Module 2 Switching concepts

Ingress

The port where a frame enters a network device.

Egress

The port where a frame leaves a network device.

Switch forwarding decision

A Layer 2 switch forwards frames based on the ingress port and destination MAC address.

MAC address table

A table that maps MAC addresses to the switch ports where devices are connected.

CAM

Content Addressable Memory; special memory used by switches to store and quickly search the MAC address table.

CAM table

Another name for the switch MAC address table.

How a switch learns MAC addresses

A switch examines the source MAC address and incoming port of each frame.

Learn process

The switch examines the source MAC address and associates it with the incoming port.

Forward process

The switch examines the destination MAC address and determines the outgoing port.

Source MAC address

The MAC address a switch uses to learn which device is connected to which port.

Destination MAC address

The MAC address a switch uses to determine where to forward a frame.

Source MAC memory trick

Source MAC = Learn.

Destination MAC memory trick

Destination MAC = Forward.

Known unicast

A frame whose destination MAC address is already in the switch MAC address table.

Unknown unicast

A frame whose destination MAC address is not in the switch MAC address table.

Unknown unicast behavior

A switch floods an unknown unicast frame out all ports except the incoming port.

Broadcast behavior

A switch floods a broadcast frame out all ports except the incoming port.

Multicast behavior

In the provided material, a switch floods a multicast frame out all ports except the incoming port.

Same-port forwarding

A switch never forwards an Ethernet frame back out the same port on which it was received.

MAC address table aging

Most Ethernet switches keep a learned MAC address table entry for about five minutes by default.

ASIC

Application-Specific Integrated Circuit; hardware used by switches to make Layer 2 forwarding decisions quickly.

Store-and-forward switching

A switching method that receives the entire frame, checks the FCS for errors, and then forwards the frame.

Store-and-forward process

Receive entire frame → check FCS → forward if valid → drop if invalid.

FCS

Frame Check Sequence; a value used to detect errors in an Ethernet frame.

CRC

Cyclic Redundancy Check; mathematical error-checking mechanism used to detect frame errors.

Store-and-forward advantage

It checks the entire frame for physical and data-link errors before forwarding it.

Store-and-forward invalid frame behavior

A switch drops a frame that fails the FCS check.

Cut-through switching

A switching method that begins forwarding after determining the destination MAC address and egress port.

Cut-through advantage

Cut-through switching provides lower latency because it does not wait for the entire frame.

Cut-through disadvantage

Cut-through switching can forward frames containing errors because it does not perform a full FCS check first.

Cut-through memory trick

Cut-through = Quick forwarding.

Store-and-forward memory trick

Store-and-forward = Store it, check it, then forward it.

Fragment-free switching

A modified form of cut-through switching that provides better error checking with very little additional latency.

Switching method using FCS

Store-and-forward switching.

Switching method with lower latency

Cut-through switching.

Switching method that can forward invalid frames

Cut-through switching.

Cisco primary LAN switching method

Store-and-forward switching.

Frame buffering

Temporarily storing frames in switch memory to help handle congestion and speed differences.

Why frame buffers are useful

They temporarily hold traffic when the ingress port is faster than the egress port.

Speed mismatch example

A 1 Gbps ingress port sending traffic to a 100 Mbps egress port may require buffering.

Collision domain

A network segment where devices share a medium and collisions can occur.

Hub collision domain

A hub creates one shared collision domain for connected devices.

Switch collision domains

A switch creates separate collision domains for its ports.

Half-duplex communication

A communication mode where devices can transmit or receive, but not both simultaneously.

Full-duplex communication

A communication mode where devices can transmit and receive simultaneously.

Half-duplex collisions

Collisions are possible when Ethernet operates in half-duplex.

Full-duplex collisions

Collisions do not occur when Ethernet operates in full-duplex.

Full-duplex advantage

Full-duplex allows both ends to transmit and receive simultaneously and increases effective bandwidth.

Gigabit Ethernet duplex requirement

1 Gbps Ethernet and higher speeds require full-duplex operation.

Auto-negotiation

Ethernet devices automatically negotiate compatible speed and duplex settings.

Highest common bandwidth

When devices negotiate, they select the highest bandwidth supported by both devices.

Broadcast domain

A collection of devices that receive Layer 2 broadcast frames from a host.

Device that divides a Layer 2 broadcast domain

A router divides Layer 2 broadcast domains.

Switch and broadcast domains

Interconnected switches normally form one Layer 2 broadcast domain.

Router and collision domains

A router also separates collision domains because each interface is a separate network segment.

Broadcast MAC address

The Ethernet broadcast destination MAC address is FF:FF:FF:FF:FF:FF.

Layer 2 broadcast behavior

A switch forwards a Layer 2 broadcast out every port except the ingress port.

Effect of connecting two switches

Connecting switches together increases the size of the Layer 2 broadcast domain.

Broadcast congestion

Too many broadcasts consume bandwidth and can contribute to network congestion.

Network congestion

Network congestion occurs when traffic load exceeds the network's ability to efficiently handle the traffic.

Fast port speeds

Higher-speed switch ports can move more data and help reduce congestion.

Fast internal switching

Switches use a fast internal bus or shared memory to process frames efficiently.

Large frame buffers

Large buffers temporarily store more received frames and help prevent frame loss during congestion.

High port density

The number of available ports on a switch.

Port density example

A 48-port switch has greater port density than a 24-port switch.

Why high port density helps

It can reduce the number of switches needed and help keep traffic local.

1 Gbps host connected to 100 Mbps switch port

The connection autonegotiates to 100 Mbps because the switch port is limited to 100 Mbps.

Layer 2 switch function

A Layer 2 switch determines the forwarding interface based on the destination MAC address.

Hub vs switch

A hub shares one collision domain, while a switch creates separate collision domains for its ports.

Unknown destination MAC address

A switch floods the frame out all ports except the port where the frame was received.

Congestion from 10 Gbps to 1 Gbps

Frame buffering helps alleviate congestion when a faster port forwards traffic to a slower port.

Port density definition

Port density is the number of available ports on a switch.

MAC table information used for learning

A switch uses the source MAC address and incoming port to maintain its MAC address table.

Full-duplex effective bandwidth

Full-duplex increases effective bandwidth because both directions can transmit simultaneously.

Full-duplex simultaneous transmission

Full-duplex allows both ends to transmit and receive simultaneously.

Address used to build MAC table

The source MAC address is used to build the switch MAC address table.

Correct store-and-forward description

Store-and-forward ensures that a frame is free of physical and data-link errors before forwarding it.

Device that divides a Layer 2 broadcast domain

A router can serve as a boundary that divides a Layer 2 broadcast domain.

Purpose of frame buffers

Frame buffers hold traffic temporarily to help alleviate network congestion.

Device used to eliminate Ethernet collisions

A switch operating in full-duplex eliminates collisions on its individual Ethernet links.

Which port speed will be autonegotiated between a host with a 1 Gbps NIC connecting to a Cisco Catalyst 2960 switch with a 100 Mbps port?

100 Mbps.

What is one function of a Layer 2 switch?

Determines which interface is used to forward a frame based on the destination MAC address.

What is the significant difference between a hub and a Layer 2 LAN switch?

A switch creates many smaller collision domains, and a hub increases the size of a single collision domain.

What will a Cisco LAN switch do if it receives an incoming frame and the destination MAC address is not listed in the MAC address table?

Forward the frame out all ports except the port where the frame is received.

Which switch characteristic helps alleviate network congestion when a 10 Gbps port is forwarding data to a 1 Gbps port?

Frame buffering.

Which switching method makes use of the FCS value?

Store-and-forward.

What does port density represent for an Ethernet switch?

The number of available ports.

Which information does a switch use to keep the MAC address table information current?

The source MAC address and the incoming port.

Which two statements are true about half-duplex and full-duplex communications?

Full duplex increases the effective bandwidth, and full duplex allows both ends to transmit and receive simultaneously.

Which type of address does a switch use to build the MAC address table?

Source MAC address.

Which option correctly describes a switching method?

Store-and-forward ensures that the frame is free of physical and data-link errors.

Which network device can serve as a boundary to divide a Layer 2 broadcast domain?

Router.

What is the purpose of frame buffers on a switch?

They hold traffic, thus alleviating network congestion.

Which network device can be used to eliminate collisions on an Ethernet network?

Switch.