【理论】交换的基本工作原理.ppt

交换机的启动顺序 启动加载器是存储在 NVRAM中的小程序，并且在交换机第一次开 启时运行。 以太网交换机的功能 以太网的MAC地址 MAC地址有48位，但它通常被表示为12位的点分十六进制数。 MAC地址全球唯一，由 IEEE对这些地址进行管理和分配。每个地址由两部分组成，分别是供应商代码和序列号。其中前2 4位二进制代表该供应商代码。剩下的24位由厂商自己分配。 如果48位全是1，则表明该地址是广播地址。 如果第8位是1，则表示该地址是组播地址。 MAC地址表 交换机学习地址 交换机学习地址 过滤数据帧 过滤数据帧 广播帧和组播帧 HTTP://WWW.CISCO.COM * ? 2010 Cisco Systems.Inc All rights reserved. Cisco Systems v1.0 Emphasize: The next few slides discuss the basic function of a bridge/switch: 1. How it learns the location of the hosts by reading the source MAC address of incoming frames. 2. How it makes forwarding/filtering decisions. There are three conditions in which a switch will flood a frame out on all ports except to the port on which the frame came in, as follows: Unknown unicast address Broadcast frame Multicast frame 3. How STP is used to avoid loops in a switched/bridged network. Layer 3 of 3 Note: 64 bytes is the minimum Ethernet frame size. The command to switch the mode on the 1900 is: wg_sw_a(config)#switching-mode ? fragment-free Fragment Free mode store-and-forward Store-and-Forward mode Slide 1 of 3 Emphasize: The 1900en max MAC address table size is 1024. Once the table is full, it will flood all new addresses until existing entries age out. The command to change the MAC address table aging time is, as follows: 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. Slide 2 of 3 Slide 3 of 3 Emphasize: Once C replies, the switch will also cache station C’s MAC address to port E2, as shown in the next slide. 交换的基本工作原理 地址学习 转发和过滤数据帧 环路避免 00e0.fc39.8034 Cut-Through 交换机一旦检测到数据帧的目的MAC地址就立即开始转发帧,不做任何帧的校验. Fragment-Free 无碎片交换 数据帧经常在首部的64字节发生错误和冲突,如果头64字节没有CRC错误,则快速转发帧,这种方法一般认为是直通方式的一个特例. Store and Forward 在转发数据帧之前,首先完成帧的缓存和CRC校验,如果没有错误,再按照目标MAC转发数据帧 转发方式 初始化MAC地址表是空的 站点A发送一个数据帧到站点C； 交换机根据数据帧的源地址在接口E0上学习到站点A的MAC地址； 这个数据帧(A-C)被泛洪到所有的交换机接口