ccna r&s book pdf

ccna r&s book pdf.Store-and-forward receives the entire frame, checks the FCS, and then exchanges the frame. This increases the switching latency but reduces the number of invalid frames in the network. Bridges only support this method of switching, as do Catalyst 2960 Series switches. Straight-through switching starts after the frame's destination MAC address (the first 14 bytes) is received. Fragment-free switching begins after 64 bytes are received, which avoids exchanging stub frames.Hands-On ExperienceAlthough the Catalyst 2960 Series switches do not support straight-through switching and fragment-free switching like the 1900 Series switches, their frame switching speed is still faster (500,000 fps for the 1900 Series switches) because the ASICs of the 2960 Series switches are much faster than the ASICs of the 1900 Series switches. Therefore, a switch should not be judged solely on the basis of the supported switching method, but rather on a combination of factors such as price, performance, and features.4.3.2 Duplex ModeDuplex mode determines the way in which a device can send and receive frames over Ethernet. There are two types of duplex modes: half-duplex and full-duplex. In half-duplex mode, the device either sends data or receives data, and cannot send and receive data at the same time. Half-duplex mode is used for shared media, such as 10Base2, 10Base5, and Ethernet hub connections. In this environment, when one device in the conflict domain sends data, all other devices listen and receive it. In such a shared environment, Ethernet segment utilization is typically 40%.60%; note, however, that utilization varies with each case, and 40% to 60% refers to the general case.Practical experience In a half-duplex environment, if network utilization is below 40%-60%, or if conflicts exceed 2% of the total traffic, consider using full-duplex mode, increasing the speed of the link (using Fast Ethernet or Gigabit Ethernet), or using a switch to split the conflict domain.Unlike half-duplex, full-duplex allows devices to send and receive frames simultaneously, but only when there are only two devices on the connection (such as connecting a PC to a switch or a switch to a router). This is referred to as a point-to-point connection. In a full-duplex connection, a hub cannot be used. To establish a full-duplex connection, both devices must support full-duplex. Table 4.2 compares half-duplex and full-duplex.Exam Points Keep in mind the information listed in Table 4.2 q Also, don't forget that full duplex provides higher bandwidth than half duplex because both devices can transmit and receive data at the same time without causing conflicts.As Table 4.2 points out, one of the main advantages of a full-duplex connection over a half-duplex connection is that no conflicts occur. Basically, the transmit circuit at one end is connected to the receive circuit at the other end, and vice versa. In this case, the Ethernet NIC disables the conflict detection mechanism because it is not needed. The following media types all support full duplex connections: 1 OBaseT. 1 OOBaseTX. 1 OOBaseFX and Gigabit Ethernet. Connections using 10Base5, lOBaseFL, and 10Base2 support only half-duplex. Note that some older 1 OBaseTNICs may not support full duplex; one such example is the 1OBaseT interface of the Cisco 2500 series router, which supports only half duplex (the 2500 series router has been retired and is no longer sold by Cisco).Table 4.2 compares full-duplex and half-duplex half-duplex full-duplex