Note A bridge has a table used in filtering decisions. 网桥工作在物理层和数据链路层 作为物理层设备时，重新生成收到的信号 ●作为数据链路层设备时，可以检查帧所包含的 MAC地址。 网桥具有过滤作用，检查帧的目的地址 网桥具有一个用作过滤决策的表 15.11
15.11 A bridge has a table used in filtering decisions. Note 网桥工作在物理层和数据链路层 作为物理层设备时，重新生成收到的信号 作为数据链路层设备时，可以检查帧所包含的 MAC地址。 网桥具有过滤作用，检查帧的目的地址 网桥具有一个用作过滤决策的表
Figure 15.5 A bridge connecting two LANs 5 5 4 3 2 32 Address Port 71:2B:13:45:61:41 71:2B:13:45:61:42 Bridge Table 64:2B:13:4561:12 2 64:2B:13:45:61:13 2 71:2B:13:45:61:4171:2B:13:45:61:42 64:2B:13:45:61:1264:2B:13:45:61:13 Bridge LAN 1 LAN2 15.12
15.12 Figure 15.5 A bridge connecting two LANs
Note A bridge does not change the physical (MAC)addresses in a frame. 0 网桥不改变帧中的MAC地址 透明网桥是站点完全不知道桥的存在 802.1d规范了透明网桥的标准 。帧必须能从一个站点转发到另外一个站点 。学习帧中的地址，自动建立转发表 避免形成循环Loop)问题 15.13
15.13 A bridge does not change the physical (MAC) addresses in a frame. Note 网桥不改变帧中的MAC地址 透明网桥是站点完全不知道桥的存在 802.1d规范了透明网桥的标准 帧必须能从一个站点转发到另外一个站点 学习帧中的地址，自动建立转发表 避免形成循环(Loop)问题
Figure 15.6 A learning bridge and the process of learning LAN1☐ Bridge LAN2☐ LAN 3 Address Port Address Port Address Port Address Port A A A 1 E B 1 a.Original b.After A sends C.After E sends d.After B sends a frame to D a frame to A a frame to C 15.14
15.14 Figure 15.6 A learning bridge and the process of learning
Figure 15.7 Loop problem in a learning bridge AIDI LAN1 LAN 1 Address port Bridge 1 Bridge 2 Address port Address port Bridge 1 Bridge 2 Address port A1 2 LAN 2 AD LAN 2 c" D AD☐ D Two copies of the frame on LAN 2 Station A sends a frame to station D Both bridges forward the frame A Two copies of A the frame on LAN 1 B AD AD LAN1 LAN 1 1 Address port Address port Address port Address port Bridge 1 Bridge 2 Bridge 1 Bridge 2 A 2 A2 A1 2 2 2 C LAN 2 AD☐ AD☐ LAN 2 Two copies of the frame on LAN 2 Both bridges forward the frame d. Both bridges forward the frame 15.15
15.15 Figure 15.7 Loop problem in a learning bridge