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CCNA: Fast Pass phần 9

Điều khiển liên kết logic (LLC) 802,2 này lớp con có trách nhiệm xác định các giao thức lớp mạng và sau đó đóng gói chúng. Một tiêu đề LLC cho các lớp liên kết dữ liệu phải làm gì với một gói tin sau khi nhận được khung dữ. Nó hoạt động như thế này: máy chủ nhận được một khung hình và nhìn vào | 296 Chapter 4 Technology Logical Link Control LLC 802.2 This sublayer is responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this a host receives a frame and looks in the LLC header and finds out that the packet is destined for say the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits. Data-link layer devices Switches and bridges both work at the Data Link layer and filter the network using hardware MAC addresses. Layer 2 switching is considered hardware-based bridging because it uses specialized hardware called an application-specific integrated circuit ASIC . ASICs can run up to gigabit speeds with very low latency rates. Latency is the time measured from when a frame enters a port to the time it exits. Bridges and switches read each frame as it passes through the network. The Layer 2 device then puts the source hardware address in a filter table and keeps track of which port the frame was received on. This information logged in the bridge s or switch s filter table is what helps the machine determine the location of the specific sending device. The real estate business is all about location location location and it s the same way for both Layer 2 and 3 devices. Though both need to be able to negotiate the network it s crucial to remember that they re concerned with very different parts of it. Primarily routers or Layer-3 machines need to locate specific networks whereas Layer 2 machines switches and bridges need to locate specific devices. So networks are to routers as individual devices are to switches and bridges. And routing tables that map the internetwork are for routers as filter tables that map individual devices are for switches and bridges. After a filter table is built on the Layer 2 device it will only forward frames to the segment where the destination hardware .