Bài giảng Mạng máy tính căn bản: Chương 8 - Phan Vĩnh Thuần

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Bài giảng Mạng máy tính căn bản - Chương 8: Ethernet switching giúp người học xác định cầu nối và chuyển đổi, xác định và mô tả các nội dung địa chỉ bộ nhớ (CAM), xác định độ trễ, mô tả lưu trữ và chuyển tiếp qua chuyển đổi chế độ, xác định va chạm, chương trình phát sóng, lĩnh vực va chạm và các lĩnh vực phát sóng.

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Nội dung Text: Bài giảng Mạng máy tính căn bản: Chương 8 - Phan Vĩnh Thuần

Chương 8: ETHERNET SWITCHING Overview • Shared Ethernet works extremely well under ideal conditions. When the number of devices trying to access the network is low, the number of collisions stays well within acceptable limits. Phan Vĩnh Thuần Computer Network 1
• when the number of users on the network increases, the increased number of collisions can cause intolerably bad performance. Bridging was developed to help ease performance problems that arose from increased collisions. Switching evolved from bridging to become the key technology in modern Ethernet LANs. Phan Vĩnh Thuần Computer Network 2
• The concept of collision domains and broadcast domains is concerned with the ways that networks can be designed to limit the negative effects of collisions and broadcasts. This module explores the effects of collisions and broadcasts on network traffic and then describes how bridges and routers are used to segment networks for improved performance. Phan Vĩnh Thuần Computer Network 3
• Students completing this module should be able to: – Define bridging and switching. – Define and describe the content-addressable memory (CAM) table. – Define latency. – Describe store-and forward and cut-through switching modes. – Explain Spanning-Tree Protocol (STP). – Define collisions, broadcasts, collision domains, and broadcast domains. – Identify the Layer 1, 2, and 3 devices used to create collision domains and broadcast domains. – Discuss data flow and problems with broadcasts. – Explain network segmentation and list the devices used to create segments. Phan Vĩnh Thuần Computer Network 4
8.1. Ethernet Switching 8.1.1 Layer 2 bridging • As more nodes are added to an Ethernet physical segment, contention for the media increases. Ethernet is a shared media, which means only one node can transmit data at a time. The addition of more nodes increases the demands on the available bandwidth and places additional loads on the media. Phan Vĩnh Thuần Computer Network 5
• By increasing the number of nodes on a single segment, the probability of collisions increases, resulting in more retransmissions. A solution to the problem is to break the large segment into parts and separate it into isolated collision domains. • To accomplish this a bridge keeps a table of MAC addresses and the associated ports. The bridge then forwards or discards frames based on the table entries. The following steps illustrate the operation of a bridge: Phan Vĩnh Thuần Computer Network 6
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– The bridge has just been started so the bridge table is empty. The bridge just waits for traffic on the segment. When traffic is detected, it is processed by the bridge. – Host A is pinging Host B. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the packet. – The bridge adds the source address of the frame to its bridge table. Since the address was in the source address field and the frame was received on port 1, the frame must be associated with port 1 in the table. Phan Vĩnh Thuần Computer Network 8
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– The destination address of the frame is checked against the bridge table. Since the address is not in the table, even though it is on the same collision domain, the frame is forwarded to the other segment. The address of Host B has not been recorded yet as only the source address of a frame is recorded. – Host B processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain. Both Host A and the bridge receive the frame and process it. Phan Vĩnh Thuần Computer Network 10
– The bridge adds the source address of the frame to its bridge table. Since the source address was not in the bridge table and was received on port 1, the source address of the frame must be associated with port 1 in the table. Phan Vĩnh Thuần Computer Network 11
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– Host A is now going to ping Host C. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the frame. Host B discards the frame as it was not the intended destination. – The bridge adds the source address of the frame to its bridge table. Since the address is already entered into the bridge table the entry is just renewed. Phan Vĩnh Thuần Computer Network 13
– The destination address of the frame is checked against the bridge table to see if its entry is there. Since the address is not in the table, the frame is forwarded to the other segment. The address of Host C has not been recorded yet as only the source address of a frame is recorded. – Host C processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain. Both Host D and the bridge receive the frame and process it. Host D discards the frame, as it was not the intended destination. Phan Vĩnh Thuần Computer Network 14
– The destination address of the frame is checked against the bridge table to see if its entry is present. The address is in the table but it is associated with port 1, so the frame is forwarded to the other segment. – The bridge adds the source address of the frame to its bridge table. Since the address was in the source address field and the frame was received on port 2, the frame must be associated with port 2 in the table. Phan Vĩnh Thuần Computer Network 15
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– When Host D transmits data, its MAC address will also be recorded in the bridge table • These are the steps that a bridge uses to forward and discard frames that are received on any of its ports. Phan Vĩnh Thuần Computer Network 17
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8.1.2. Layer 2 switching • Generally, a bridge has only two ports and divides a collision domain into two parts. All decisions made by a bridge are based on MAC or Layer 2 addressing and do not affect the logical or Layer 3 addressing. Thus, a bridge will divide a collision domain but has no effect on a logical or broadcast domain. Phan Vĩnh Thuần Computer Network 19
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