CCNA ITN: Module 7

Ethernet

family of LAN technologies using wired communication defined by IEEE 802.2 and 802.3

LLC sublayer

places info in frame to identify network layer protocol

MAC sublayer

handles data encapsulation, addressing, error detection, and media access control

modern Ethernet

operates in full-duplex, legacy uses CSMA/CD

invalid Ethernet frames

less than 64 bytes (runt) or more than 1500 bytes (jumbo/baby giant)

Preamble and SFD

8-byte fields for synchronization and attention of receiving node

Destination MAC Address

6-byte field identifying the intended receiver (unicast, multicast, broadcast)

Source MAC Address

6-byte field identifying sender

Type/Length field

2-byte field identifying encapsulated upper-layer protocol

Data field

contains 46–1500 bytes of payload, may be padded

FCS field

4-byte field for CRC error detection

MAC address

48-bit unique binary value shown in 12 hexadecimal digits

MAC uniqueness

combination of IEEE OUI and vendor-assigned value

frame delivery

only matching destination MAC processes the frame, others discard it

unicast frame

has one destination MAC

broadcast MAC address

FF-FF-FF-FF-FF-FF, flooded out all ports, not forwarded by routers

multicast frame

forwarded only if router is configured to allow it

switch forwarding decisions

based only on MAC address, protocol unaware

MAC address table

learned dynamically using source MAC and incoming port

MAC table timer

default timeout is 5 minutes for entries

unknown unicast

frame is forwarded out all ports except source port if destination is unknown

multiple MACs per port

occurs when switch is connected to another switch or multiple devices

store-and-forward switching

entire frame received and CRC checked before forwarding

cut-through switching

frame forwarded before full reception

fast-forward switching

forwards after destination address read, lowest latency

fragment-free switching

stores first 64 bytes to avoid forwarding collisions

port-based buffering

each port has its own queue, one frame can delay others

shared memory buffering

common memory space dynamically allocated as needed

auto-negotiation

automatically selects best speed and duplex settings

auto-MDIX

auto-detects cable type (crossover or straight-through) and configures ports

mdix auto

command to enable auto-MDIX

best cable practice

use correct cable type even if auto-MDIX is enabled