SRWE_Module_2.pptx Networking Courses IT

Module 2: Switching
Concepts
Switching, Routing, and
Wireless Essentials v7.0
(SRWE)

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© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential
Module Objectives
Module Title: Switching Concepts
Module Objective: Explain how Layer 2 switches forward data.
Topic Title Topic Objective
Frame Forwarding Explain how frames are forwarded in a switched network.
Switching Domains Compare a collision domain to a broadcast domain.

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2.1 Frame Forwarding

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Frame Forwarding
Switching in Networking
Two terms are associated with frames entering or
leaving an interface:
• Ingress – entering the interface
• Egress – exiting the interface
A switch forwards based on the ingress interface
and the destination MAC address.
A switch uses its MAC address table to make
forwarding decisions.
Note: A switch will never allow traffic to be
forwarded out the interface it received the traffic.

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Frame Forwarding
The Switch MAC Address Table
A switch will use the destination MAC address to determine the egress interface.
Before a switch can make this decision it must learn what interface the destination is
located.
A switch builds a MAC address table, also known as a Content Addressable Memory
(CAM) table, by recording the source MAC address into the table along with the port
it was received.

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Frame Forwarding
The Switch Learn and Forward Method
The switch uses a two step process:
Step 1. Learn – Examines Source Address
• Adds the source MAC if not in table
• Resets the time out setting back to 5 minutes if source is in the table
Step 2. Forward – Examines Destination Address
• If the destination MAC is in the MAC address table it is forwarded out the specified port.
• If a destination MAC is not in the table, it is flooded out all interfaces except the one it was
received.

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Frame Forwarding
Video – MAC Address Tables on Connected Switches
This video will cover the following:
• How switches build MAC address tables
• How switches forward frames based on the content of their MAC address tables

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Frame Forwarding
Switch Forwarding Methods
Switches use software on application-specific-integrated circuits (ASICs) to make
very quick decisions.
A switch will use one of two methods to make forwarding decisions after it receives a
frame:
• Store-and-forward switching - Receives the entire frame and ensures the frame
is valid. Store-and-forward switching is Cisco’s preferred switching method.
• Cut-through switching – Forwards the frame immediately after determining the
destination MAC address of an incoming frame and the egress port.

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Frame Forwarding
Store-and-Forward Switching
Store-and-forward has two primary characteristics:
• Error Checking – The switch will check the Frame Check Sequence (FCS) for CRC errors.
Bad frames will be discarded.
• Buffering – The ingress interface will buffer the frame while it checks the FCS. This also
allows the switch to adjust to a potential difference in speeds between the ingress and
egress ports.

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Frame Forwarding
Cut-Through Switching • Cut-through forwards the frame immediately
after determining the destination MAC.
• Fragment (Frag) Free method will check the
destination and ensure that the frame is at
least 64 Bytes. This will eliminate runts.
Concepts of Cut-Through switching:
• Is appropriate for switches needing latency
to be under 10 microseconds
• Does not check the FCS, so it can
propagate errors
• May lead to bandwidth issues if the switch
propagates too many errors
• Cannot support ports with differing speeds
going from ingress to egress

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2.2 Switching Domains

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Switching Domains
Collision Domains
Switches eliminate collision domains and
reduce congestion.
• When there is full duplex on the link the
collision domains are eliminated.
• When there is one or more devices in
half-duplex there will now be a collision
domain.
• There will now be contention for the
bandwidth.
• Collisions are now possible.
• Most devices, including Cisco and
Microsoft use auto-negotiation as the
default setting for duplex and speed.

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Switching Domains
Broadcast Domains
• A broadcast domain extends across all Layer 1
or Layer 2 devices on a LAN.
• Only a layer 3 device (router) will break the
broadcast domain, also called a MAC
broadcast domain.
• The broadcast domain consists of all devices
on the LAN that receive the broadcast traffic.
• When the layer 2 switch receives the broadcast
it will flood it out all interfaces except for the
ingress interface.
• Too many broadcasts may cause congestion
and poor network performance.
• Increasing devices at Layer 1 or layer 2 will
cause the broadcast domain to expand.

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Switching Domains
Alleviated Network Congestion
Switches use the MAC address table and full-duplex to eliminate collisions and avoid
congestion.
Features of the switch that alleviate congestion are as follows:
Protocol Function
Fast Port Speeds Depending on the model, switches may have up to 100Gbps port speeds.
Fast Internal
Switching
This uses fast internal bus or shared memory to improve performance.
Large Frame Buffers This allows for temporary storage while processing large quantities of
frames.
High Port Density This provides many ports for devices to be connected to LAN with less cost.
This also provides for more local traffic with less congestion.

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2.3 Module Practice and Quiz

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Module Practice and Quiz
What did I learn in this module?
Frame Forwarding
• Ingress is the entry port, egress is the exit port.
• The switch builds a MAC address table to forward frames on the LAN.
• The switch can use either the store-and-forward or cut-through method of switch forwarding.
Switching Domains
• Ethernet ports in half-duplex will be a part of a collision domain.
• Full-duplex will eliminate collision domains.
• A switch will flood out all interfaces except the ingress port if the frame is a broadcast or if
the unicast destination MAC is unknown.
• Broadcast domains may be broken up by a layer 3 device, like a router.
• Switches extend broadcast domains, but can eliminate collision domains and relieve
congestion.

SRWE_Module_2.pptx Networking Courses IT

SRWE_Module_2.pptx Networking Courses IT

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