change with each hop.

43. The default administrative distances are as below:
Directly connected ----- 0
Static Route ------------- 1
EIGRP Summary---------5
External BGP ------------20
EIGRP --------------------- 90
IGRP -----------100
OSPF ----------- 110
ISIS ------------115
RIP ------------ 120
Unreachable ------------255

An administrative distance of 0 represents highest trustworthiness of the route.

An administrative distance of 255 represents the lowest trustworthiness of the route.

Routed and Routing protocols:

Routing protocols job is to maintain routing tables and route packets appropriately. Examples of routing protocols are RIP, IGRP, EIGRP, OSPF. Routers can support multiple independent routing protocols and can update and maintain routing tables for each protocol independently.

Routed protocols are used to transport user traffic from source node to destination node. Examples of routed protocols are IP, IPX, AppleTalk.

44. There are three ways a router learns how to forward a packet:

1. Static Routes - Configured by the administrator manually. The administrator must also update the table manually every time a change to the network takes place. Static routes are commonly used when routing from a network to a stub (a network with a single route) network.
The command is
ip route network mask address/interface [distance]
ex: ip route 165.44.34.0 255.255.255.0 165.44.56.5
Here, 165.44.34.0 is the destination network or subnet
255.255.255.0 is the subnet mask
165.44.56.5 is the default gateway.

2. Default Routes - The default route (gateway of last resort) is used when a route is not known or is infeasible. The command is
ip route 0.0.0.0 0.0.0.0 165.44.56.5
The default gateway is set to 165.44.56.5

3. Dynamic Routes - As soon as dynamic routing is enabled, the routing tables are automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with other routers. Each route entry includes a subnet number, the interface out to that subnet, and the IP address of the next router that should receive the packet. The commands to enable rip are:
router rip
network <major network number>.

45. To enable the Cisco IOS to forward packets destined for obscure subnets of directly connected networks onto the best route, you use "ip classless" command.

46. There are broadly three types of routing protocols:

1. Distance Vector (Number of hops) - Distance vector routing determines the direction (vector) and distance to any link in the internetwork. Typically, the smaller the metric, the better the path. EX: Examples of distance vector protocols are RIP and IGRP. Distance vector routing is useful for smaller networks. The limitation is that any route which is greater than 15 hops is considered unreachable. Distance vector protocols listen to second hand information to learn routing tables whereas, Link state protocols build routing tables from first hand information. Routers with distance vector protocols send its entire routing table to each of its adjacent neighbors.

2. Link State Routing: Link State algorithms are also known as Shortest Path First (SPF) algorithms. SPF generates the exact topology of the entire network for route computation, by listening to the first hand information. Link State protocols take bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more efficient for larger networks. Bandwidth and delay are the most widely used metrics when using Link-State protocols. EX: OSPF and NLSP.

Benefits of Link State protocols:

Allows for a larger scalable network
Reduces convergence time
Allows "supernetting"

3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. EX: EIGRP.

47. 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!

48. RIP (and IGRP) always summarizes routing information by major network numbers. This is called classfull routing.

49. Convergence is the term used to describe the state at which all the internetworking devices, running any specific routing protocol, are having identical information about the internetwork in their routing tables. The time it takes to arrive at identical information of the internetwork is called Convergence Time.

50. RIP,RIP2, and IGRP use distance vector algorithms.

RIP2 transmits the subnet mask with each route. This feature allows VLSM (Variable Length Subnet Masks) by passing the mask along with each route so that the subnet is exactly defined.

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