Switching Concepts: Frame Forwarding

Module 2: Switching Concepts

• Explains how Layer 2 switches forward data.
• Frame forwarding is explained
• Collision domains are compared to broadcast domains.

Frame Forwarding

• A switch forwards based on the ingress interface and the destination MAC address
• A switch uses its MAC address table to make forwarding decisions.
• A switch will never allow traffic to be forwarded out the interface it received.
• Ingress is the term for entering the interface
• Egress is the term for exiting the interface
• The switch uses the destination MAC address to determine the egress interface.
• A switch builds a MAC address table, or Content Addressable Memory (CAM) table.
• The MAC address table is built by recording the source MAC address into the table along with the port it was received.
• The switch utilizes a two step process to learn and forward
  • Step 1: Learn - Examines the Source Address
  • Adds the source MAC if it's not in the table.
  • Resets the time out setting back to 5 minutes if the source is in the table.
  • Step 2: Forward – Examines the Destination Address
  • If the destination MAC is in the MAC address table, the frame gets forwarded out the specified port.
  • If a destination MAC isn't in the table, it's flooded out all interfaces except the one it was received.
• Types of Switching
  • Store-and-forward switching
  • Cut-through switching
• Store-and-forward switching receives the entire frame and verifies that the frame is valid, it is a preferred method from Cisco
• Store-and-forward has two primary characteristics:
  • Error Checking: The switch checks the Frame Check Sequence (FCS) for CRC errors and discards bad frames.
  • Buffering: The ingress interface buffers the frame while checking the FCS, allowing adjustment to speed differences between ingress/egress ports.
• Cut-through forwards frames immediately after determining the destination MAC.
• Fragment (Frag) Free method checks destination and ensures the frame is at least 64 Bytes, eliminating runts.
• Cut-through switching is appropriate for switches needing <10 microseconds latency.
• Cut through has some drawbacks:
  • It does not check the FCS, allowing it to propagate errors.
  • It may cause bandwidth issues if the switch propagates too many errors.
  • It cannot support ports with differing speeds going from ingress to egress

Switching Domains

• Switches eliminate collision domains and reduce congestion
• When full duplex is enabled on the link, collision domains are eliminated
• In half-duplex with one or more multiple devices, contention and collisions can occur for bandwidth
• Most devices, including Cisco and Microsoft, use auto-negotiation for duplex and speed by default
• In Broadcast domains, broadcast extends across all Layer 1 / Layer 2 devices on a LAN
• Only a Layer 3 device (router) breaks the broadcast domain, also a MAC broadcast domain
• The Broadcast domain includes all devices on the LAN that receive broadcast traffic
• When a Layer 2 switch receives a broadcast, it floods out all interfaces except the ingress interface
• Too many broadcasts may result in congestion and reduced network performance
• Increasing devices at Layer 1 or Layer 2 will cause the broadcast domain to expand
• Switches use both a MAC address table and full-duplex to eliminate collisions and avoid congestion
• Switch performance is impacted by:
  • Fast Port Speeds: Switches may have up to 100Gbps port speeds, depending on the model
  • Fast Internal Switching: Utilizes fast internal bus or shared memory to optimize performance
  • Large Frame Buffers: Allows for temporary data storage while processing large quantities of frames.
  • High Port Density: This provides many ports for devices to be connected to LAN with less cost, and provides for more local traffic with less congestion.

Module Recap

• Ingress is the entry port of the interface, egress is the exit port
• Switches build a MAC address table for forwarding frames on the LAN
• Switches employ either the store-and-forward method or the cut-through method for switching frames
• In half-duplex, Ethernet ports are part of a collision domain; full-duplex eliminates collision domains
• For broadcasts or unknown unicast destinations, a switch floods all interfaces except the ingress port
• Layer 3 devices (routers) break up broadcast domains
• Switches expand broadcast domains, but reduce collision domains to relieve congestion.

Terms to Know

• Content accessible memory (CAM)
• MAC address table
• Store-and-forward switching
• Cut-through switching
• Automatic buffering
• Fragment free switching
• Collision domains
• Broadcast domains

Module 3: VLANs

• Explains how network protocols enable devices to access resources locally and remotely.

Overview of VLANs

• Describes the purpose of VLANs in a switched network.
• VLANs are logical connections with similar devices
• Benefits include:
  • Segmentation of different device groups on the same switches
  • Improved, more manageable organization
  • Keeping broadcasts, multicasts and unicasts isolated to the individual VLAN.
  • Allowing Each VLAN its own its own range of IP addresses
  • Overall results are smaller broadcast domains

Benefits of VLAN design

• Improved Security
• Reduce Broadcast Domains
• Improvement of IT efficiency
  • by grouping devices by a set of specific requirements like faculty and students
• Reduced costs
  • 1 switch can provide many VLAN's
  • allows for reduction in bandwidth traffic
• Improved performance
• Simpler management of similar groups with application sets

Types of VLAN's

• Default VLAN
• One VLAN for Data
• Native
• Management
• Voice

Information about VLAN 1

• a Default VLAN
• VLAN which cannot be renamed
• It is recommended to assign default features to native VLAN's

Data VLAN information

• dedicated to users as per email and web traffic
• VLAN 1 is the default data VLAN as all interfaces are assigned it.

Native VLAN

• It is for a trunk link only
• Frames are Tagged on an 802.1Q
• Frames are trunked except the native VLANS

Management VLAN

• used when using the SSH/TELNET application
• Should not be carried with end user traffic
• The Switch Virtual Interface layer 2 should be assigned to Typical VLAN

Voice VLAN

• A separate VLAN is required due to voice traffic
• Traffic that requires a safe bandwidth plus a high QoS priority
• Designed to make Ability to avoid congestion that is Less then 150ms from source to destination as a requirement to support traffic

VLAN Trunks

• A Trunk is a point between 2 network devices
• Cisco Trunk function allows VLAN
• extend the VLAN across the entire network
• By default support all VLAN's
• Supports 802.1Q

Networks

• Broadcast Traffic - connected to the broadcast without VLAN traffic

Layer 2 network

• The Layer two broadcast sends a broadcast frame to configure VLAN10 ( PC 1 configures switches just forwards broadcast when VLAN is configure)
• the IEEE 802.1Q header is 4 Bytes
• once the tag is created ,the FCS has to equal it
• when sent to end device the Tag must be removed the FCS must calculate back to original number.
• tagging is used on all VLANS(802. 1Q)
• Both ends of the Trunk have to be configured with the native VALN
• Each Trunk is configured separate and can have different Native VLANs using separate trunks

Voice VLAN

• tag it's own traffic
• the phone has access to the layer
• Phone can tag the from's from a PC but only the VOIP frames
• the show interface command can assist in showing us two sets of data VLAN that are assigned to an interfac

Command examples for VLAN

• Switchport - enabled
• Administrative mode- static access
• Vlan mode- VLAN and voice

Catalyst switches

• 2960 and 3650 can support over 4000+VLAN's

Normal and Extend VLAN

• Range of 1 to 1005 is used for small business and legacy VLAN's 1005 is created by the system and can't be deleted(1 and 1002-1005) ,data gets stored in the flash and the VTP syncs between switches. range of 1006-4095
• Range of 1006-4095 are used for service providers
• Are in running config mode

Command IOS'

• Switch config T gives access /assign VLAN id/switch config name / then you end process

Port assignment commands

• Access Interface is needed for vlan interface
• Enter config T/Then you enter interface assignment/the ports get assigned/then use end command

Data and Voice VLANS

• an Access port can be assigned to a VLAN but is also assigned a Voice VLAN for when an IP phone is on the device .
• A VLAN will want create and name data and voice VLAN's the newer catalyst switch will create existing device that are assigned.

Commands to Verify /Show VLAN

• summary/ name is equal to a string from 1-32/show run/interface data to be displayed

Commands to Change VLANS

• to assign you have to reenter the command's
• to place interface back in VLAN 1 you have to use a no switch command.

Command to check assignment

• Sh int fast and switch port is the command

Command to Delete

Caution-reassign all member ports to a different VLAN.

Trunk configuration command

• interface that is set to default can be reverted by using switch port
• must be set to a permanent trunk mode switch can be a local VLAN

Verification

Encapsulation must be verified using dot1q

Dynamic Trunking Protocol

• DTP is a Cisco proprietor
• it has Dynamic Auto(is set to default)
• can be turn off (noneogiate command)

More on interfaces

Actively seeks to become a trunk using auto or other desirable interfaces

Learn how Layer 2 switches forward data based on the destination MAC address. Understand MAC address tables. Explore how switches learn and forward traffic, examining source addresses and building their MAC address tables.