40. Default administrative distances some important routing protocols are as below:

Route Source                        Default Distance

Directly connect Interface        0

Static Route                            1

IGRP                                    100

RIP                                      120

Unknown                              255

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

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

41.  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.

42.  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.

The default gateway is set to 165.44.56.5

3. Dynamic Routes - In dynamic routing, the routing tables are automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with other routers. 

     The commands to enable rip are:

router rip

network <major network number>  

43.  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.

44.  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:  

1. Allows for a larger scalable network

2. Reduces convergence time

3. 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

45. 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!

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

47. 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.

48. 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.

49.Routing metrics used by IGRP:

Bandwidth, MTU, Reliability, Delay, and Load.

1.      Bandwidth: This is represents the maximum throughput of a link.

2.      MTU (Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets.

3.      Reliability: This is a measurement of reliability of a network link. It is assigned by the administrator or can be calculated by using protocol statistics.

4.      Delay: This is affected by the band width and queuing delay.

5.      Load: Load is based among many things, CPU usage, packets processed per sec.

50. The metric limit for link-state protocols is 65,533

51. Following are the possible solutions for preventing routing loops.

1.      Split Horizon - based on the principle that it is not useful to send the information about a route back in the direction from which the information originally came.

2.      Poison Reverse - A router that discovers an inaccessible route sets a table entry consistent state (infinite metric) while the network converges.

3.      Hold-down Timers - Hold down timers prevent regular update messages from  reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route.

4.      Triggered Updates - Normally, new routing tables are sent to neighboring routers at regular intervals (IP RIP every 30 sec / and IPX RIP every 60 sec). A triggered update is an update sent immediately in response to some change in the routing table. Triggered updates along with Hold-down timers can be used effectively to counter routing loops.

52.  IP RIP based networks send the complete routing table during update. The default update interval is 30 seconds. IGRP update packet is sent every 90 seconds by default.

53.  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. OSPF, and IGRP use AS numbers.

54.  CDP stands for Cisco Discovery Protocol.  This protocol is proprietary of Cisco. CDP runs SNAP (Sub network Access Protocol) at the Data Link Layer. Two Cisco devices running two different Network layer protocol can still communicate and learn about each other.

55. Show IP protocol: This command will show information on RIP timers including routing update timer (30sec default), hold-down timer (default 180sec). It also displays the number of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.

1.      Show IP route: This command will display the IP routing table entries. In addition, it displays the Gateway of last resort (if one is assigned). It also displays the codes used for various types of routes. Some of the important codes are:

C: directly connected;

S: Statically connected

 I   : IGRP

 R : RIP

2.      Show IP interface: This command shows you  interface-wise information such as IP address assigned to each interface, whether the interface is up, MTU etc.

3.      Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous list of routing updates as they are sent and received. This leads to lot of overhead, which is the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with debugging.

56.  Cisco router boot configuration commands:

1.  boot system - This is a global command that allows you to specify the source of the IOS software image to load. If you configure more than one source, attempts are made to load the IOS from the first command in the configuration to the last successively. If the first fails, the second boot command is used.

2.  boot system rom - Loads  IOS from ROM.

3.  boot system flash - Loads the first file from flash memory.

4. boot system tftp <file name>  < tftp_address >  - Loads IOS with a filename <file name>  from a TFTP server.

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