三层交换机和路由器的区别[转贴]

pxinyu

不过确实是个好东东！应该进入精华区！~

wolfop

原帖由 "tutuzhang" 发表：


现在是在说网桥，没有涉及到三层的东西。一知半解，好好学习学习！

呵呵，有人连广播风暴到底是那层引起的的都没有弄清楚还说别人一知半解，哈哈。连碰撞和广播都弄混淆的人，还要别人学习。
行了，你去问问随便一个通过CCNP 甚至CCIE的人，看看那个人敢在比试的时候选择bdrige 可以隔离broadcast。

苦行僧

原帖由 "tutuzhang" 发表：


现在是在说网桥，没有涉及到三层的东西。一知半解，好好学习学习！

交换机相当于多端口网桥，网桥隔离碰撞域， 能转发广播。

wolfop

原帖由 "pxinyu" 发表：
vlan 不是第三层的东西！网桥我没用过，不过看过点相关的东东。
好想不能隔离广播!只能对mac地址判断所属范围。

当然不能隔离广播，二层交换机盒网桥只有端口和MAC地址的对照表，当发生局域网广播时候，目标MAC地址是FFFFFFFFFFFF，网桥和没有划分VLAN的交换机会向所有没有被SPANNING TREE禁止的端口和源端口转发。

tutuzhang

《计算机网络》，第四版中的解释
A third issue is broadcasting. Most LANs support broadcasting, and many upper-layer protocols use this feature extensively. For example, when a user wants to send a packet to an IP address x, how does it know which MAC address to put in the frame? We will study this question in Chap. 5, but briefly summarized, the answer is that it broadcasts a frame containing the question: Who owns IP address x? Then it waits for an answer. And there are many more examples of where broadcasting is used. As more and more LANs get interconnected, the number of broadcasts passing each machine tends to increase linearly with the number of machines.

Related to broadcasts is the problem that once in a while a network interface will break down and begin generating an endless stream of broadcast frames. The result of this broadcast storm is that (1) the entire LAN capacity is occupied by these frames, and (2) all the machines on all the interconnected LANs are crippled just processing and discarding all the frames being broadcast.

At first it might appear that broadcast storms could be limited in scope by separating the LANs with bridges or switches, but if the goal is to achieve transparency (i.e., a machine can be moved to a different LAN across the bridge without anyone noticing it), then bridges have to forward broadcast frames.

Having seen why companies might want multiple LANs with restricted scope, let us get back to the problem of decoupling the logical topology from the physical topology. Suppose that a user gets shifted within the company from one department to another without changing offices or changes offices without changing departments. With hubbed wiring, moving the user to the correct LAN means having the network administrator walk down to the wiring closet and pull the connector for the user's machine from one hub and put it into a new hub.

In many companies, organizational changes occur all the time, meaning that system administrators spend a lot of time pulling out plugs and pushing them back in somewhere else. Also, in some cases, the change cannot be made at all because the twisted pair from the user's machine is too far from the correct hub (e.g., in the wrong building).

In response to user requests for more flexibility, network vendors began working on a way to rewire buildings entirely in software. The resulting concept is called a VLAN (Virtual LAN) and has even been standardized by the 802 committee. It is now being deployed in many organizations. Let us now take a look at it. For additional information about VLANs, see (Breyer and Riley, 1999; and Seifert, 2000).