Luminous Preamble:
In the whole sky, among hundreds of millions of stars, look how small we are, however, our small, but it can see the whole expanse of sky.
text:
Link layer switch
The last century 90 late years, Ethernet uses a star topology-based hub.
Hub (Hub) is a physical layer device, it is not process the bit frame. When you represent a 0 or a 1 bit when the interface reaches a hub, which broadcasts this bit, the bit out to all other transmission interfaces. Thus, the hub has a star topology of Ethernet is a broadcast LAN .
Now Ethernet LAN star topology is connected using a central switch.
Frame arrival rate of any one of the output interface of the switch may temporarily exceed the capacity of the link interface. To solve this problem, switch the output interface has a cache, working in a manner very similar to the router interface.
Switch is Plug and Play devices, because they do not require a network administrator or user intervention .
The switch is duplex, which means that for any of the links connected to the switch node, the nodes and switches can be transmitted simultaneously without collision.
A switch forwarding and filtering
I decided to switch over is a special frame is forwarded to an interface should be discarded or function.
A frame forwarding decision which interface should be directed, and to move the frame to the function of these interfaces.
Switch filtering and forwarding by means of forwarding completed . The forwarding table includes LAN some nodes but not necessarily on the entry of all nodes. A table entry typically contains:
①节点的 MAC 地址，
②与节点相连的交换机接口，
③表项有效时间。
交换机转发分组基于 MAC 地址而不是基于 IP 地址。
交换机转发数据帧时，从 x 接口接收到帧，目的地 D，转发时可能存在 3 种情况:
如果表中没有表项将 D 与任何一个端口相连，那么进行广播。
如果 D 与端口 x 关联，说明目的地在 x 接口所在网段(考虑到交换机级联的情况，一个端口对应的表项可能有多个，就是说一个端口可能对应多个主机)则进行过滤，丢弃该帧。
如果表中有一个表项将 D 与一个除 x 以外的端口联系起来。在这种情况下，该帧需要被转发到与这个端口相连的 LAN 网段。交换机通过将该帧放到这个端口前面的输出缓存完成转发功能。
2 转发表的建立：自学习的过程
交换机的表是自动地、动态地、自治地建立的，即没有来自网络管理员或配置协议的任何干预。
以如下方式实现的:
1)交换机表初始为空。
2)对于在某接口接收到的每个入帧，该交换机在其表中存储:
①帧源地址字段中的 MAC 地址，②帧到达的接口，③当前的时间。
3)如果在一段时间后，交换机没有接收到以该 MAC 址作为源地址的帧，就在表中删除这个地址的表项。
3 交换机的性质
消除碰撞，在使用交换机构建的局域网中，没有因碰撞而浪费的带宽~~
交换机将异质的链路彼此隔离，局域网中的不同链路能够以不同的速率运行在不同介质