1、1啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊(fm.DocWiki CISCO / zslcn编译注)四Bridging and Switching Basics 桥接和交换的基本原理http:/ page was last modified on 17 December 2009, at 21:50四Bridging and Switching Basics 桥接和交换的基本原理From DocWikiJump to: navigation, searchThis article introduces the technologies employed in devic
2、es loosely referred to as bridges and switches. Topics summarized here include general link layer device operations, local and remote bridging, ATM switching, and LAN switching. Chapters in Part 5, “Bridging and Switching,” address specific technologies in more detail.本章介绍用于,不严格地称为桥和交换机的设备,的技术。综述在这里
3、的论题包括一般链路层设备的操作，本地和远程的桥接，ATM 交换，和 LAN交换。在手册第 5部分“桥接和交换“的各章中将更详细地讲述具体技术。Contentshide1 What Are Bridges and Switches? 什么是网桥和交换机？2 Link Layer Device Overview 链路层设备概述3 Types of Bridges 网桥的类型3.1 Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas 图 3.1 本地和远程网桥在指定区连接局域网段3.2 Figure: A M
4、AC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 Networks 图 3.2 MAV层网桥连接 IEEE 802.3和 IEEE 802.5网络4 Types of Switches 交换机的类型4.1 ATM Switch ATM交换机Contents联网技术手册 指导目录Part 1 Internetworking Basics 互联网络联网基本原理Part 2 LAN Technologies 局域网技术part 3 WAN Technologies广域网技术part 4 Internet Protocols 互联网协议par
5、t 5 Bridging and Switching 桥接和交换part 6 Routing 路由part 7 Network Management 网络管理part 8 Voice/Data Integration Technologies 声音/数据集成技术part 9 Wireless Technologies 无线技术part 10 Cable Access Technologies 有线接入技术part 11 Dial-up Technology 拨号技术part 12 Security Technologies 安全技术part 13 Quality of Service Netw
6、orking 联网服务质量part 14 Network Caching Technologies 网络缓存技术part 15 IBM Network Management IBM网络管理part 16 Multiservice Access Technologies 多业务接入技术24.1.1 Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells 图 4.1.1 多局域网交换信元时可以使用基于 ATM的骨干网4.2 LAN Switch 局域网交换机4.2.1 Figure: A LAN S
7、witch Can Link 10-Mbps and 100-Mbps Ethernet Segments 图 4.2.1 局域网交换机可以连接 10-Mbps和 100-Mbps以太网段5 Review Questions 复习问答6 For More Information 参阅更多信息1. What Are Bridges and Switches? 什么是桥和交换机?Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference m
8、odel. As such, they are widely referred to as data link layer devices. 桥和交换机都是通信设备,它们主要是在 OSI参考模型的第二层上操作,所以它们被广泛地称之为数据链路层设备。Bridges became commercially available in the early 1980s. At the time of their introduction, bridges connected and enabled packet forwarding between homogeneous networks. More
9、recently, bridging between different networks has also been defined and standardized. 商业上使用桥始于 1980年初。当时,桥用来在同质的网络间连接并实现数据包转发。最近,在不同网络间的桥接,已经被定义和标准化了。 Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while s
10、ource-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridg
11、ing and source-route bridging to enable communication in mixed Ethernet/Token Ring environments. 有几种桥接已经被证实很重要,就像互联网络联网设备那样。透明的桥接主要在以太网环境中能找到,而源路由桥接则主要发生在令牌环网的环境中。透明的桥接提供格式间的转换和不同媒体类型(通常是以太网和令牌环网)的转送规则。源路由透明桥接则是组合了上述两种桥接的算法,以实现在以太网/令牌环网的混合环境中进行通信。Today, switching technology has emerged as the evolut
12、ionary heir to bridging-based internetworking solutions. Switching implementations now dominate applications in which bridging technologies were implemented in prior network designs. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed t
13、o the emergence of switches as replacement technology for bridges and as complements to routing technology. 今天,交换技术已经显露成为基于桥接的互联网络联网解案的革命性的继承技术。交换实现现在主要的应用是在那些以前的网络设计中曾用桥接技术实现的地方。超级的吞吐性能,较高的接口密度,更低的单口代价,和更大的灵活性等,由于交换机 s作为桥接的替代技术和路由技术的补充而出现了。2. Link Layer Device Overview 链路层设备概述Bridging and switching
14、 occur at the link layer, which controls data flow, handles transmission errors, provides physical (as opposed to logical) addressing, and manages access to the physical medium. Bridges provide these functions by using various link layer protocols that dictate specific flow control, error handling,
15、addressing, and media-access algorithms. Examples of popular link layer protocols include Ethernet, Token Ring, and FDDI. 桥接和交换发生在链路层。链路层控制数据流,处理传送错误,提供物理寻址,以及管理对物理介质的访问。桥提供这些功能是通过使用各种链路层协议来实现的。协议安排特殊的流量控制,错误处理,寻址,和媒体访问算法。流行的链路层协议的例子包括以太网,令牌环网,和 FDDI。, Bridges and switches are not complicated device
16、s. They analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames 3toward the destination. In some cases, such as source-route bridging, the entire path to the destination is contained in each frame. In other cases, such as transparent br
17、idging, frames are forwarded one hop at a time toward the destination. 桥和交换机并非复杂设备。它们解析收入的帧,根据包含在帧中的信息,作出转发决策,并将帧向目的地转发。在某些情况下,如源路由桥接,到目的地的整个通路都包含在每个帧中。在其他情况下,如透明桥接,帧是向着目的地,一次转发一跳。Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types opera
18、te at the link layer, they are not required to examine upper-layer information. This means that they can rapidly forward traffic representing any network layer protocol. It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic between two or more networks. 对上层协议的透明性是
19、桥接和交换的主要优点。由于两种类型的设备都操作在链路层,它们并不需要检查上层信息。这意味着它们可以迅速转发表达任何网络层协议的信流。对于一个桥而言,在两个或更多的网络间移动 AppleTalk, DECnet, TCP/IP, XNS, 和其它信流并不罕见。 Bridges are capable of filtering frames based on any Layer 2 fields. For example, a bridge can be programmed to reject (not forward) all frames sourced from a particular
20、network. Because link layer information often includes a reference to an upper-layer protocol, bridges usually can filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcast and multicast packets. 桥有能力过滤基于任何层 2字段的帧。例如,一个桥可以编程以拒绝(不转发)源自一个具体网络的所有帧。由于链路层信息通常包括一个
21、对一个上层协议的引用,桥常可过滤该参数。此外,过滤器可以帮助处理不必要广播和组播数据包。 By dividing large networks into self-contained units, bridges and switches provide several advantages. Because only a certain percentage of traffic is forwarded, a bridge or switch diminishes the traffic experienced by devices on all connected segments. T
22、he bridge or switch will act as a firewall for some potentially damaging network errors and will accommodate communication between a larger number of devices than would be supported on any single LAN connected to the bridge. Bridges and switches extend the effective length of a LAN, permitting the a
23、ttachment of distant stations that was not previously permitted. 通过把大网络划分成自含单元,桥和交换机就能提供若干优点。因为只有部分信流被转发,一个桥或交换机就减少了在所有连接的网段上的设备不必要再历经信流。桥和交换机可以作为防火墙用于防止某些潜在地损坏网络的错误,容许大量设备间的通信,而不必对连接到桥的单个 LAN,一个一个地支持。桥和交换机扩展了 LAN的有效长度,允许附加以前不允许的远处站点。Although bridges and switches share most relevant attributes, seve
24、ral distinctions differentiate these technologies. Bridges are generally used to segment a LAN into a couple of smaller segments. Switches are generally used to segment a large LAN into many smaller segments. Bridges generally have only a few ports for LAN connectivity, whereas switches generally ha
25、ve many. Small switches such as the Cisco Catalyst 2924XL have 24 ports capable of creating 24 different network segments for a LAN. Larger switches such as the Cisco Catalyst 6500 can have hundreds of ports. Switches can also be used to connect LANs with different media-for example, a 10-Mbps Ether
26、net LAN and a 100-Mbps Ethernet LAN can be connected using a switch. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. Finally, switches reduce collisions on network segments because they prov
27、ide dedicated bandwidth to each network segment. 虽然桥和交换机大多数有关的属性,若干特帧区分了这些技术。桥通常用于将 LAN分段成几个较小段。桥一般只有几个端口用于 LAN连接,而交换机通常有很多。小交换机如 2924XL有 24端口,对一个 LAN有能力产生24段不同的网络段。大交换机如 6500可有百数端口。交换机也可以用于将 LAN与不同的媒体相连接-例如,一个 10Mbps和一个 100Mbps以太网 LANs可以使用一个交换机连接起来。某些交换机支持交换切断,减少了网络中的潜伏和延迟,而桥只支持存储-和-转发信流交换。最后,交换机减少
28、了网络段上的碰撞,因为它们提供了专用的带宽给每个网络段。 43. Types of Bridges 桥的种类Bridges can be grouped into categories based on various product characteristics. Using one popular classification scheme, bridges are either local or remote. Local bridges provide a direct connection between multiple LAN segments in the same area
29、. Remote bridges connect multiple LAN segments in different areas, usually over telecommunications lines. 桥可以根据各种产品的特性组成类别。使用一个流行的分类方案,桥可以分成本地的或远处的。本地桥 s在同一域的多个 LAN段之间提供一个直接连接。远处桥在不同域中,通常在电信线路上,连接多个 LAN段。图4-1举例说明了这两种配置。 Figure: Local and Remote Bridges Connect LAN Segments in Specific Areas illustra
30、tes these two configurations. Figure: Local and Remote Bridges Connect LAN Segments in Specific AreasRemote bridging presents several unique internetworking challenges, one of which is the difference between LAN and WAN speeds. Although several fast WAN technologies now are establishing a presence i
31、n geographically dispersed internetworks, LAN speeds are often much faster than WAN speeds. Vast differences in LAN and WAN speeds can prevent users from running delay-sensitive LAN applications over the WAN. 远程桥接存在若干少见的互联网络联网的挑战,其中之一是 LAN和 WAN间速度不同。虽然在地理上分布的互联网络 s中,若干快速 WAN技术现在正在出现,然而 LAN的速度通常总是十分快
32、于 WAN速度。LAN 与 WAN在速度上的巨大差别可能阻止用户在 WAN上运行 LAN的应用。Remote bridges cannot improve WAN speeds, but they can compensate for speed discrepancies through a sufficient buffering capability. If a LAN device capable of a 3-Mbps transmission rate wants to communicate with a device on a remote LAN, the local bri
33、dge must regulate the 3-Mbps data stream so that it does not overwhelm the 64-kbps serial link. This is done by storing the incoming data in onboard buffers and sending it over the serial link at a rate that the serial link can accommodate. This buffering can be achieved only for short bursts of dat
34、a that do not overwhelm the bridges buffering capability. 远程桥不能改善 WAN的速度,然而它们通过其足够的缓冲能力来补偿速度上的(巨大)差别。例如,The Institute of Electrical and Electronic Engineers (IEEE) differentiates the OSI link layer into two separate sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC
35、) sublayer. The MAC sublayer permits and orchestrates media access, such as contention and token passing, while the LLC sublayer deals with framing, flow control, error control, and MAC sublayer addressing. IEEE将 OSI链路层分成两个子层：MAC(媒体访问控制)子层和 LLC子层(逻辑链路控制)。MAC 子层允许和安排媒体访问,如竞争和令牌传递,而 LLC子层处理成帧,流量控制,错误控
36、制,和 MAC子层寻址。 Some bridges are MAC-layer bridges, which bridge between homogeneous networks (for example, IEEE 802.3 and IEEE 802.3), while other bridges can translate between different link layer protocols (for example, IEEE 802.3 and IEEE 802.5). 某些桥是 MAC子层桥,处于同质网络之间(例如 IEEE 802.3和 IEEE 802.3),而其它桥
37、则处在不同链路层协议之间,(例如,IEEE 802.3 和 IEEE 802.5),桥中可以实现转换。这样的一种转换的基本机制图示在图 4.2中。The basic mechanics of such a translation are shown in Figure: A MAC-Layer Bridge Connects the 5IEEE 802.3 and IEEE 802.5 Networks. Figure: A MAC-Layer Bridge Connects the IEEE 802.3 and IEEE 802.5 NetworksThe above figure illu
38、strates an IEEE 802.3 host (Host A) formulating a packet that contains application information and encapsulating the packet in an IEEE 802.3-compatible frame for transit over the IEEE 802.3 medium to the bridge. At the bridge, the frame is stripped of its IEEE 802.3 header at the MAC sublayer of the
39、 link layer and is subsequently passed up to the LLC sublayer for further processing. After this processing, the packet is passed back down to an IEEE 802.5 implementation, which encapsulates the packet in an IEEE 802.5 header for transmission on the IEEE 802.5 network to the IEEE 802.5 host (Host B
40、). 图 4-2举例说明了一个连接主机 sIEEE 802.3和 IEEE 802.5的 MAC层桥的工作过程。主机(主机 A)将包含应用信息的数据包进行安排,用 IEEE 802.3兼容的帧格式包装数据包,然后经由 IEEE 802.3介质送至桥处。在桥的 MAC子层,帧IEEE 802.3头部被剥离,并上传到 LLC子层作进一步处理。处理后,数据包回传到 IEEE 802.5实现,封装IEEE 802.5头部,出桥,再经 802.5介质,传送到 802.5主机 B。 A bridges translation between networks of different types is n
41、ever perfect because one network likely will support certain frame fields and protocol functions not supported by the other network. 桥在不同类型网络间的转换是不会完美的,因为一个网络支持某种帧字段 s和协议功能 s,但不一定为其它网络所支持。4. Types of Switches 交换机 s的种类Switches are data link layer devices that, like bridges, enable multiple physical L
42、AN segments to be interconnected into a single larger network. Similar to bridges, switches forward and flood traffic based on MAC addresses. Any network device will create some latency. Switches can use different forwarding techniques-two of these are store-and-forward switching and cut-through swi
43、tching. 交换机 s是数据链路层设备 s,像桥 s那样,能使多个物理 LAN段连成单一大网络。也类似于桥 s,能按照 MAC6地址 s转发信流,也会淹没信流。任何网络设备都会产生某些潜伏。交换机 s可以使用不同的转发技术-其中的两个是转发交换和穿越交换。In store-and-forward switching, an entire frame must be received before it is forwarded. This means that the latency through the switch is relative to the frame size-the
44、larger the frame size, the longer the delay through the switch. Cut-through switching allows the switch to begin forwarding the frame when enough of the frame is received to make a forwarding decision. This reduces the latency through the switch. Store-and-forward switching gives the switch the oppo
45、rtunity to evaluate the frame for errors before forwarding it. This capability to not forward frames containing errors is one of the advantages of switches over hubs. Cut-through switching does not offer this advantage, so the switch might forward frames containing errors. Many types of switches exi
46、st, including ATM switches, LAN switches, and various types of WAN switches. 在存储-转发交换中,必须接收到完整的一帧才能转发。这意味着通过交换机的潜伏时间,帧越大,延迟就越长。穿越交换则允许交换机在接收到部分的,但已达到指定大小的帧时,就按转发决策转发。这样就减少了通过交换机的潜伏时间。存储-转发交换使交换机有机会在转发前检查帧的错误。比较集线器 s,这是交换机 s的优点之一。, 穿越交换则不提供这个优点,所以交换机可能转发包含错误的帧 s。有很多种类的交换机 s,包括 ATM交换机s,LAN交换机 s,以及各种 W
47、AN交换机 s。ATM Switch ATM交换机Asynchronous Transfer Mode (ATM) switches provide high-speed switching and scalable bandwidths in the workgroup, the enterprise network backbone, and the wide area. ATM switches support voice, video, and data applications, and are designed to switch fixed-size information un
48、its called cells, which are used in ATM communications. ATM(异步传输模式)交换机 s提供高速交换和可缩放带宽予工作组,企业网络骨干,和广域网。ATM 交换机 s支持声音,视频,和数据应用 s。ATM 设计了固定尺寸的信息单元,叫做信元,用于 ATM通信 s中作为交换的基本单元。图 4-3举例说明了一个企业网络,由多个 LAN互联,通越一个 ATM骨干网组成。 Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching Cells illustrates
49、 an enterprise network comprised of multiple LANs interconnected across an ATM backbone. Figure: Multi-LAN Networks Can Use an ATM-Based Backbone When Switching CellsLAN Switch LAN交换机LAN switches are used to interconnect multiple LAN segments. LAN switching provides dedicated, collision-free communication between network devices, with support for multiple simultaneous 7conversations. LAN switches are designed to switch data frames at high speeds. LAN交换机 s用于互联多个
