SWITCH交换机基础知识

Switch

一、桥，可以隔离冲突域。

1、学习MAC功能，基于源学习。建立桥电路。每个mac表的每一条都有一个计时器，默认从300秒开始递减，递减到0，该条将被删减。如果该条又发送数据，则300秒重计。可以试验一个交换机下面接一个hub。Hub有4种。 2、数据转发/过滤

3、环路避免（avoidance）：如果成环，将启动避免机制

二、传输帧的方式 1、cut-through Switch checks destination address and immediately begins forwarding frame.检查目的地址后立即转发

只检查目的MAC地址，就转发出去。速度快，无校验；只转换到能找到目的mac地址为止，然后立即转发。

2、store and forward Complete frame is received and checked before forwarding 接收完整帧，校验然后转发 存储转发，对帧进行校验，重新计算校验和，和接收到帧的校验和比较，如果相同，进行转发。否则，认为该帧被破坏或出错，丢弃该帧。Tcp包，通过三次握手的计时器来让源知道该帧丢失；udp包一般不丢弃，通过上层来纠错。还可以有其它的操作。速度慢，延迟慢，有校验；

3、fragment-free Switch checks the first 64 bytes,then immediately begins forwarding frame

Fragment-Free:折中，读最先的64个字节（因为以太网的最小帧是64个字节，出现的机率大），进行校验，然后开始转发。当帧>64字节时，就没有办法校验帧了，但是有头部校验和。

三、学习MAC表

unknown unicasts are flooded：不知道的单播洪泛。 当HOST A发送到HOST B时，交换机将学会A的MAC地址，但是不会学到B的MAC地址。

交换机只学习源的MAC地址，不学习目的地址。

而广播和多播的地址是不会出现在源地址中的，所以交换机不会学习到他们。

The age time，1900默认是每5分钟refresh刷新一次mac-address-table，因为mac表的大小有限。

广播和多播，Broadcast and multicast frames are flooded to all ports other than the originating port. 发送到除发送者之外的其它所有端口。

四、生成树协议 SPT 1、Spanning-Tree Protocol

为了避免冗余交换机线路带来的问题

出发点：想办法把环路打断。当一台交换机坏了，它又自动打开。它每隔2秒发bpdu桥协议数据单元，如果它20秒没有返回，在我认为它坏掉了，则取消Block状态。

2、Spanning-Tree 操作：

? One root bridge per network每个网络（指一个广播域）必须有一个根桥（交换机），

根桥上所有端口为指定端口（DP）

? One root port per nonroot bridge每个非根桥选一个根端口

? One designated port per segment每个网段（指冲突域）选一个DP

? Nondesignated ports are unused。指定端口为转发状态，其它端口为阻塞状态。(阻

塞状态的端口不转发数据，但是它依然可以接收数据)

BPDU=Bridge Protocol Data Unit 桥协议数据单元。默认每2秒发一次。就像是hello包。 当我收到比我桥ID还小的，我就不发bpdu了，最后只有最小的那个在发。

1）怎么确定根桥？

――选最小桥ID的交换机为根桥。 根桥的每个端口都是指定端口. 桥ID＝桥优先级.MAC地址 两部分组成。

――优先级0～64k，缺省为32k。MAC地址就是交换机上最大的mac地址。 ――选择桥id最小的为根桥 （树根放在网络中心。）

一个vlan是一个广播域，可以认为是一棵树。

在一台交换机启动时，出现端口状态有block,listen,learn,forward共4种状态。 z

启动交换机的头50秒过程：

20秒，只听BPDU； ※交换机最多7个串连，每个延迟2秒，所以，7个×2秒＋3次×2秒＝20秒※

监听状态15秒，也收听BPDU；就是spt过程。 15秒学习状态；

（20＋15＋15）50秒后，开始转发状态。

2）怎么选择根端口？ 选择根端口是从该桥到根桥的 path cost值最小为根端口。 最短路径

What value is primarily used to determine which port becomes the root port on each non root switch in a spanning-tree topology? A. path cost

B. lowest port MAC address C. VTP revision number

D. highest port priority number

E. port priority number and MAC address

同一cost值时，比较端口id，交换机端口优先级是0～255，默认是128。再加上mac地址，谁的mac地址低，就选谁。

3）每个网段（冲突域）选一个指定端口，到根桥cost值最小的端口为指定端口。

3、Rapid Spanning-Tree Protocol：快速的Spanning-Tree Protocol RSTP

AlternatePort备份端口（DIS）：监听，你断开了，我就把自己设置成 转发状态

30秒可以实现。

五、配置交换机

show spantree Show vlan

1、配置交换机ip地址

29以上的交换机，都要在vlan1接口模式下，配置交换机管理ip地址。 如果你给vlan2配ip地址，则vlan1的ip就down掉了。 3550是三层交换机，每个接口都能配ip。

#interface vlan 1

(config-if)#ip address 10.5.5.11 255.255.255.0

显示交换机管理ip

wg_sw_2950#show interface vlan 1

2、配置缺省网关

缺省网关，当别人从远程telnet上到交换机，那么要返回给远程用户信息时，把数据传到缺省网关上。

# ip default-gateway 10.5.5.3

2950只能通过show run来看缺省网关。

3、接口模式下设置端口全双工状态

wg_sw_2950(config-if)#duplex {auto | full | half}

29缺省是auto

用show Interface slot/port命令查看端口双工模式

论坛上有个千分的说他考试的时候选的BE

Half Duplex (CSMA/CD)

? Unidirectional data flow ? Higher potential for collision ? Hubs connectivity

Full Duplex

? Point-to-point only ? Attached to dedicated switched port ? Requires full-duplex support on both ends ? Collision-free

? Collision detect circuit disabled 监听的功能就关掉了

冲突检测用的是本地环电路(local loop circuit)检测CSMA/CD

Nodes that are attached to hubs that share their connection to a switch port must operate in half-duplex mode because the end stations must be able to detect collisions.

4、管理mac-address-table 1）Show mac-address-table

wg_sw_a(config)#mac-address-table aging-time ? <10-1000000> Aging time value The default is 300 sec.

The MAC address table is also referred to as the CAM table (Content Address Memory) on some switches.

2）配置永久的mac地址

wg_sw_1900&2950(config)#mac-address-table permanent {mac-address type module/port}

wg_sw_2950_only(config)#mac-address-table static mac_addr {vlan vlan_id} [interface int1 [int2 ... int15]]

3）Setting a Restricted Static MAC Address on the Catalyst 2950

这个端口只能接这个mac地址的机器。就是mac和端口绑定??? 对于EI版ios还可以规定由谁可以给它发数据

wg_sw_2950(config)#mac-address-table secure mac-addr interface [vlan vlan-id] ————

wg_sw_2950#mac-address-table secure 0003.3333.3333 fa 0/1 vlan 1 wg_sw_2950#show mac-address-table

Dynamic Address Count: 1 Secure Address Count: 1 Static Address (User-defined) Count: 1 System Self Address Count: 25 Total MAC addresses: 28

Maximum MAC addresses: 8192 Non-static Address Table:

Destination Address Address Type VLAN Destination Port ------------------- ------------ ---- --------------------

0050.0f02.3372 Dynamic 1 FastEthernet0/2 0003.3333.3333 Secure 1 FastEthernet0/1

5、配置端口安全

wg_sw_2950(config)#interface fa0/1 wg_sw_2950(config-if)#port security

wg_sw_2950(config-if)#port security max-mac-count 10 这个端口只能接10个mac地址

port secure max-mac-count 1：这个端口只能和1个mac地址进行捆绑。 默认时一个接口下可以接132个mac。 Port security 或者portswitch security

校验交换机端口安全设置：

wg_sw_2950#show mac-address-table secure

wg_sw_2950(config-if)#port security action {shutdown | trap} wg_sw_2950#show port-security

port security action {shutdown | trap} 关闭/陷阱－你没有任何感觉，但是交换机把该信息传到管理员哪里了。如果你用了ciscoworks之类的软件。

怎么设置，监视哪些信息，trap什么：ccna不要求掌握。要设置snmp（基于udp。有v1、v2、v3三个版本）服务器。 6、

wg_sw_2950#copy startup-config tftp://host/dst_file

wg_sw_2950#erase startup-config ；删除startup-config，回复到出厂模式

7、show spantree 8、show vlan

What type of errors can indicate a duplex mode mismatch?

--FCS and late collision errors

What happens if a frame arrives at a switch destined for a MAC address that is not already in the MAC table,and the table is full?

-- the frame is forwarded through all ports except the receiving port.

Which two functions can LAN switches and bridges provide?(choose two)

a. packet routing b.jitter avoidance c.address learning d.store and forward decision e.loop avoidance using the Spanning Tree Protocol answer:c e

what feature do switches and bridges implement so that they can receive and transmit frames independently on each port?

a.loop avoidance b.buffering memory c.strore and forward mode d.spanning tree protocol answer:b

collisions usually happen within the first ? bytes of a frame. a.16 b.32 c.64 d.128 answer:b

which three frame types are flooded to all ports except the source port on a switch? a.unicast frames b.multicast frames c.broadcast frames d.frames with a known destination address e.frames with an unknown destination address answer:b.c.e

with STP, how is the designated port selected on a segment? a.the port that has the lowest path cost to the root bridge b.the port that has the highest path cost to the root bridge

c.the port that has the lowest path cost to the closest nonroot bridge d. the port that has the highest path cost to the closest nonroot bridge

if a Catalyst switch port is in autonegotiate mode and an attached device that does NOT support autonegotiation is operating in full-duplex mode,whtat will the switch do?

a.disable the port b.suspend the port c.set the port to full-duplex d.set the port to half-duplex answer:d

what does the mac-addrress-table restricted static 1111.2222.1111 e0/3 e0/1 command do?

Answer: specifies that traffic to the address 1111.2222.1111 on e0/3 will be allowed only from interface e0/1

What is the default maximum number of address allowed on a secure port? Answer:256

Which three frame types are flooded to all ports except the source port on a switch? a.unicast frames b.multicast frames c.broadcast frames

d.frames with a known destination address e.frames with an unknown destination address flood是在交换机mac表里面找不到目的地址的时候的一种处理方式 ???

CCNA笔记：2层交换

Layer 2 Switching Switching Services

路由协议有在阻止层3的循环的过程.但是假如在你的switches间有冗余的物理连接,路由协议并不能阻止层2循环的发生,这就必须依靠生成树协议(Spanning Tree Protocol,STP)

不像bridges使用软件来创建和管理MAC地址过滤表,switches使用ASICs来创建和管理MAC地址表,可以把switches想象成多端口的bridges

层2的switches和bridges快于层3的router因为它们不花费额外的时间字查看层3包头信息,相反,它们查看帧的硬件地址然后决定是转发还是丢弃.每个端口为1个冲突域,所有的端口仍然处于1个大的广播域里

层2交换提供:

1.基于硬件的桥接(ASIC) 2.线速(wire speed) 3.低延时(latency) 4.低耗费

Bridging vs. LAN Switching

桥接和层2交换的一些区别和相似的地方: 1.bridges基于软件,switches基于硬件 2.switches和看作多端口的bridges

3.bridges在每个bridge上只有1个生成树实例,而switches可以有很多实例

4.switches的端口远多于bridges 5.两者均转发层2广播

6.两者均通过检查收到的帧的源MAC地址来学习 7.两者均根据层2地址来做转发决定 Three Switch Function at Layer 2

2层交换的一些功能:

1.地址学习(address learning):通过查看帧的源MAC地址来加进1个叫做转发/过滤表的MAC地址数据库里

2.转发/过滤决定(forward/filter decisions):当1个接口收到1个帧的时候,switch在MAC地址数据库里查看目标MAC地址和出口接口,然后转发到符合条件的那个目标端口去

3.循环避免(loop avoidance):假如有冗余的连接,可能会造成循环的产生,STP就用来破坏这些循环 Spanning Tree Protocol(STP) Spanning Tree Terms

Digital Equipment Coporation(DEC)在被收购和重命名为Compaq的时候,创建了原始的STP,之后IEEE创建了自己的STP叫做802.1D版本的 STP.和之前的DEC的STP不兼容.STP的主要任务是防止层2的循环,STP使用生成树算法(spanning-tree algorithm,STA)来创建个拓扑数据库,然后查找出冗余连接并破坏它。

我们来看些关于STP的术语:

1.STP:bridges之间交换BPDU信息来检测循环,并通过关闭接口的方式来破坏循环

2.根桥(root bridge):拥有最好的bridge ID即为根桥,网络中的一些诸如哪些端口被堵塞(block)哪些端口作为转发模式的决定都由根桥来决定

3.BPDU:Bridge Protocol Data Unit,所有的switches通过交换这些信息来选择根switch

4.bridge ID:用于STP跟踪网络中的所有switches,这个ID由bridge优先级(priority)和MAC地址符合而成,优先级默认为32768,ID最低的即为根桥 5.非根桥(nonroot bridge):不是根桥的全为非根桥,非根桥交换BPDUs来更新STP拓扑数据库

6.根端口(root port):与根桥直接相连的端口,或者是到根桥最短的接口.如果到根桥的连接不止1条,将比较每条连接的带宽,耗费(cost)低的作为根端口;如果耗费相同就比较bridge ID,ID低的将被选用

7.指定端口(designated port):耗费低的端口,作为转发端口 8.端口耗费(port cost):带宽来决定

9.非指定端口(nondesignated port):耗费较高,为堵塞模式(blocking mode),即不转发帧

10.转发端口(forwarding port):转发端口用来转发帧

11.堵塞端口(blocked port):不转发帧,用来防止循环的产生,虽然不转发,但是它可以监听(listen)帧 Spanning Tree Operations

之前说过:STP的任务就是查找出网络中的所有连接,并关闭些会造成循环的冗余连接.STP首先选举1个根桥,用来对网络中的拓扑结构做决定. 当所有的switches认同了选举出来的根桥后,所有的bridge开始查找根端口.假如在switches之间有许多连接,只能有1个端口作为指定端 口 Selecting the Root Bridge

bridge ID用来在STP域里选举根桥和决定根端口,这个ID是8字节长,包含优先级和设备的MAC地址,IEEE版本的STP的默认优先级是32768.决定谁是根桥,假如优先级一样,那就比较MAC地址,MAC地址小的作为根桥 Selecting the Designated Port

假如不止1个连接到根桥，那就开始比较端口耗费,耗费低的作为根端口,下面是一些典型的耗费标准: 1.10Gbps:2 2.1Gbps:4 3.100Mbps:19 4.10Mbps:100

Spanning-Tree Port States

运行STP的bridges和switches的5种状态:

1.堵塞(blocking):不转发帧,只监听BPDUs,主要目的是防止循环的产生.默认情况下,当switch启动时所有端口均为blocking状态

2.监听(listening):端口监听BPDUs,来决定在传送数据帧之前没有循环会发生,STP过程。

3.学习(learning):监听BPDUs和学习所有路径,学习MAC地址表,不转发帧 4.转发(forwarding):转发和接收数据帧

5.禁用(disabled):不参与帧的转发和STP,一般在这个状态的都是不可操作的。

一般来说,端口只处于转发和堵塞状态,如果网络拓扑发生了变化,端口会进入监听和学习状态,这些状态是临时的 Convergence

汇聚,也叫收敛(convergence):当所有端口移动到非转发或堵塞状态时,开始收敛,在收敛完成前,没有数据将被传送.收敛保证了所有的设备拥有相同的数据库达到一致.一般来说从堵塞状态进入到转发状态需要50秒 Spanning Tree Example

我们来看1个STP的例子,拓扑图给出了已知的MAC地址,并且所有优先级均为32768:

注意A的MAC地址最小,优先级均为32768,所以A作为根桥,并且要注意的是根桥的所有端口均作为转发模式(指定端口).接下来决定根端口,直 接与根桥相连的作为根端口,而且作为转发模式,所以可以判定出根端口个指定端口,bridge ID决定指定和非指定端口.但是注意D和E之间,由于D的bridge ID小,所以D的为指定端口,E的作为非指定端口,如下:

LAN Switch Types

LAN的交换类型决定了当switch的端口接收到1个帧的时候如何去处理 延时(latency):指数据包进入一个网络设备到离开该设备的出口接口所花的时间,这个根据不同的交换模式也不一样 3种交换模式:

1.cut-through(fastforward):Cisco称这种模式叫

cut-through,fastforward或者real time模式,使用这种模式的时候,LAN switch只读取到帧的目标地址为止,减少延时,但是不适合与高偏向错误率的网络

2.fragmentfree(modified cut-through):和cut-through类似,但是LAN switch读取到数据(data)部分的前64字节,这个是Catalyst 1900的默认模式 3.store-and-forward:在这个模式下,LAN switch复制整个帧到它的缓冲区里,然后计算CRC,也帧的长短可能不一样,所以延时根据帧的长短而变化.如果CRC不正确,帧将被丢弃;如果正确, LAN switch查找硬件目标地址然后转发它们 3种模式对帧的读取程度具体如下:

Configuring the Catalyst 1900 and 2950 Switches

1900是款低端switch产品,分为1912和1924系列:1912是有12个10BaseT端口而1924是有24个10BaseT端口 1900 and 2950 Switch Startup

当1900启动后,先运行POST,此时每个端口的发光二级管(LEDs)是绿色的,当POST完成后,LEDs开始闪烁然后熄灭;假如 POST发现某个端口出问题了的话,系统LED和那个出问题的端口的LED开始变黄.假如你的console线缆接好了的话,POST开始显示菜单,如 下:

1 user(s) now active on Management Console. User Interface Menu

[M] Menus

[I] IP Configuration Enter Selection:K

CLI session with the switch is open To end the CLI session, enter [Exit]. >

而2950的启动,有点像router,先进入setup模式.但是默认你可以不对其进行配置,启动如下:

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]: no

Press RETURN to get started!

00:04:53: %LINK-5-CHANGED: Interface Vlan1, changed state to administratively down

00:04:54: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down Switch>

Setting the Passwords

配置密码:

1.登陆密码(用户模式):防止未授权用户登陆 2.启用密码(特权模式):防止未授权用户修改配置 Setting the User Mode and Enable Mode Passwords

1900下,输入K进入CLI,输入enable进入特权模式,再输入config t进入全局配置模式,如下: >en

#config t (config)#

当进入全局配置模式后,使用enable password命令配置登陆密码个启用密码,如下:

(config)#enable password ? level Set exec level password (config)#enable password level ? <1-15> Level Number

level1为登陆密码,level15为启用密码,密码长度范围是4到8字符之间,如下:

(config)#enable password level 1 nocoluvsnoko Error: Invalid password length.

Password must be between 4 and 8 characters 重配置并验证:

(config)#enable password level 1 noco (config)#enable password level 15 noko (config)#exit #exit

2950下的配置和配置router有点类似,如下: Switch>en Switch#conf t

Switch(config)#line ? <0-16> First Line number

console Primary terminal line vty Virtual terminal

Switch(config)#line vty ? <0-15> First Line number

Switch(config)#line vty 0 15 Switch(config-line)#login

Switch(config-line)#password noko Switch(config-line)#line con 0 Switch(config-line)#login

Switch(config-line)#password noco

Switch(config-line)#exit Switch(config)#exit Switch#

Set the Enable Secret Password

enable secret比enable password更安全,而且同时设置了2者的话,只有前者起作用1900下的配置,注意在1900下,enable secret和enable password可以设置成一样的,如下:

(config)#enable secret noko

2950下的配置和router类似,但是enable secret和enable passwor不可以设置成一样的,如下:

Switch(config)#enable password noko Switch(config)#enable secret noko The enable secret you have chosen is the same as your enable password. This is not recommended. Re-enter the enable secret. Switch(config)#enable secret noco Switch(config)# Setting the Hostname

给1900配置主机名,使用hostname命令,如下: (config)#hostname Noko Noko(config)#

给2950配置主机名,使用hostname命令,如下: Switch(config)#hostname Noco Noco(config)#

Setting IP Information

你可以不配置IP信息,直接把线缆插进端口,一样可以工作.配置IP地址信息有2点原因:

1.通过telnet或其他软件方式来管理switch 2.配置VLANs和其他等网络功能

默认下,没有IP地址和默认网关信息配置,在1900下,使用show ip命令查看默认IP配置,如下: 1900#sh ip

IP Address: 0.0.0.0 Subnet Mask: 0.0.0.0 Default Gateway: 0.0.0.0 Management VLAN: 1 Domain name:

Name server 1: 0.0.0.0

Name server 2: 0.0.0.0 HTTP server: Enable HTTP port: 80 RIP: Enable

在1900下使用ip address和ip default-gateway命令来配置IP地址信息和默认网关信息,如下:

1900(config)#ip address 172.16.10.16 255.255.255.0 1900(config)#ip default-gateway 172.16.10.1 1900(config)#

2950下的配置是在VLAN1接口下配置,VLAN1是管理VLAN,默认所有接口均是VLAN1的成员,配置如下: 2950(config)#int vlan1

2950(config-if)#ip address 172.16.10.17 255.255.255.0 2950(config-if)#no shut 2950(config-if)#exit

2950(config)#ip default-gateway 172.16.10.1 2950(config)#

注意2950的IP地址配置是在VLAN1接口下,另外要注意打开接口

Configuring Interface Description

配置描述信息,在接口配置模式下使用description命令,1900下的描述信息不能使用空格键,如下: 1900(config)#int e0/1

1900(config-if)#description Cisco_VLAN 1900(config-if)#int f0/26

1900(config-if)#description trunk_to_building_4 1900(config-if)# 息

2950下的描述就可以使用空格键,如下: 2950(config)#int fa 0/1

2950(config-if)#description Sales Printer 2950(config-if)#^Z

可以使用show interface和show running-config命令来查看这些描述信

Erasing the Switch Configuration

1900和2950的配置文件是存储在NVRAM里的,但是1900里你不能查看NVRAM或startup-config的内容,只能查看 running-config的内容,在1900里,你对配置所进行的修改自动被复制到NVRAM里,所以没有copy run start这样的命令;但是2950就有startup-config和running-config,使用copy run start来保

存配置到NVRAM里,擦除2950里startup-config文件使用erase startup-config命令;擦除1900里的使用delete命令.如下:

1900#delete ?

nvram NVRAM configuration

vtp Reset VTP configuration to defaults 1900#delete nvram

2950:

2950#erase startup-config

Refer to the exhibit. The Serial0/0 interface of the Tampa router connects to the Orlando router. Which two statements are true about the connection between these two routers? (Choose two.) A. Routing updates exchanged between the Orlando and Tampa routers are authenticated. B. Data exchanges between the Orlando and Tampa routers are encrypted.

C. The on device witch the Tampa router will negotiate a data link is the Orlando router. D. The link uses a two-way handshake for authentication. E. The link uses a three-way handshake for authentication.

F. The link is addressed on the \ Answer: C,E

Refer to the exhibit. A technician has configured the FastEthernet 0/1 interface on Sw11 as an access link in VLAN 1. Based on the output from the show vlan brief command issued on Sw12, what will be the result of making this change on Sw11?

A. Only the hosts in VLAN 1 on the two switches will be able to communicate with each other. B. The hosts in all VLANs on the two switches will be able to communicate with each other. C. Only the hosts in VLAN 10 and VLAN 15 on the two switches will be able to communicate with each other.

D. Hosts will not be able to communicate between the two switches. Answer: A

在VLAN环境中，，如果没有置TRUNK，那么它默认会传送VLAN1 也就是管理LAN 的信息

配置成trunk的端口，不会出现在vlan列表中。

Answer: E. 可以这么说 当时老师讲的时候讲了一大堆 但结果还是mac低的留下。。。

You have a server that's directly connected to a Cisco switch by way of its Fa0/1 port, and you don't want any other MAC addresses from any other servers to access this port. How would you accomplish this? (Select two answer choices) A. Configure port Fa0/1 to accept connections only from the static IP address of the server.

B. Employ a proprietary connector type on Fa0/1 that is incompatible with other host connectors.

C. Configure the MAC address of the server as a static entry associated with port Fa0/1.

D. Bind the IP address of the server to its MAC address on the switch to prevent other hosts from spoofing the server IP address.

E. Configure port security on Fa0/1 to reject traffic with a source MAC address other than that of the server.

F. Configure an access list on the switch to deny server traffic from entering any port other than Fa0/1. Answer: C, E

Explanation: You can configure a MAC address to be associated only with a particular port, with the restriction that frames destined to that MAC address have to enter through that particular port. So answer choice C is correct.

By default, which of the following factors determines the spanning-tree path

cost?

A. It is the individual link cost based on latency B. It is the sum of the costs based on bandwidth C. It is the total hop count

D. It is dynamically determined based on load Answer: B

There are 4 STP states that a bridge port can be in: Blocking, Listening, Learning, and Forwarding:

The Testking switched LAN is displayed in the network below:

In the network shown in the diagram, which ports on Switch2 are receiving BPDUs?

A. Fa 0/1 only B. Fa 0/2 only C. Fa 0/3 only

D. Fa 0/1 and Fa 0/2 only E. Fa 0/1 and Fa 0/3 only F. All three ports

Answer: E ??F

In which Spanning-Tree states does a switch port learn MAC addresses? (Select two)

A. Blocking B. Listening C. Forwarding D. Learning E. Relaying Answer: C, D

* Listening - Listens to incoming Hello messages to ensure that there are no loops, but does not forward traffic or learn MAC addresses on the interface.

* Learning -learns MAC addresses and builds a filter table but does not forward frames.

* Forwarding - Sends and receives all data on the bridged port. * Blocking - are used to prevent network loops.

On a half-duplex Ethernet LAN, two hosts attempt to send data simultaneously, resulting in a collision. Following this collision, what will the hosts do? (Select all valid answers)

A. The destination host sends a request to the source for retransmission. B. The jam signal indicates that the collision has been cleared.

C. The hosts will attempt to resume transmission after a time delay has expired.

D. An electrical pulse indicates that the collision has cleared.

E. The router on the segment will signal that the collision has cleared.

F. The hosts will do nothing, as the higher layers are responsible for data error correction and re-transmission. Answer: C Explanation:

When a host on an Ethernet LAN has information to send, the following steps are taken:

1. A device with a frame to send listens until Ethernet is not busy. 2. When the Ethernet is not busy, the sender begins sending the frame. 3. The sender listens to make sure that no collision occurred.

4. Once the senders hear the collision, they each send a jamming signal, to ensure that all stations recognize the collision.

5. After the jamming is complete, each sender randomizes a timer and waits that long. 6. When each timer expires, the process starts over with step 1.

half-duplex and full-duplex Ethernet? (Select two answer choices.) A. Full-duplex Ethernet uses CSMA/CD to prevent collisions.

B. Half-duplex Ethernet uses a loopback circuit to detect collisions. C. A full-duplex Ethernet card allows 20Mbps for data transmission. D. Full-duplex Ethernet makes use of two pairs of wires for data.

E. An Ethernet hub can operate both half and full duplex simultaneously. Answer: B, D

What are the differences between full-duplex Ethernet and half-duplex Ethernet? (Select

all that apply)

A. Half-duplex Ethernet operates in a shared collision domain. B. Full-duplex Ethernet has a lower effective throughput.

C. Half-duplex Ethernet operates in a private collision domain. D. Full-duplex Ethernet allows two-way communication.

E. Half-duplex Ethernet operates in a private broadcast domain. Answer: A, D

Two stations on a LAN transmit at the same time, resulting in a collision. What happens when a collision occurs on the network? (Choose all that apply) A. Each device on the Ethernet segment stops transmitting for a short time. B. A jam signal informs all devices that a collision occurred.

C. When data transmission resumes, the devices that were involved in the collision have priority to transmit.

D. The devices that are involved in the collision stops transmitting for a short time. E. The collision invokes a random back-off algorithm. Answer: B, D, E

With regard to Ethernet media access methods, which of the following are true? (Choose all that apply.)

A. A device waits for an electronic signal before transmitting.

B. A device listens and waits until the media is not busy before transmitting.

C. All devices on an Ethernet segment see data that passes on the network medium. D. Only the sender and the receiver devices see data that passes on the network medium.

E. Ethernet networks allow you to configured devises with higher transmission priority.

Answer: B, C

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