for my own reference
plaigerized directly from the ccna
Ethernet Frame
Ethernet operates in the data link layer and the physical layer. Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.
Ethernet uses the LLC and MAC sublayers of the data link layer to operate.
Data encapsulation includes the following: Ethernet frame, Ethernet addressing, and Ethernet error detection.
Ethernet LANs use switches that operate in full-duplex.
The Ethernet frame fields are: preamble and start frame delimiter, destination MAC address, source MAC address, EtherType, data, and FCS.
Ethernet MAC Address
Binary number system uses the digits 0 and 1. Decimal uses 0 through 9. Hexadecimal uses 0 through 9 and the letters A through F.
The MAC address is used to identify the physical source and destination devices (NICs) on the local network segment.
MAC addressing provides a method for device identification at the data link layer of the OSI model.
An Ethernet MAC address is a 48-bit address expressed using 12 hexadecimal digits, or 6 bytes.
An Ethernet MAC address consists of a 6 hexadecimal vendor OUI code followed by a 6 hexadecimal vendor assigned value. When a device is forwarding a message to an Ethernet network,
the Ethernet header includes the source and destination MAC addresses. In Ethernet, different MAC addresses are used for Layer 2 unicast, broadcast, and multicast communications.
The MAC Address Table
A Layer 2 Ethernet switch makes its forwarding decisions based solely on the Layer 2 Ethernet MAC addresses.
The switch dynamically builds the MAC address table by examining the source MAC address of the frames received on a port.
The switch forwards frames by searching for a match between the destination MAC address in the frame and an entry in the MAC address table.
As a switch receives frames from different devices, it is able to populate its MAC address table by examining the source MAC address of every frame.
When the MAC address table of the switch contains the destination MAC address, it is able to filter the frame and forward out a single port.
Switch Speeds and Forwarding Methods
Switches use one of the following forwarding methods for switching data between network ports: store-and-forward switching or cut-through switching.
Two variants of cut-through switching are fast-forward and fragment-free. Two methods of memory buffering are port-based memory and shared memory.
There are two types of duplex settings used for communications on an Ethernet network: full-duplex and half-duplex. Autonegotiation is an optional function found on most Ethernet switches and NICs.
It enables two devices to automatically negotiate the best speed and duplex capabilities. Full-duplex is chosen if both devices have the capability along with their highest common bandwidth.
Most switch devices now support the automatic medium-dependent interface crossover (auto-MDIX) feature.
When enabled, the switch automatically detects the type of cable attached to the port and configures the interfaces accordingly.
MAC and IP
Layer 2 physical addresses (i.e., Ethernet MAC addresses) are used to deliver the data link frame with the encapsulated IP packet from one NIC to another NIC on the same network.
If the destination IP address is on the same network, the destination MAC address will be that of the destination device. When the destination IP address (IPv4 or IPv6) is on a remote network,
the destination MAC address will be the address of the host default gateway (i.e., the router interface). Along each link in a path, an IP packet is encapsulated in a frame.
The frame is specific to the data link technology associated that is associated with that link, such as Ethernet. If the next-hop device is the final destination, the destination MAC address will be that of the device Ethernet NIC.
How are the IP addresses of the IP packets in a data flow associated with the MAC addresses on each link along the path to the destination? For IPv4 packets, this is done through a process called ARP. For IPv6 packets,
the process is ICMPv6 ND.
ARP
Every IP device on an Ethernet network has a unique Ethernet MAC address. When a device sends an Ethernet Layer 2 frame, it contains these two addresses: destination MAC address and source MAC address.
A device uses ARP to determine the destination MAC address of a local device when it knows its IPv4 address. ARP provides two basic functions: resolving IPv4 addresses to MAC addresses
and maintaining a table of IPv4 to MAC address mappings. The ARP request is encapsulated in an Ethernet frame using this header information: source and destination MAC addresses and type.
Only one device on the LAN will have an IPv4 address that matches the target IPv4 address in the ARP request. All other devices will not reply. The ARP reply contains the same header fields as the request.
Only the device that originally sent the ARP request will receive the unicast ARP reply. After the ARP reply is received, the device will add the IPv4 address and the corresponding MAC address to its ARP table.
When the destination IPv4 address is not on the same network as the source IPv4 address, the source device needs to send the frame to its default gateway. This is the interface of the local router.
For each device, an ARP cache timer removes ARP entries that have not been used for a specified period of time. Commands may also be used to manually remove some or all of the entries in the ARP table.
As a broadcast frame, an ARP request is received and processed by every device on the local network, which could cause the network to slow down. A threat actor can use ARP spoofing to perform an ARP poisoning attack.
Neighbor Discovery
IPv6 does not use ARP, it uses the ND protocol to resolve MAC addresses. ND provides address resolution, router discovery, and redirection services for IPv6 using ICMPv6.
ICMPv6 ND uses five ICMPv6 messages to perform these services: neighbor solicitation, neighbor advertisement, router solicitation, router advertisement, and redirect.
Much like ARP for IPv4, IPv6 devices use IPv6 ND to resolve the MAC address of a device to a known IPv6 address.
Configure Initial Router Settings
The following tasks should be completed when configuring initial settings on a router.
Configure the device name.
Secure privileged EXEC mode.
Secure user EXEC mode.
Secure remote Telnet / SSH access.
Secure all passwords in the config file.
Provide legal notification.
Save the configuration.
Configure Interfaces
For routers to be reachable, the router interfaces must be configured. The Cisco ISR 4321 router is equipped with two Gigabit Ethernet interfaces: GigabitEthernet 0/0/0 (G0/0/0) and GigabitEthernet 0/0/1 (G0/0/1).
The tasks to configure a router interface are very similar to a management SVI on a switch. Using the no shutdown command activates the interface.
The interface must also be connected to another device, such as a switch or a router, for the physical layer to be active.
There are several commands that can be used to verify interface configuration including the show ip interface brief and show ipv6 interface brief, the show ip route and show ipv6 route,
as well as show interfaces, show ip interface and show ipv6 interface.
Configure the Default Gateway
For an end device to communicate over the network, it must be configured with the correct IP address information, including the default gateway address.
The default gateway address is generally the router interface address for the router that is attached to the local network of the host.
IP address of the host device and the router interface address must be in the same network. To connect to and manage a switch over a local IP network,
it must have a switch virtual interface (SVI) configured. The SVI is configured with an IPv4 address and subnet mask on the local LAN.
switch must also have a default gateway address configured to remotely manage the switch from another network. To configure an IPv4 default gateway on a switch,
use the ip default-gateway ip-address global configuration command. Use the IPv4 address of the local router interface that is connected to the switch.