CCIE安全指南V2第1部分：基础网络概念解析

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"CCIE Security Guide V2 Part1 涵盖了网络基础、TCP/IP、交换与桥接（包括VLANs和Spanning Tree等）、路由协议（如RIP、EIGRP、OSPF和BGP）、点对点协议（PPP）、IP多播、综合业务数字网（ISDN）、异步通信、接入设备（如Cisco AS5300系列）、电信最佳实践以及无线最佳实践。书中还介绍了这些主题在CCIE Security笔试蓝图中的重要性，该考试包含大约50%的安全问题和50%的网络基础知识问题。" 在《CCIE Security Guide V2 Part1》中，我们首先会深入探讨General Networking Topics，这是一个关键的学习领域，对于准备CCIE Security认证的考生至关重要。以下是对这些知识点的详细说明： 1. **网络基础**：这部分内容涉及网络的基本概念，如网络层次模型、网络设备（如路由器和交换机）的作用、网络地址分配以及数据传输的基本原理。 2. **TCP/IP**：TCP/IP协议族是互联网的基础，包括IP（网络层）、TCP（传输层）和应用层协议（如HTTP、FTP等）。理解TCP/IP的工作原理，如IP地址、子网掩码、端口号以及TCP连接的三次握手和四次挥手过程，是网络工程师必备的知识。 3. **交换与桥接**：这部分涵盖交换机和桥接器的功能，包括VLAN（虚拟局域网）用于隔离广播域，以及Spanning Tree Protocol（STP）防止环路的形成。理解VLAN配置、VTP（ VLAN Trunking Protocol）和STP的计算逻辑是至关重要的。 4. **路由协议**：RIP（Routing Information Protocol）、EIGRP（Enhanced Interior Gateway Routing Protocol）、OSPF（Open Shortest Path First）和BGP（Border Gateway Protocol）都是网络中常见的路由协议，分别用于内部和外部路由选择。理解它们的工作机制、路由算法和部署策略是网络规划和故障排查的基础。 5. **点对点协议（PPP）**：PPP是一种在点对点链路上建立连接的协议，常用于拨号连接。学习PPP的LCP（Link Control Protocol）、NCP（Network Control Protocols）和认证机制（如PAP、CHAP）是必要的。 6. **IP多播**：IP多播允许一次性发送数据到多个接收者，节省网络资源。理解多播地址范围、多播组管理和多播路由协议（如PIM）对高效多媒体传输非常重要。 7. **ISDN**：ISDN是一种提供高速数据和语音通信的电话网络技术，包括B信道和D信道。理解ISDN的组成部分、接口类型和速率对于电信集成是必要的。 8. **异步通信**：异步通信是指数据传输速率不固定，适用于低速设备的连接，例如调制解调器。 9. **接入设备**：如Cisco AS5300系列，这类设备用于连接远程用户或分支机构，通常涉及到拨号访问、虚拟私有网络（VPNs）和安全配置。 10. **电信与无线最佳实践**：这部分内容涉及电信网络的设计原则和无线网络的安全及优化策略，如802.11标准、WPA/WPA2加密、无线覆盖规划等。了解并掌握这些主题是通过CCIE Security笔试的关键，因为它不仅测试你的安全知识，还评估你对网络基础设施的全面理解。考生需要通过实践和理论相结合的方式，加深对这些概念的理解，以确保在考试中取得成功。在备考过程中，访问Cisco.com获取最新的考试大纲更新，持续学习和练习是必不可少的。

Integrated Services Digital Network (ISDN)

z Synchronous Data Link Control (SDLC)

z High-Level Data Link Control (HDLC)

z Point-to-Point Protocol (PPP)

z Frame Relay

z Ethernet version 2

z Bridge protocol data units (BPDUs) in the Spanning Tree Protocol (STP)

Layer 3: The Network Layer

The network layer determines the best path to a destination. Device addressing, packet fragmentation, and routing all occur at the

network layer. Information at this layer is processed in what are commonly known as packets. Examples of network layer protocols

include the following:

z Internet Protocol (IP)

z Internetwork Packet Exchange (IPX)

Routing protocols, such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border

Gateway Protocol (BGP), provide the information required to determine the topology of the internetwork and the best path to a remote

destination. A routed protocol is one that is transported by a routing protocol (such as Routing Information Protocol [RIP]). For

example, IP is a routed protocol that can be advertised by a number of routing algorithms, such as RIP, OSPF, and BGP. The Layer-3

field format of protocol type defines to the higher layers what protocol is being carried inside the IP packet. For example, OSPF has an

IP protocol number of 89; EIGRP has an IP protocol number of 88. Technically, OSPF and EIGRP are not Layer-3 mechanisms.

Note

Layer 3 protocols, such as IP, are commonly referred to as connectionless protocols, whereas Layer 4 protocols, such as TCP, are

commonly referred to as connection-oriented protocols.

A connection-oriented protocol, such as TCP, ensures delivery of all information, whereas a connectionless protocol, such as IP,

packages the data but sends it without guaranteeing delivery. Connection-oriented protocols exchange control information (also called

Handshake) before transmitting data. A telephone call can be considered a connection-oriented service because the call is established

before conversation can take place, much the same way that TCP sets up a data connection before data is sent. FTP is another example

of a connection-oriented protocol.

Layer 4: The Transport Layer

The transport layer is responsible for segmenting upper-layer applications and establishing end-to-end connections between devices.

Other transport layer functions include providing data reliability and error-free delivery mechanisms. Information at this layer is

processed in what are commonly known as segments. Examples of transport layer protocols include the following:

z Transmission Control Protocol (TCP)

z Real-Time transport protocol (RTP)

z User Datagram Protocol (UDP)

RTP has some important properties of a transport Layer-4 protocol; however, it also runs on end systems. RTP differs from transport

protocols like TCP in that it (currently) does not offer any form of reliability or a protocol-defined flow/congestion control. IP voice is

an example of RTP operating at Layer 4 of the OSI model.

Layer 5: The Session Layer

The session layer performs several major functions, including managing sessions between devices and establishing and maintaining

sessions. Examples of session layer protocols include the following:

z Database SQL

z NetBIOS Name Queries

z H.323 (supports video as well; it is the packet-switched voice standard)

z Real-Time Control Protocol (RTCP)

Layer 6: The Presentation Layer

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The presentation layer handles data formats and code formatting. The layer's functions are normally transparent to the end user because

this layer takes care of code formats and presents them to the application layer (Layer 7), where the end user can examine the data.

Examples of presentation layer protocols include the following:

z Graphics Interchange Format (GIF)

z Joint Photographic Experts Group (JPEG)

z American Standard Code for Information Interchange (ASCII)

z Moving Picture Experts Group (MPEG)

z Tagged Image File Format (TIFF)

z Musical Instrument Digital Interface (MIDI)

z Hypertext Markup Language (HTML)

Layer 7: The Application Layer

The application layer is closest to the end user, which means that the application will be accessed by the end user. This layer's major

function is to provide services to end users. Examples of application layer services include the following:

z File Transfer Protocol (FTP)

z Telnet

z Ping

z Trace route

z Simple Mail Transfer Protocol (SMTP)

z Mail clients

TCP/IP and OSI Model Comparison

TCP/IP is the most widely used networking protocol and is often compared to the industry-defined OSI model.

Figure 1-2 displays the TCP/IP model in relation to the OSI model and shows where the protocol suite of TCP/IP lines up with the ISO

standard. This comparison is provided to demonstrate that TCP/IP does not conform exactly to the OSI model. For example, the TCP/IP

model has no Layer 5 or 6.

Figure 1-2. OSI and TCP/IP Models

Example of Peer-to-Peer Communication

Each layer of the OSI or TCP model has its own functions and interacts with the layer above it and layer below it. Furthermore, the

communication between each layer's end devices also establishes peer-to-peer communication; this means that each layer of the OSI

model communicates with the corresponding peer. For example, Layer 3 of Host A in Figure 1-3 will communicate with the

corresponding Layer 3 (IP) device host B.

Figure 1-3. Peer-to-Peer Communication Example

[View full size image]

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Consider the normal communication that occurs between two IP hosts over a WAN running Frame Relay, as displayed in Figure 1-3.

The data from Host A is encapsulated inside a TCP header and passed down to Layer 3 (the IP layer) for address configuration, where

an IP header is also added. Information included here is the source IP address and destination address. Layer 3 (the network layer)

passes the data to the local router acting as the gateway via the Ethernet connection in raw binary.

Router A strips the 802.3 header and encapsulates the IP, TCP, and data in a Frame Relay packet for delivery over the WAN. A CRC is

added here to ensure the packet is not corrupted over the WAN. Because Frame Relay is connectionless, if an error occurs, it's up to the

upper layers to retransmit; Frame Relay will not retransmit the packet. Similarly, HDLC (Layer 2 protocol) is connectionless and

depends on upper layers to resubmit damaged data packets. PPP (connection-oriented), on the other hand, resubmits packets damaged in

transmission over the WAN.

Router B receives the Layer 2 frames, strips the Frame Relay header/CRC, and encapsulates the IP, TCP, and data frame back into an

802.3 header (with its own CRC, Ethernet checks only for errors and cannot repair them; once more, upper layers, such as TCP, ensure

data delivery) for binary transmission across the Ethernet to Host B. The data is passed up the layers through IP, TCP, and finally to the

application, where the application layer reads and acts upon the data.

The good news for security candidates is that Token Ring and legacy technologies are not covered in the written exam, so this chapter

concentrates only on Ethernet switching. Before covering switching, the next section summarizes the evolution of Ethernet so that you

are aware of the standards that have developed since Xerox Corporation first introduced Ethernet.

Ethernet Overview

Ethernet networks are based on a development made by Xerox, Digital Equipment Corporation, and Intel Corporation. The two versions

of Ethernet are commonly referred to as Ethernet I and Ethernet II (or version 2).

Ethernet uses carrier sense multiple access collision detection (CSMA/CD) to transmit frames on the wire. In an Ethernet environment,

all hosts can transmit as long as no other devices are transmitting. CSMA/CD is used to detect and warn other devices of any collisions,

and colliding stations use a backoff algorithm and wait a random amount of time before trying again. Colliding devices send a jam

signal to advise all stations that a collision has occurred. When a jam signal is sent (a jam signal is detected by all devices, because the

voltage is that of the combined colliding devices), all stations also stop transmitting. A device attempts to transmit up to 16 times before

a user is notified of the collisions; typically, an application error informs the user that data could not be delivered. Microsoft's famous

words are "Network is busy."

Note

The only situation in which CSMA/CD is not used is in a full-duplex connection, because collisions are not possible when one pair of

unshielded twisted-pair cable (UTP, the physical cable connection) is used to transmit data (one pair of twisted-pair cable) and receive

data (a second pair of twisted-pair cable). In other words, devices connected in full-duplex mode can send and receive data at the same

time without the possibility of collision.

Table 1-2 lists some of the common Ethernet media specifications and the characteristics of each.

Table 1-2. Ethernet Media Formats

Media Type Characteristics

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