Module 2: Switching Concepts
Switching, Routing, and Wireless Essentials v7.0 (SRWE)
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
2.1 Frame Forwarding
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8
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
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9
2.2 Switching Domains
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13
2.3 Module Practice and Quiz
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14
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.
© 2019 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15