tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 26

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 26 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 219 Tuesday May 27 2003 9 01 AM LAN Connection Devices 219 the geographical area past what a single LAN can support as shown in Figure 4-32. The devices that are used to connect network segments together include bridges switches routers and gateways. Switches and bridges operate at the data link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network. Bridges can also be used to connect dissimilar protocols and media as with wireless bridges interconnecting Ethernet LANs in a metropolitan area. Figure 4-32 Bridges Segmenting a Network Segment 1 Computer A Computer B Computer C 00-50-DA-0D-F5-2D 00-50-04-7C-2B-01 00-50-F1-12-8A-00 Segment 2 When a bridge receives a frame on the network the destination MAC address is looked up in the bridge table to determine whether to filter flood or copy the frame onto another segment. This decision process occurs as follows If the destination device is on the same segment as the frame the bridge blocks the frame from going on to other segments as shown in Figure 4-33. This process is known as filtering. If the destination device is on a different segment the bridge forwards the frame to the appropriate segment as shown in Figure 4-34. Page 220 Tuesday May 27 2003 9 01 AM 220 Chapter 4 Cable Testing and Cabling LANs and WANs Figure 4-33 Bridges Segmenting a Network Filtering In this example a data packet originates from Computer V and its destination is Computer Xc. The packet reaches its final destination and is not broadcast to other segments of the network. Figure 4-34 Bridges Segmenting a Network Forwarding F_ L_ Ct O U B A XX Rk Hh F O L B Ct XX Hh Q V Bh Xc In this example a data packet originates from Computer V and its destination is Computer Hh. The bridge checks its table to determine whether or not to allow the signal to continue to other segments of the network. Page 221 Tuesday May