When frame reached the interface of the router, the router would check the destination address field in the frame. When the destination address is the address of router interface or broadcast address, router will strip out pocket from frame and transmit to network layer. Then the destination address in the packet would be checked, and protocol field be checked, finally it is sent to appropriate process. If the packet allows to be routed, routers will check its own routing table to find corresponding routing entries, which would contain at least the following two elements:
1、destination address: this address must be an address that the router is able to arrive
2、The indicator arrived at the destination address: this indicator is the Via we can see in the routing table, or next-hop.

Routers do the checking work in the routing table according to the destination address field in the packet. The accuracy of the inquiry decreases in the following order:

1、 host address
2、 subnet address
3、 Summary of network number
4、 The main categories network number
5、 （super net）
6、 The default route

If you cannot find an appropriate entry after all the inquiries, drop the packet, and returns an ICMP unreachable message to the sender.
you can use orders under EXEC mode when checking routing table on the CISCO routers: show IP route
R1#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  ia - IS-IS inter area, * - candidate default, U - per-user static route
  o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.123.0/24 is directly connected, FastEthernet0/0
  1.0.0.0/24 is subnetted, 3 subnets
C 1.1.1.0 is directly connected, Loopback0
C 1.1.2.0 is directly connected, Loopback1
C 1.1.3.0 is directly connected, Loopback2
C 192.168.14.0/24 is directly connected, Serial1/2
If you only want to check static routing entry, you can add static parameter to show ip route, and so on…

Configuration static routes
1、determine the address of each link
2、find non-direct link for each router
3、Write static entries for every non-direct link of each router(direct link need not)
Note: send the data text (destined to unknown network) to direct connections or the known network.
R1(config)#ip route 2.2.1.2 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.2.2 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.3.2 255.255.255.0 192.168.12.2
Which one is its unknown? ——all the non-direct link
Which one is its known? ——all the direct link
Two choices on the direct link: next-hop and outbound interface
See their differences
Next-hop is the interface address of direct linked router, while outbound interface is its own interface name. It’s like the entrances and exits of a tunnel: exit is next-hop, and entrance is outbound interface.
R1 and R2 pass through FA0/0——FA0/0，S1/1——S1/0
IP of FA0/0: 192.168.123.1/192.168.123.2
IP of S1/1：192.168.12.1
IP of S1/0：192.168.12.2
Each of them has three direct connected segments
1.1.1.1
1.1.2.1
1.1.3.1
2.2.1.2
2.2.2.2
2.2.3.2

Analysis:
As to R1, its known are:
192.168.123.0/24
192.168.12.0/24
1.1.1.0/24
1.1.2.0/24
1.1.3.0/24

As to R2, its known are:
192.168.123.0/24
192.168.12.0/24
2.2.1.0/24
2.2.2.0/24
2.2.3.0/24
Apart from known entry, the rest are their unknown. Now we have two methods to configure static routes
Using next-hop
R2(config)#ip route 1.1.1.0 255.255.255.0 192.168.123.1
R2(config)#ip route 1.1.2.0 255.255.255.0 192.168.123.1
R2(config)#ip route 1.1.3.0 255.255.255.0 192.168.123.1
Let’s see the routing table
R2#sh ip route static
1.0.0.0/24 is subnetted, 3 subnets
S 1.1.1.0 [1/0] via 192.168.123.1
S 1.1.2.0 [1/0] via 192.168.123.1
S 1.1.3.0 [1/0] via 192.168.123.1
Arrive at three unknown sites, which are all transmitted to via 192.168.123.1. note that the number in the [] is [1/0].
Using interface:
R1(config)#ip route 2.2.1.0 255.255.255.0 serial 1/1
R1(config)#ip route 2.2.2.0 255.255.255.0 serial 1/1
R1(config)#ip route 2.2.3.0 255.255.255.0 serial 1/1
Let’s see the routing table
R1#sh ip route static
2.0.0.0/24 is subnetted, 3 subnets
S 2.2.1.0 is directly connected, Serial1/1
S 2.2.2.0 is directly connected, Serial1/1
S 2.2.3.0 is directly connected, Serial1/1
Here [] is replaced by directly connected, which shows that router1 takes these entries for directly connected entries.
When choosing routes, routers would consider which one is the better way.
Suppose that you connect it with the same routing entry using the first method, let’s see what would happen:
R1(config)#ip route 2.2.1.0 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.2.0 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.3.0 255.255.255.0 192.168.12.2
R1#sh ip route sta
R1#sh ip route static
2.0.0.0/24 is subnetted, 3 subnets
S 2.2.1.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
S 2.2.2.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
S 2.2.3.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
R1 consider there are now two ways to arrive at the direct link network behind R2, but in fact, there is only one path while R1 think there are two. Thus these two methods should be used according to situation, otherwise routing problems would appear.
Floating Static Routes
Set a static route with longer management distance. When normal link is DOWN, this route with longer management distance will be UP immediately to work in substitute.
(1)Router(config)#ip route 1.1.1.0 255.255.255.0 192.168.123.2
(2)Router(config)#ip route 1.1.1.0 255.255.255.0 192.168.12.2 50
As you see, when (1) is down, (2) would be up automatically, replacing (1).

Pass4sure practice exam No.147
147.Users on the 172.17.22.0 network cannot reach the server located on the 172.31.5.0 network. The network administrator connected to router Coffee via the console port, issued the show ip route command, and was able to ping the server. Based on the output of the show ip route command and the topology shown in the graphic, what is the cause of the failure?

A: The network has not fully converged.

B: IP routing is not enabled.

C: A static route is configured incorrectly.

D: The FastEthernet interface on Coffee is disabled.

E: The neighbor relationship table is not correctly updated.

F: The routing table on Coffee has not updated .

Correct Answers:  C

In this exercise, users on 172.18.22.0 network cannot access the server located on the 172.31.5.0 network. The network administrator sees the following routing table records via show ip route command. Which one is correct?

First compare the two direct-connected routing entries, normal; the next-hop address of the following static default route is wrong, because the address 172.19.22.2 does not exist. Suppose now we send message from host A on 172.17.22.0 to the server on 172.31.5.0, when the data text enters Coffee, and Coffee would find that this entry does not exist after checking the routing table. Then the message would be matched the default router, and be transmitted to next-hop 172.19.22.2. But then Coffee realizes it does not know how to get to 172.19.22.2, so the data text is dropped.

When frame reached the interface of the router, the router would check the destination address field in the frame. When the destination address is the address of router interface or broadcast address, router will strip out pocket from frame and transmit to network layer. Then the destination address in the packet would be checked, and protocol field be checked, finally it is sent to appropriate process. If the packet allows to be routed, routers will check its own routing table to find corresponding routing entries, which would contain at least the following two elements:
1、destination address: this address must be an address that the router is able to arrive
2、The indicator arrived at the destination address: this indicator is the Via we can see in the routing table, or next-hop.

Routers do the checking work in the routing table according to the destination address field in the packet. The accuracy of the inquiry decreases in the following order:

1、 host address
2、 subnet address
3、 Summary of network number
4、 The main categories network number
5、 （super net）
6、 The default route

If you cannot find an appropriate entry after all the inquiries, drop the packet, and returns an ICMP unreachable message to the sender.
you can use orders under EXEC mode when checking routing table on the CISCO routers: show IP route
R1#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  ia - IS-IS inter area, * - candidate default, U - per-user static route
  o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.123.0/24 is directly connected, FastEthernet0/0
  1.0.0.0/24 is subnetted, 3 subnets
C 1.1.1.0 is directly connected, Loopback0
C 1.1.2.0 is directly connected, Loopback1
C 1.1.3.0 is directly connected, Loopback2
C 192.168.14.0/24 is directly connected, Serial1/2
If you only want to check static routing entry, you can add static parameter to show ip route, and so on…

Configuration static routes
1、determine the address of each link
2、find non-direct link for each router
3、Write static entries for every non-direct link of each router(direct link need not)
Note: send the data text (destined to unknown network) to direct connections or the known network.
R1(config)#ip route 2.2.1.2 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.2.2 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.3.2 255.255.255.0 192.168.12.2
Which one is its unknown? ——all the non-direct link
Which one is its known? ——all the direct link
Two choices on the direct link: next-hop and outbound interface
See their differences
Next-hop is the interface address of direct linked router, while outbound interface is its own interface name. It’s like the entrances and exits of a tunnel: exit is next-hop, and entrance is outbound interface.
R1 and R2 pass through FA0/0——FA0/0，S1/1——S1/0
IP of FA0/0: 192.168.123.1/192.168.123.2
IP of S1/1：192.168.12.1
IP of S1/0：192.168.12.2
Each of them has three direct connected segments
1.1.1.1
1.1.2.1
1.1.3.1
2.2.1.2
2.2.2.2
2.2.3.2

Analysis:
As to R1, its known are:
192.168.123.0/24
192.168.12.0/24
1.1.1.0/24
1.1.2.0/24
1.1.3.0/24

As to R2, its known are:
192.168.123.0/24
192.168.12.0/24
2.2.1.0/24
2.2.2.0/24
2.2.3.0/24
Apart from known entry, the rest are their unknown. Now we have two methods to configure static routes
Using next-hop
R2(config)#ip route 1.1.1.0 255.255.255.0 192.168.123.1
R2(config)#ip route 1.1.2.0 255.255.255.0 192.168.123.1
R2(config)#ip route 1.1.3.0 255.255.255.0 192.168.123.1
Let’s see the routing table
R2#sh ip route static
1.0.0.0/24 is subnetted, 3 subnets
S 1.1.1.0 [1/0] via 192.168.123.1
S 1.1.2.0 [1/0] via 192.168.123.1
S 1.1.3.0 [1/0] via 192.168.123.1
Arrive at three unknown sites, which are all transmitted to via 192.168.123.1. note that the number in the [] is [1/0].
Using interface:
R1(config)#ip route 2.2.1.0 255.255.255.0 serial 1/1
R1(config)#ip route 2.2.2.0 255.255.255.0 serial 1/1
R1(config)#ip route 2.2.3.0 255.255.255.0 serial 1/1
Let’s see the routing table
R1#sh ip route static
2.0.0.0/24 is subnetted, 3 subnets
S 2.2.1.0 is directly connected, Serial1/1
S 2.2.2.0 is directly connected, Serial1/1
S 2.2.3.0 is directly connected, Serial1/1
Here [] is replaced by directly connected, which shows that router1 takes these entries for directly connected entries.
When choosing routes, routers would consider which one is the better way.
Suppose that you connect it with the same routing entry using the first method, let’s see what would happen:
R1(config)#ip route 2.2.1.0 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.2.0 255.255.255.0 192.168.12.2
R1(config)#ip route 2.2.3.0 255.255.255.0 192.168.12.2
R1#sh ip route sta
R1#sh ip route static
2.0.0.0/24 is subnetted, 3 subnets
S 2.2.1.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
S 2.2.2.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
S 2.2.3.0 is directly connected, Serial1/1
[1/0] via 192.168.12.2
R1 consider there are now two ways to arrive at the direct link network behind R2, but in fact, there is only one path while R1 think there are two. Thus these two methods should be used according to situation, otherwise routing problems would appear.
Floating Static Routes
Set a static route with longer management distance. When normal link is DOWN, this route with longer management distance will be UP immediately to work in substitute.
(1)Router(config)#ip route 1.1.1.0 255.255.255.0 192.168.123.2
(2)Router(config)#ip route 1.1.1.0 255.255.255.0 192.168.12.2 50
As you see, when (1) is down, (2) would be up automatically, replacing (1).

Pass4sure practice exam No.147
147.Users on the 172.17.22.0 network cannot reach the server located on the 172.31.5.0 network. The network administrator connected to router Coffee via the console port, issued the show ip route command, and was able to ping the server. Based on the output of the show ip route command and the topology shown in the graphic, what is the cause of the failure?

A: The network has not fully converged.

B: IP routing is not enabled.

C: A static route is configured incorrectly.

D: The FastEthernet interface on Coffee is disabled.

E: The neighbor relationship table is not correctly updated.

F: The routing table on Coffee has not updated .

Correct Answers:  C

In this exercise, users on 172.18.22.0 network cannot access the server located on the 172.31.5.0 network. The network administrator sees the following routing table records via show ip route command. Which one is correct?

First compare the two direct-connected routing entries, normal; the next-hop address of the following static default route is wrong, because the address 172.19.22.2 does not exist. Suppose now we send message from host A on 172.17.22.0 to the server on 172.31.5.0, when the data text enters Coffee, and Coffee would find that this entry does not exist after checking the routing table. Then the message would be matched the default router, and be transmitted to next-hop 172.19.22.2. But then Coffee realizes it does not know how to get to 172.19.22.2, so the data text is dropped.

Before study this charpter, we have to know what’s our aim on career and how will us to begin our career, and before study anything, we must make an aim for ourselves, none but we have the aim, then we know what to do and how to do, then avoid the useless work.
Many persons regard the IT certificate as the most popular thing, but I think:
1, Interest is the best teacher
2.If the certificate is unrelated to the career planning, then it’s not worth to take it!
For this series of articles, they are only written for those who have been restricted their career goals — network interconnect technology engineer or related specialists reference.
Why we choose IT certification?
The company is now recruiting staff, repairs being employees have the skills, computer proficiency examination is often the focus of IT-related jobs especially. Any company can have a complete examination system, evaluation staff for the professional skills and the ability to work. At this point, and will focus on the third-party certification, certification result, various emerged. So, with 12 IT certification for IT career it is
For current compines, they pay more attention on the employees’ abilities and the skills on computer is often the focus of the interview, especially for those IT-related jobs. At this point, they will focus on the third-party certifications. so it’s very important to get some IT certifications for IT career.
In general, IT (information technology) certification divided into three categories : professional IT certification (such as Cisco and Microsoft certification of the manufacturers); IT skills certification (such as Adobe certification); Ability to use computer certification (such as computer application ability examinations). In doing occupational position, it must be based on their own career planning choices for their certification, so as to avoid detours.
More than 300 IT certifications in the world, why should we choose CISCO?
1.CISCO company has more than 20,000 employees in the world, its integrated technology, customer support and the financial ability to balance the standards ranked the top of “Netword” magazine, and ranked fifth on the latest release of the “world’s most valuable company in 1000″ from “Business Week” , and fifth of ” IT 100 “.

2.CISCO systems integration company has been regarded as the leading vendors on networking solutions, and their solutions are the foundation of thousands of companies, universities, government departments and enterprises to establish networks around the world.
3.CISCO systems integration company is building the backbone of the Internet, on the Internet and over 80% of backbone routers is Cisco’s products.
4.Many people think : to use of a CISCO product is a shortcut to get rich.

This series of articles aime at Knowledge Analysis on CISCO certification–the leading hardware vendors, so as to facilitate the the candidates to review before the examination.
This series of articles aim at the analysis on the combination of knowledge point for Cisco’s new version of CCNA (640-802).

Network functions and role
*Resource sharing
*Information transmission

When network change, the attendant question is : increasingly large scale networks, LAN traffic will surge, which have intermittent obstruction, users will feel slow response. At this point, you need some equipments to divided this overburdened network into some small networks, we call this: network segments. It can be achieved by using routers, switches and bridges.

Following reasons may cause obstruction of LAN:
*Too many hosts
*Broadcast Storm
*Low bandwidth
*There is sharing equipment in network (such as hubs)
*A large number of ARP or IPX traffic

Broadcast Domain
Broadcast Domain refers to the collection of all equipments, the equipments tune in all of the network broadcasting. Broadcasting does harm on network greatly, but it is also unavoidable, so the best way is to limit the broadcast domain to the minimum so as not to affect the entire network.

Divide up the broadcasting domain
Routers are used to separate the broadcasting domain. Routers can isolate the broadcasting. When router’s interfaces intercept radio traffic, it will discard the broadcasting packet and not continue to transmit it. Each router interface counterparts a broadcasting domain. (Refer to example 1)

Example 1，

Question
If the router R1 has a packet with a destination address 192.168.1.255 , what describes the operation of the network?
A. R1 will drop this packet because this it is not a valid IP address
B. R1 will forward the packet out all interfaces.
C. As R1 forwards the frame containing this packet, Sw-A will add 192.168.1.255 to its MAC table
Note: The router will filter the radio traffic which attempts to get through. Not like the switch flows out broadcasting traffic to all the interfaces, once the router received the radio traffic, it will do a “discarded”, and will not transmit to the lower reaches. So, B and C are the wrong options. The correct answer is A (R1 will discard the packet, because the address is invalid - it’s a broadcast address

What gave birth to OSI reference model?
When the network has just come into being the rudiment, in the typical, it only could communicate between the computers made by the same manufacturer. For example, it only can achieve the IBM solutions or DECNET solutions but can not combine the two.
In the late of 1970s, International Organization for Standardization, ISO set up a Standardization Development System Interconnection Reference Model, thereby broke the barriers that the network solutions not compatible with for different vendors.

The role of the OSI Reference Model
The OSI model is created to help suppliers under an agreement to build interoperable network equipments and softwares for different vendors’ network to work with each other. This is a very idealized but as the world peace, it may never fully realized, it is still a great goal

Working mechanisms of OSI reference model

OSI network model is the most basic level of the structure model. It describes how the data and network information are transferred from a computer application to the other computer applications through the network media. The OSI reference model is achieved by layered.

Advantages of Layered:
*The process of network communication will be divided into smaller, simpler components, thus contributing to the various components of development, design and troubleshooting.

*Through the standardization of network components, allowing multiple suppliers to develop

*By definition the function of each model to encourage industry standardization

*Allow all types of network hardware and mutual communication of software

*To prevent the changes of a floor to affect other layers, so as to conducive to the development

How to make the Layered of OSI reference model?
OSI reference model is divided into seven layers 2 groups; The highest three layers define how users communicate with each other; the rest four defines how to transmit the data from end to end.
The highest three layers, also known as the upper (upper layer), are not concerned about the specific network, such work is completed by the bottom four layers.

The whole reference model is descending into:
1.Application
2.Presentation
3.Session
4.Transport
5.Network
6.Data link
7.Physical

The tasks of each layer of OSI reference model :
The tasks of each layer of OSI reference model:
1.Application:Providing user interface
2.Presentation:Expression data; The operation of the data such as encryption, compression, etc.
3.Session:A conversation, segregate different applications of data
4.Transport:Provide reliable and unreliable data delivery; former correct the errors in data transmission
5.Network:Provide logical address, the path chosen for routers
6.Data Link:The nature of the byte packets composed frame; According to the MAC address transmission media visits; Implementation of error detection, error correction but not implemented
7.Physical：Transmission equipment between bits (bit); Definition of voltage, wire-speed, the physical-pin standard

The functions of each layer of OSI reference model (refer to following sample) :
1.Application:The documents, print, database, application procedures and other services
2.Presentation:Data encryption, compression and translation, etc.
3.Session:Session control
4.Transport:Provide end-to-end connectivity
5.Network:routing
6.Data Link:Frame components
7.Physical:Define the physical topology

8.Which of the following are associated with the application layer of the OSI model? (Choose two.)
A: ping
B: FTP
C: Telnet
Correct Answers: B C
Note: This question checks that what the CISCO candidates have understood the OSI model. Which is the application layer of the OSI Application Layer (the most senior)?
Ping is using ICMP, work at the network layer, only telnet and FTP are working in the OSI Application Layer. The correct answers are B and C.