[CCNA] CCNA1 - Chapter5

What happens to runt frames received by a Cisco Ethernet switch?

• In an attempt to conserve bandwidth and not forward useless frames, Ethernet devices drop frames that are considered to be runt (less than 64 bytes) or jumbo (greater than 1500 bytes) frames.

Ethernet frame

• The minimum Ethernet frame is 64 bytes.
• The maximum Ethernet frame is 1518 bytes.
• A network technician must know the minimum and maximum frame size in order to recognize runt and jumbo frames.

Ethernet

• Ethernet is the most common LAN protocol in the world.
• It operates at Layer 1 and 2, but is not required for Internet communication.

OSI model

• The OSI model is used to describe how networks operate.

WAN

• A WAN connects multiple sites located in different countries.

LLC

• Logical link control is implemented in software and enables the data link layer to communicate with the upper layers of the protocol suite.
• Logical link control is specified in the IEEE 802.2 standard.

MAC (Media Access Control) sublayer

• The MAC (Media Access Control) sublayer is responsible for the placement and retrieval of frames on and off the media.
• The MAC sublayer is also responsible for adding a header and a trailer to the network layer protocol data unit (PDU).

MAC address

• A MAC address is composed of 6 bytes.
• The first 3 bytes are used for vendor identification and the last 3 bytes must be assigned a unique value within the same OUI.
• MAC addresses are implemented in hardware.
• A NIC needs a MAC address to communicate over the LAN.
• The IEEE regulates the MAC addresses.
• A switch builds a MAC address table by inspecting incoming Layer 2 frames and recording the source MAC address found in the frame header.
• The discovered and recorded MAC address is then associated with the port used to receive the frame.

store-and-forward switching

• always stores the entire frame
• checks the CRC before forwarding
• checks the frame length before forwarding
• When the store-and-forward switching method is used, the switch receives the complete frame before forwarding it on to the destination.
• The cyclic redundancy check (CRC) part of the trailer is used to determine if the frame has been modified during transit.

cut-through switch

• low latency
• may forward run frames
• begins forwarding when the destination address is received
• cut-through switch forwards the frame once the destination Layer 2 address is read.
• Two types of cut-through switching methods are fast-forward and fragment-free.

Auto-MDIX

• Auto-MDIX is a feature that is enabled on the latest Cisco switches and that allows the switch to detect and use whatever type of cable is attached to a specific port.​​

port-based memory buffering

• Frames are buffered in queues linked to specific ports.
• frames are stored in queues that are linked to specific incoming and outgoing ports.

ARP spoofing attack

• In an ARP spoofing attack, a malicious host intercepts ARP requests and replies to them so that network hosts will map an IP address to the MAC address of the malicious host.

RUNT frame

• A runt frame is a frame of fewer than 64 bytes, usually generated by a collision or a network interface failure.

potential network problems that can result from ARP operation

• On large networks with low bandwidth, multiple ARP broadcasts could cause data communication delays.
• Network attackers could manipulate MAC address and IP address mappings in ARP messages with the intent of intercepting network traffic.

The switches are in their default configuration. Host A needs to communicate with host D, but host A does not have the MAC address for its default gateway. Which network hosts will receive the ARP request sent by host A?

• Since host A does not have the MAC address of the default gateway in its ARP table, host A sends an ARP broadcast.
• The ARP broadcast would be sent to every device on the local network.
• Hosts B, C, and router R1 would receive the broadcast.
• Router R1 would not forward the message.

ARP tables

• The switch ARP table keeps a mapping of Layer 2 MAC addresses to Layer 3 IP addresses.
• These mappings can be learned by the switch dynamically through ARP or statically through manual configuration.