CCNA考试主要知识点指导
添加时间: 2008-2-21 16:22:45 作者: Cisco指导 阅读次数:33 来源: http://www.d9soft.com
(三)
在CCNA考试中，要分清楚下面的devices在什么layer工作。
Layer 3 devices: router
Layer 2 devices: Switch, bridge
Layer 1 devices: repeater, Hub
Switching is not very important for the CCNA exam. However you need to know the basic switching concepts.
To solve the network congestion problem
1. Create a smaller collision domain.
Ethernet uses CSMA/CD, here CD means collison detection. When two computers try to send the packet at the same time. Collison is found in the network. So none of the packet will be sent. They wait random
period of the time to resend the packet. If computers are in the same subnet, they are in the same collision domain. Creating a smaller collision domain means creating multiple subnets in your network.
2. increase the bandwidth, for example, upgrade your network to 100Mbps fast Ethernet.
LAN segmentation:
LAN segmentation means use bridge, router or switch to segment the network and create a smaller collision domain.
You need to know how a bridge, router, switch works. If you don't know how they work, take the MCSE networking essentials exam first.
Both bridge and router use the routing table to distinguish if the computer are in the same segment. (not subnet). A bridge connects two network segments of the same subnet. A router can connect two or more network segments of the different subnets.
For example, the subnet mask for your network is 255.255.255.0
(1.1.1.1 - 1.1.1.10) Bridge (1.1.1.100 - 1.1.1.120)
(1.1.1.1 - 1.1.1.10) router (1.1.2.1 - 1.1.2.10)
Bridge works on the MAC layer, it does not care of the upper layer protocol. You can use a bridge to connect the network using either routable protocol (TCP/IP, IPX/SPX) or unroutable protocol. You can only use a router to connect the network using the routable protocol(TCP/IP, IPX/SPX)
A bridge does not stop broadcasting, while using a router can prevent broadcasting.
A router supports multiple pathes, however, a bridge only supports spanning-tree.
区分 Switch & bridge
1. both layer 2 device
2. switch supports multiple ports, while bridge only supports two ports
3. bridge: hardware-optimized for speedy. concurrent switching, which reduces latency compared to transparent bridging.
Switching mode:
** The Ethernet header is 14 bytes.
1.Store-and-Forward: Store the entire packet and forward it. You can also minimize the error by checking CRC. Latency varies depending on frame size;
2.Cut-Through (Real-Time): Constant latency; no error checking
3.Fragment-Free: Waits 64 bytes before forwarding a frame – Constant latency (because the most possible errors occur in the first 64 bytes)
Low-end switch only supports store-and-forward.
full-duplex Ethernet
An extension to 10BaseT Ethernet that is implemented in a switched Ethernet environment, which has a dedicated line between the station and switch. It is built into the network adapter (NIC) and switch,
providing bi-directional transmission that boosts bandwidth from 10 to 20 Mbps.
Fast Ethernet (802.3u)
still use CSMA/CD
Gigabit Ethernet
1000BaseT (IEEE 802.3ab) : 2-pair CAT 5 UTP (100 m) UTP
1000BaseSX (IEEE 802.3z) :2-strand multi-mode optical fiber (220-550m)
1000BaseLX (IEEE 802.3z):2-strand multi-mode or single-mode optical fiber (3000 m)
如果你有多个switch的话，如果你 网络 设计的时候有loop的时候，packets会一直在网络里面循环的传，产生broadcast storm.请比较一下用router 连的network, 因为有TTL (time to live)这个参数，所以到一定的时候packets就自动丢掉了。而switch的network里面没有TTL这个概念，所以如果你在switch network 里面如果有loop的话，就会一直传下去。
所以一般设计的时候采用spanning -tree的结构。但是spanning-tree没有fault-tolernce, 所以用spanning-tree protocol to provide layer2 redundence并解决loop的问题。
spanning-tree protocol其实也没什么，就是只enable一个port, 让现在的网络只支持 spanning-tree的结构，如果那个port出问题的话，再enable另外一个port来做fault-tolernce.
spanning-tree protocol(STP)有下面的特点：
1. IEEE 802.1d
2. prevent routing loops in a bridge or switched network
3. STP enable the port when one switch is down
STP Convergence Steps:
1.Elect one Root Bridge
2.Elect one Root Port per non-Root Bridge
3.Elect one Designated Port per segment
VLAN
create a broadcast domain (在CCNA考试中，broadcast domain和IP subnet是一个概念) by one or more switches
VLAN的关键是在switch连接支持trunk link. Cisco 的switch支持trunk link,但不是所有的switch都支持的。所以并不是所有的switch都支持VLAN。
Switch switch
trunk link is a configuration in switch.use Frame tagging to add VLAN ID number to the packet.
TCP/IP
familiar with the following terms:
IP, ARP, RARP, BootP, ICMP
IP address schema
IP address = network ID + subnet ID + host ID
||
| network ID | subnet ID | host ID |
||
Class A, begin with 0, subnet mask: 255.0.0.0
class B, begin with 10, subnet mask: 255.255.0.0
class C, begin wiht 110, subnet mask: 255.255.255.0
For an IP address, first of all, check it begins with 0, 10 or 110 to
see if it is a class A, class B or class C network.
Secondly, ANDING the subnet mask, you can get the network ID.
Determine the Number of Required Network IDs
1.One for each subnet
2.One for each wide-area network connection
Cisco supports IP unumbered, so you don't need to have one network ID for each network connection. But other vendors might not support IP unumbered.
For the CCNA exam, you need to know how to calculate the class, subnet mask and broadcast address.如何从二进制转成10进制，10进制转成2进制。
2进制转10进制比较简单。
由右往左，
第一位乘以 2的0次方，
第二位乘以 2的1次方，
第三位乘以 2的2次方，
第四位乘以 2的3次方，
。。。
把所有的数字加起来就好了。
10进制转2进制稍微麻烦一些。但一次一次的除以2，就可以得出结果。因为老美的算术的普遍水平连国内的小学生都比不了，(90%以上的人都不会9 x9乘法口诀)所以大家不用太紧张。
：P
Broadcast address:
subnet ID
inverse the subnet ID 0 --> 1 , 1 -->0
IP ANDING the inversed subnet ID
Q: What is the valid IP address range in the subnet of 172.37.2.56 with
12 subnet bits?
1. 172 begin with 10, so it is a class B network.
2. -> default subnet mask is 16 bits.
3. new subnet mask = 16 bits + 12 bits = 28 bits
4. convert the 172.37.2.56 to binary
5. begin IP address: change the last 4 bits to 0001, change the last 4 bits to 1110.
Configure IP address in Cisco
Because IP address is interface specific, you need to go to interface
Router>nable
Router#config t
Router(config)#int s0
Router(config-if)#ip address 172.16.20.2 255.255.255.0
Router(config-if)#int e0
Router(config-if)#ip address 172.16.10.1 255.255.255.0
Router(config-if)#^Z
在cisco router里面 ping return result 有下面几种:
! --> OK
. --> Time out
u --> unreachable
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