ccna qos pdf

ccna qos pdf.PC-A PC-BAt the network layer, one of the functions of the router is to route packets to their destinations. To do this, the router looks at the destination in the packetdestination logical address and extracts the network number from it. The router then compares that network number with the entry in its routing table. If no matching entry is found, the router discards the packet; if it finds one, it forwards the packet out the outbound interface (the local interface specified in the routing table).To forward the packet, the router passes it down to the data-link layer, which encapsulates it using the correct data-link layer frame format. For example, in the case of an Ethernet frame, the source MAC address will be the router's MAC address, and the destination MAC address will be the MAC address of the PC. Next, the data link layer passes the frame down to the physical layer, which in turn converts the bits into physical layer signals.When sending data to devices located in different network segments, the source MAC address of the Layer 2 frame is the MAC address of the source device, and the destination MACaddress is the MAC address of the default gateway; the source address of a Layer 3 packet is the Layer 3 address of the source device, but the destination address is not the defaultgateway's Layer 3 address, but the destination address is not the Layer 3 address of the default gateway, but the Layer 3 address of the actual destination. Don't forget that the Layer 2 address is used for packets that are in the same physicalor logical network segment/network, while Layer 3 addresses are used to communicate between devices located on different networks (segments).In other words, the MAC address may change when data is moved from one link to another, but the Layer 3 logical address does not.Note that routers are used to separate physical or logical network segments, whereas bridges and switches have no such function. Therefore, when sending data to PC-B, PC-A uses the router's MAC (layer 2) address to move the data to the exit of the current segment, while using PC-B's logical (layer 3) address to tell the router that the data is not being sent to it, but to a device in another segment. This process is discussed in more detail in Chapters 3 and 9.