Module 7: Ethenet Switching

What 2 layers does Ethernet operate on?
(OSI Model)

- Ethernet operates in the data link layer and the physical layer

What 2 sublayers of the data link layer do Ethernet and 802 LAN/MAN standards operate on?

- LLC sublayer (IEEE 802.2) ; places info in the frame to identify which network layer protocol is used for the frame
- MAC sublayer (IEEE 802.3, 802.11 or 802.15) ; Responisble for data encapsulation and media access control (MAC) and provides data link layer addressing

What happens during MAC sublayer data encapsulation and access the media?

- IEEE 802.3 Data encapsulation includes the following;
- Ethernet Frame ( internal structure of ethernet frame)
- Ethernet Addressing
- Ethernet Error Detection

What is Ethernet Addressing?

- The ethernet frame includes both a source + destination MAC address to deliver the Ethernet Frame from ethernet NIC to ethernet NIC on the same LAN

What is Ethernet Error Detection?

- The ethernet frame includes a frame check sequence (FSC) trailer used for error detection

What is media access in MAC sublayer?

- Includes specifications for different ethernet communications standards over various types of media including copper and fibre

- Ethernet LANS use switches that operate in full duplex
- They dont require access control through CSMA/CD

- Legacy ethernet using bus topology or hubs is shared half duplex medium
-Ethernet over a half duplex medium uses CSMA/CD

What is the minimum and maximum ethernet frame size and what are they considered as?

- Min = 64 bytes
- Max = 1518 bytes
- 64 bytes

What does an Ethernet MAC address consist of?

- 48 bit binary value / 12 hexadecimal value

Ethernet MAC address

- Everey network device is connected to the same shared Media
- Provides method for device identification at data link layer of OSI model

What is frame processing?

- Ethernet header = Source MAC address and Destination MAC address
- When a NIC recieves an ethernet frame it examines the destination MAC address to see if it matchesthe physical MAC address stored in RAM
- If theres no match the device discards the frame
- If there is a match thenit passes the frame up the OSI layers where de encapsulation occurs

Unicast MAC Address

- A unique MAC address which is used when a frame is sent from a signel transmitting device to a single destination device

Broadcast MAC Address

- A MAC address in which all bits are set to 1 (FF:FF:FF:FF:FF:FF).

Multicast MAC address

- There is a destination MAC address of 01-00-5E when the encapsulated data is an IPv4 multicast packet and a destination MAC address of 33-33 when the encapsulated data is an IPv6 multicast packet.

- There are other reserved multicast destination MAC addresses for when the encapsulated data is not IP, such as Spanning Tree Protocol (STP).

- It is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping. It is not forwarded by a router, unless the router is configured to route multicast packets.

- Because multicast addresses represent a group of addresses (sometimes called a host group), they can only be used as the destination of a packet. The source will always be a unicast address.

- As with the unicast and broadcast addresses, the multicast IP address requires a corresponding multicast MAC address.

What are switch fundementals?

- A layer 2 ethernet switch uses a layer 2 mac address to make forwarding decisions
- An ethernet switch examines its MAC address table to make out a forwarding decision for each frame
- When a switch is turned on a MAC addresss table is empty

Examining the source MAC address

- Every frame that enters a switch is checked for new info to learn
- It does this by examining the source MAC address of the frame and port no' where the frame entered the switch
- If the source MAC address doesn't exist the switch updates the refresh timer for that entry
- Usually 5 mins on ethernet switches

Finding the Destination MAC address

- If the destination MAC address == unicast
- the switch will look for a match between the destination mac address of the frame an entry in it's MAC address table.
- If the destination MAC address is in the table it will forward the frame out the specified port
- Else; Switch forwards frame out of all ports except the incoming port
- This is called unknown unicast

How are frames filtered?

- As a swtich recieves frame from different devices
- It is able to populate it's mac address table by examing the source MAC of every frame
- When the mac addresss table of the switch contains the destination MAC it's able to filter the frame and forward out of a single port

The 2 forwarding methods for switching data between network ports?

- Store and forward switching
- Cut through switching

What is store and forward switching?

- This method recieves the entire frame and computes the CRC
- If valid the switch looks up the destination MAC address which determines the outgoing interface
- Then the frame is forwarded out of the correct port

What is cut through switching?

It is a technique that optimizes performance by examing only the first 6 bytes (destination MAC address) of an Ethernet frame before making a forwarding decision. The switch does a MAC address table lookup for the destination MAC address and forwards the frame.

What are the 2 variations of Cut through Switching?

- Fast forward switching
- Fragment free switching

What is fragment free switching?

- A compromise between high latency and high integrity of the store and forward switching and the low latency and reduced integrity of fast forward switching
- The switch stores and performs an error check on the 1st 64 bytes of the frame before forwarding
- Because this is where most network errors and collisions occur

What is fast forward switching?

- offers the lowest lvl of latency by immediately forwarding a packet after reading the destination adddres
- BEcause fast forward switching starts forwarding before the entire packet has been recieved there may be times when packets are relayed with errors
- The Destination NIC discards the faulty packet upon receipt
- Fast forward switching is the typical cut through method of switching

What is the post based memory buffering technique?

- Frames are stored in queues that are linked to specific incoming and outgoing ports
- A frame is transmitted to the outgoing port only when all the frames ahead in the queue have been successfully transmitted
- It is possible for a single frame to delay the transmission of all the frames in the memory because of a busy destination port
- This delay occurs even if the other frames could be trasnmistted top open destination ports

What is the shared memory buffering technique?

- Deposits all frames into a common memory buffer shared by all switch ports and the amount of buffer memory required by a port is dynamically allocated
- The frames in the buffer are dynamicaly linked to the destination port enabling a packet to be recieved on 1 port and then transmitted on another port w/o it moving to a different queue
- Shared memory buffering also results in larger frames that can be transmitted with fewer dropped frames. This is important with asymmetric switching which allows for different data rates on different ports. Therefore, more bandwidth can be dedicated to certain ports (e.g., server port).

What are the 2 types of duplex settings used for communications on an ethernet network?

- Full duplex - both ends can send and recieve info simultaneously
- Half duplex - only 1 end of connection can send at a time

What is auto-negotiation?

- 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

What is duplex mismatch?

- common cause of performance issues on 100/10Mbps ethernet links
- Occurs when 1 port on the link = half duplex but other is full duplex

How does duplex mismatch occur?

- This can occur when 1 or both ports on a link are reset
- The autonegotiation process doesn't result in both link paterns having same config
- Also can occur when users reconfig 1 side of a link and forget to reconfig other

Auto-MDIX

- Automatic Medium-Dependent Interface Crossover
- A feature that detects the type of cable, and configures the interfaces to allow the connection