ccna professional certification

ccna professional certification.link layer. Data link layer protocols and standards include IEEE802.2 and 802.3, EthernetII, HDLC, PPP, and Frame Relay.The data link layer detects errors, but does not correct them (instead of retransmitting, it simply discards the damaged frames).The data link layer is also responsible for defining the format of Layer 2 frames and the mechanism by which devices communicate through the physical layer. The data link layer is responsible for the following.. Defining the MAC (hardware) address; . Defining the physical (hardware) topology of the connection; . Defining how to encapsulate network layer protocols in data link layer frames; . Providing connectionless and connection-oriented services; . Checks the checksum of received frames to ensure that the frames are valid (and discards them if they are not). The data link layer does not normally provide connection-oriented (i.e., reliable) services (services that perform error detection and correction). o However, in the context of integrating the SystemsSystems Network Architecture (SNA) is used as a data link layer protocol, however, SNA can provide sequencing and flow control to provide guarantees for the transmission of data link layer frames. o SNA was developed by IBM to help devices in the LAN (primarily token rings) communicate with each other at the data link layer. In most cases, a reliable connection is provided by the transport layer.Don't forget that the primary function of the data link layer is to regulate how network devices using the same Layer 2 protocol communicate with each other. It also checks the checksum of incoming frames to ensure that the frames are valid. At the data link layer, frames are used to transport information to and from the port. If devices use different Layer 2 protocols (i.e., they are on different network segments), these devices will usually communicate with each other involving the network layer (Layer 3).Data Link Layer AddressingThe data link layer uses MAC (hardware) addresses to communicate. To communicate within a LAN, each host within the same segment or topology must have a unique MAC address, which is 48 bits long and expressed as a hexadecimal number. When using hexadecimal representation, such addresses are 12 characters long. For readability, MAC addresses are expressed in dotted hexadecimal format, similar to the following: FFFF.FFFF.FFFF; MAC addresses are also commonly expressed in the following format: FF:FF:FF:FF:FF:FF:FF O Since MAC addresses are expressed in hexadecimal digits, there are 16 possible values for each: 0.9 and A-Fo For example, the hexadecimal value A corresponds to to a decimal value of 10o hexadecimal will be discussed in more detail in Chapter 6. In addition to MAC addresses, there are other types of data link layer addressing; for example, Frame Relay uses Data Link Connection Identifiers (DLCIs), which are discussed in more depth in Chapter 25.The MAC address uniquely identifies the device at Layer 2. the MAC address must be unique only within the same broadcast domain, which contains all the conflicting domains of a Layer 2 connection The OMAC address allows devices located on the same physical network (Layer 2) to communicate with each other.The first 6 digits of the MAC address indicate the vendor (NIC manufacturer). These 6 numbers are often referred to as Organizational Unique Identifiers (OUIs) ,and each vendor has one or more unique OUIs. for example, Cisco has an OUI value of 0000.0C. the last 6 numbers are used to uniquely identify the NIC made by a particular vendoro In theory, each NIC's MAC address is unique; however, this may not be the case in practice. What is important to you is that the MAC address of each device's NIC must be unique within the same physical or logical network segment. A logical network segment is a VLAN, and is referred to as a broadcast domain, which is discussed in Chapter 12. Some devices allow this hardware address to be modified, while others do not.