1. Switching in an Enterprise Network
Introducing Routing and Switching in the Enterprise – Chapter 3

2. Objectives
Compare the types of switches used in an enterprise network.
Explain how Spanning Tree Protocol prevents switching loops.
Describe and configure VLANs on a Cisco switch.
Describe and configure trunking and Inter-VLAN routing.
Maintain VLANs in an enterprise network.

3. Compare the Types of Switches Used in an Enterprise Network
Switching and network segmentation
Content addressable memory (CAM) - MAC address table in high-speed memory, recreates every time switch is activated
Virtual circuit
Broadcast vs Collision
Microsegmentation – reduce the size of a collision domain to a single switch port
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4. Compare the Types of Switches Used in an Enterprise Network
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5. If a source MAC address is not in the table, it is added
Aging timer - deletes entries from the MAC address table if they are not used within a certain period of time
If a source MAC address is not in the table, it is added
Checks for the destination MAC address, forwards the frame out the port or floods the frame out every active port except the port upon which it was received.
Graphic:
Activity

6. Hardware-based Layer 2 switching
Software-based Layer-3 (multilayer) switching
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7. Store and forward switching
entire frame is read and stored in memory
checks the integrity of the bits in the frame by recalculating the cyclic redundancy check (CRC) value
Cut-through switching
Fast-forward - forwards the frames out the destination port as soon as it reads the destination MAC address
Fragment-free - reads the first 64 bytes of the frame before it begins to forward it out the destination port.
Graphic: —fast-forward about halfway through the animation, until “64 bytes” and the frames are both visible

8. Adaptive Cut Through uses Threshold Value
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9. Compare the Types of Switches Used in an Enterprise Network
Switch physical security
Switch access security
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10. Recording…..
(password: NetAcad)
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11. Reduce congestion & support high availability & load balancing
Redundancy required in the network design to maintain a high degree of reliability and eliminate any single point of failure
Accomplished by installing duplicate equipment and network links for critical areas
Reduce congestion & support high availability & load balancing
Dangers of switching loops & broadcast storms
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12. Explain How Spanning Tree Protocol Prevents Switching Loops
Create a loop-free logical topology
Potential loop detection and port blocking
Redundancy without switching loops
, graphic from right side, with both bridges visible to show how the redundant bridge takes over

13. Explain How Spanning Tree Protocol Prevents Switching Loops
, graphic from right side, with both bridges visible to show how the redundant bridge takes over

14. As a switch powers on, each port cycles through a series of four states:
Blocking
Listening
Learning
Forwarding
Disabled (indicates that the administrator has shut down the switch port)
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15. Graphic:

16. Graphic:

17. Root Bridge is the primary switch or focal point in the STP topology.
Root bridge communicates with the other switches using Bridge Protocol Data Units (BPDUs).
BPDUs are frames that multicast every 2 seconds to all other switches. BPDUs contain information such as:
Identity of the source switch
Identity of the source port
Cumulative cost of path to root bridge
Value of aging timers
Value of the hello timer
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18. Explain How Spanning Tree Protocol Prevents Switching Loops
Determining a root bridge
Bridge ID (BID)
Root ports, designated ports, and blocked ports
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19. The bridge priority value plus the MAC address creates the BID.
One root bridge on each network, and it is elected based on the bridge ID (BID)
The bridge priority value plus the MAC address creates the BID.
Bridge priority has a default value of 32,768. If a switch has a MAC address of AA-11-BB-22-CC-33, the BID for that switch would be: 32768: AA-11-BB-22-CC-33.
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20. The root bridge = the lowest BID value
If same default priority value, the switch with the lowest MAC address becomes the root bridge
As each switch powers on, it assumes that it is the root bridge, and sends out BPDUs containing its BID
STP designates three types of ports: root ports, designated ports, and blocked ports
Root port - provides the least cost path back to the root bridge
Designated Port - forwards traffic toward the root bridge but does not connect to the least cost path
Blocked Port - does not forward traffic
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21. Root port - provides the least cost path back to the root bridge
STP designates three types of ports: root ports, designated ports, and blocked ports
Root port - provides the least cost path back to the root bridge
Designated Port - forwards traffic toward the root bridge but does not connect to the least cost path
Blocked Port - does not forward traffic
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22. To specify the root bridge - configured with the lowest priority value
Range for the priority is from 0 to 65535, but values are in increments of 4096.
Default value =
To set priority:
S3(config)#spanning-tree vlan 1 priority 4096
To restore priority to default:
S3(config)#no spanning-tree vlan 1 priority
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23. STP Enhancements
Cisco proprietary - PortFast, UplinkFast, and BackboneFast
PortFast causes an access port to enter the forwarding state immediately, bypassing the listening and learning states - use PortFast on access ports that are connected to a single workstation or server
UplinkFast accelerates the choice of a new root port when a link or switch fails or when STP reconfigures itself - root port transitions to the forwarding state immediately without going through the listening and learning states
BackboneFast provides fast convergence after a spanning tree topology change occurs - used at the Distribution and Core Layers, where multiple switches connect
Graphic: —stop animation while yellow frame is visible

24. Spanning-tree verification commands
show spanning-tree - Displays root ID, bridge ID, and port states
show spanning-tree summary - Displays a summary of port states
show spanning-tree root - Displays the status and configuration of the root bridge
show spanning-tree detail - Displays detailed port information
show spanning-tree interface - Displays STP interface status and configuration
show spanning-tree blocked ports - Displays blocked ports
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25. Rapid Spanning Tree Protocol –
requires a full-duplex, point-to-point connection between switches to achieve the highest reconfiguration speed
occurs in less than 1 second, as compared to 50 seconds in STP
reduces the number of port states to three: discarding, learning and forwarding
all ports that are not discarding are part of the active topology and will immediately transition to the forwarding state.
Graphic: —go all the way to the end of the animation

26. Describe and Configure VLANs on a Cisco Switch
Virtual LANs
Logical networks
Broadcast control
Transparent to end-users
A VLAN is a logical broadcast domain that can span multiple physical LAN segments.
Network design best practice - broadcast traffic is contained to the area of the network in which it is required
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27. Describe and Configure VLANs on a Cisco Switch
VLAN functions
VLAN membership
Static – manually assigned
Dynamic – server based
Graphic: with one VLAN highlighted
VLAN has two major functions:
Contains broadcasts.
Groups devices.
Devices located on one VLAN are not visible to devices located on another VLAN.
Traffic requires a Layer 3 device to move between VLANs.

28. Describe and Configure VLANs on a Cisco Switch
VLAN 1: management VLAN - used to exchange information, such as Cisco Discovery Protocol (CDP) traffic and VLAN Trunking Protocol (VTP) traffic, with other networking devices.
VLAN numbers & names
Port assignment
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29. Describe and Configure VLANs on a Cisco Switch
VLAN verification commands
Deleting a VLAN - Switch(config)#no vlan vlan_number
Removing a port from a VLAN –
Switch(config)#interface fa0/port_number
Switch(config-if)#no switchport access vlan vlan_number
Graphic:
Lab

30. Describe and Configure VLANs on a Cisco Switch
VLAN ID
Frame tagging: IEEE 802.1Q
A switch associates each port with a specific VLAN number. As a frame enters that port, the switch inserts the VLAN ID (VID) into the Ethernet frame. The addition of the VLAN ID number into the Ethernet frame is called frame tagging. The most commonly used frame tagging standard is IEEE 802.1Q.
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31. Describe and Configure VLANs on a Cisco Switch
VLAN ID
Frame tagging: IEEE 802.1Q
Tag field increases the minimum Ethernet frame from 64 to 68 bytes. The switch recalculates the FCS because the number of bits in the frame has been modified.
802.1Q-compliant port is connected to another 802.1Q-compliant port ??? -
YES - VLAN tagging information passes between them
NO - VLAN tag is removed before the frame is placed on the media.
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32. Describe and Configure Trunking & Inter-VLAN Routing
Trunk port characteristics
Point-to-point link
Carry multiple-VLAN traffic over single link
Support for frame tagging
Trunk modes - Trunk ports are necessary to carry the traffic from multiple VLANs between devices when connecting either two switches together, a switch to a router, or a host NIC that supports 802.1Q trunking.
Graphic: , with “Trunking” button highlighted

33. Describe and Configure Trunking and Inter-VLAN Routing
Traffic may need to cross the 802.1Q configured link without VLAN ID
No VLAN ID is called untagged.
Examples of untagged traffic are Cisco Discovery Protocol (CDP), VTP, and certain types of voice traffic.
Untagged traffic minimizes the delays associated with inspection of the VLAN ID tag.
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34. Describe and Configure Trunking and Inter-VLAN Routing
To accommodate untagged traffic, a special VLAN called a native VLAN is available
Configuring a native VLAN
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35. Describe and Configure Trunking & Inter-VLAN Routing
A Layer 3 device provides connectivity between different VLANs.
Subinterfaces - logically divide one physical interface into multiple logical pathways
Router-on-a-stick – Allows each VLAN to have its own logical pathway and default gateway
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36. Maintain VLAN Structure on an Enterprise Network
VLAN Trunking Protocol (VTP) purpose and goals – automate many of the VLAN configuration functions, ensures that VLAN configuration is consistently maintained across the network
Management domain - reduces the task of VLAN management and monitoring
VTP modes: server, client, transparent
VLAN database – saved in NVRAM, contains a revision number, if a VTP receives an update message that has a higher revision number than the one stored in the database, the switch updates its VLAN database with this new information
Configuration revision number - begins at zero, as changes occur, the configuration revision number increases by one.
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37. Maintain VLAN Structure on an Enterprise Network
VTP messages
Summary advertisements –
contain the current VTP domain name and the configuration revision number , if the domain name is the same, the switch compares the configuration revision number to its own number, lower or equal, the switch ignores the packet, If the revision number is higher, an advertisement request is sent.
Subset advertisements –
follows the summary advertisement, contains a list of VLAN info
Advertisement requests –
required if the switch has been reset or the VTP domain name has been changed
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38. Maintain VLAN Structure on an Enterprise Network
Configuring VTP
Verifying VTP configuration
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39. Summary
Switches forward traffic using store and forward or cut-through techniques
Basic security features should be applied to switches
A VLAN is a way to group hosts on the same logical network even though they may be physically separated
Frame tagging allows a switch to identify the source VLAN of an Ethernet frame.
A Layer 3 device is needed to move traffic between different VLANs.
Subinterfaces allow router interfaces to support multiple VLANs.
VLAN Trunking Protocol provides centralized control, distribution and maintenance of VLANs.