Chapter 13: Switching and VLANs

1
CCNA Guide to Cisco Networking

• Chapter 13 Switching and VLANs

2
Objectives

• Explain the features and benefits of Fast
  Ethernet
• Describe the guidelines and distance limitations
  of Fast Ethernet
• Define full- and half-duplex Ethernet operations
• Distinguish between cut-through, fragment-free,
  and store-and forward LAN switching
• Define the operation of the Spanning Tree
  Protocol and its benefits

3
Objectives (continued)

• Describe the benefits of virtual LANs
• Understand the purpose of the VLAN trunking
  protocol (VTP)

4
Ethernet Operations

• CSMA/CD
• Listen to wire before transmitting
• Contention mention
• Interframe gap
• Also known as an interpacket gap
• 9.6 seconds
• Collisions
• Simultaneous frame transmission
• 32-jam signal
• Back-off period

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Ethernet Operations (continued)

• CSMA/CD (continued)
• Collision domain
• Physical topology segment in which frames may
  collide
• Layer 3, layer 2, and layer 1
• Repeaters and hubs do not microsegment
• Switches and bridges microsegment at layer 2
• Routes and gateways segment at layer 2 and layer 3

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Latency

• Latency
• Sometimes referred to as propagation delay
• Length of time to forward, send, or propagate a
  data frame
• Bit time
• Slot time
• 5-4-3 rule
• Latency differs depending on
• Resistance of transmission medium
• Number of nodes
• Amount of processing of packet

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Latency (continued)
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Ethernet Errors

• Most errors are caused by
• Defective equipment
• Incorrectly configured equipment
• Frame size errors
• Frame size minimum 64 bytes
• Frame size maximum 1518 bytes

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Ethernet Errors (continued)

• Frame error classification
• Short frame or runt
• Long frame or giant
• Jabber
• Frame check sequence error
• Alignment error

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Collision Errors

• As the number of devices increase so does the
  number of collisions
• Late collisions
• Violate the 5-4-3 rule
• Cable too long
• Slot time exceeded
• Segment with a router
• Microsegment with a switch or bridge
• Transmitting station will attempt to retransmit
  16 times
• Additional collision detections will be
  considered a NIC error

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Broadcasts

• Nodes establishing a presence
• Applications advertising a service
• IP address-to-MAC address resolution
• Broadcast storm
• Network loop
• 126 or more broadcasts per second
• Possible solutions
• Reduce the number of services on servers
• Limit the number of protocols

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Fast Ethernet

• 100 Mbps
• 10/100 Autosense
• Full-duplex or half-duplex
• Category 5 or higher cable
• IEEE 802.3u implementations
• 100Base-TX
• 100Base-T4
• 100Base-FX

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Half- And Full-Duplex Communication

• Half-duplex
• Send and receive signals separately
• Full-duplex
• Send and receive simultaneously
• No collisions
• Benefits of full-duplex
• No collisions
• No retransmissions
• Full bandwidth in both directions
• No waiting for other transmissions

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Half- And Full-Duplex Communication (continued)

• Four different duplex options on 2950 switch
• Auto
• Full
• Full-flow control
• Half

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Half- And Full-Duplex Communication (continued)
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LAN Segmentation

• Segmenting with bridges
• Filter traffic at Data Link layer
• Segment LAN into 2 or 3 major segments
• Bridges build a MAC-to-segment table
• Manual configuration
• Learn from source MAC of arriving frame
• Bridges forward frames through the bridge when
  the destination of the frame is on a different
  segment

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LAN Segmentation (continued)

• Points to remember about bridges
• Reduce collisions
• No effect on broadcasts or multicasts
• Extend physical length of LAN
• Efficient use of bandwidth

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Segmenting With Routers

• Points to remember when segmenting with routers
• Forwards packets based on layer 3 addresses
• Decrease collisions
• Reduce broadcast and multicast traffic
• Support multiple paths and routes between routers
• Efficient use of bandwidth for the newly created
  segments

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Segmenting With Routers (continued)

• Points to remember when segmenting with routers
  (continued)
• Increase security
• Increase the physical distance of the network
• Provide layer 3 routing, packet fragmentation and
  reassembly, and traffic flow control
• Provide communications between different
  technologies such as Ethernet and Token Ring or
  Ethernet and Frame Relay
• Higher latency than bridges

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LAN Switching

• Segmentation with switches
• Switches are hardware controlled
• Bridges are software controlled
• Microsegmentation
• Switched bandwidth
• Shared bandwidth
• Efficient use of bandwidth
• Able to connect segments of different speeds
• 10 Mbps to 100 Mbps
• 100 Mbps to 1Gbps

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LAN Switching (continued)
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LAN Switching (continued)
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Switch Operations

• MAC-to-switch port mapping
• Content-addressable memory (CAM)
• Learns MAC addresses automatically
• Source address from arriving frame
• Two types of memory buffering
• Port-based memory buffering
• Shared-memory buffering
• Symmetric and asymmetric switching

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Securing Switch Ports

• Configure a permanent MAC address
• Does not have a TTL
• Define a static map entry
• Restricts communication between specific ports
• Set a limit on the number of MAC addresses
• Define the action when a security violation occurs

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Switching Methods

• Four methods for processing and forwarding frames
• Store-and-forward
• Read the entire frame
• Fragment-free
• Reads first 64 bytes
• Lower latency than store-and-forward
• Also known as modified cut-through
• Minor error detection

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Switching Methods (continued)

• Four methods for processing and forwarding frames
  (continued)
• Cut-through
• Forwards frame after destination MAC is read
• First 14 bytes of frame
• Lowest latency
• No error detection
• Adaptive cut-through
• Error sensing
• Uses cut-through and store-and-forward

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Switching Methods (continued)
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Switching Methods (continued)
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Switching Methods (continued)
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Spanning Tree Protocol

• Spanning Tree Protocol (STP)
• Physical loops
• Logical loops
• Spanning Tree Algorithm (STA)
• IEEE 802.1d

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Spanning Tree Protocol (continued)
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Spanning Tree Protocol (continued)

• Build a logical path
• Election process
• Root bridge (root device)
• Bridge protocol data units (BPDU) or
  Configuration bridge protocol data units (CBPDU)
• Root ports

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Spanning Tree Protocol (continued)

• Port states
• Stable states
• Blocking Send and receive BPDUs but no data
  frames
• Forwarding Send and receive all data frames and
  learn new MAC addresses
• Disabled No frames sent or received
• Transitory states
• Listening Listening to election process only
• Learning Learning new MAC addresses

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Spanning Tree Protocol (continued)

• STP switch port process
• From bridge/switch bootup to blocking
• From blocking to listening (or to disabled)
• From listening to learning (or to disabled)
• From learning to forwarding (or to disabled)
• From forwarding to disabled (automatically or
  manually)
• Topology changes

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Virtual LANs

• Logical grouping of network devices and nodes
• Broadcast domain
• Management VLAN
• Also known as default VLAN
• Cannot be deleted
• Every port is on VLAN 1 by default
• Router are required to move traffic between VLANs

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Virtual LANs (continued)
37
Virtual LANs (continued)
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Benefits of VLANs

• VLANS provide the following benefits
• It is easier to add and move stations on the LAN
• It is easier to reconfigure the LAN
• There is better traffic control
• There is increased security

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Dynamic vs. Static VLANs

• VLANs can be configured dynamically or statically
• Static VLANs are configured port-by-port
• Dynamic VLAN ports automatically learn their VLAN
  assignment
• Software database of MAC address-to-VLAN mappings

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VLAN Standardization

• Frame filtering
• Frames can be separated into VLANs
• MAC addresses
• Network-layer protocol type
• Application type
• Frame tagging
• IEEE 802.1q
• Also known as frame identification
• Adds a four-byte field to Ethernet frame
• Inter-Switch Link (ISL) protocol
• Cisco proprietary frame-tagging method
• 26 byte header

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Creating VLANs

• VLAN configuration
• Rm410HLvlan database
• Rm410(vlan)vtp domain hudlogic
• Rm410(vlan)vtp server
• Rm410(vlan)vlan 2 name production
• Rm410(vlan)vlan 3 name accounting
• Rm410(vlan)vlan 4 name marketing

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Creating VLANs (continued)

• VLAN configuration (continued)
• Rm410configure terminal
• Rm410(config)interface f0/1
• Rm410(config-if)switchport mode trunk
• Rm410(config-if)exit
• Rm410(config)interface f0/2
• Rm410(config-if)switchport access vlan 1

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Link Types And Configuration

• Two types of links
• Trunk links
• Switch-to-switch links
• Switch-to-router links
• 100 Mbps links
• 1 Gbps links
• Access links
• Non-VLAN aware devices

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Link Types And Configuration (continued)

• Trunk links have five states
• Auto
• Desirable
• Non-negotiate
• Off
• On
• Rm410(config)interface f0/1
• Rm410(config-if)switchport mode trunk

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Trunking Protocol

• VLAN trunking protocol
• Layer 2 messaging protocol
• Manages all changes to the VLANs across networks
• VTP domains
• VTP devices are organized in to domains
• Switches can only belong to one domain
• Rm410HLvlan database
• Rm410(vlan)vtp domain hudlogic

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Trunking Protocol (continued)

• VTP device modes
• Server
• Rm410(vlan) vtp server
• Client
• Rm410(vlan) vtp client
• Transparent
• Rm410(vlan) vtp transparent
• Default to server mode
• VTP pruning
• Reduces the number of VTP updates on trunk link
• Rm410(vlan) vtp pruning

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Trunking Protocol (continued)

• Delete VLAN database
• Rm410 delete flashvlan.dat
• Switch interface descriptions
• Rm410HL(config)int f0/1
• Rm410HL(config-if)description productionVLAN
• Nonswitching hubs and VLANs

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Routers and VLANs

• Increase security
• Manage traffic between VLANs
• Subinterfaces
• Access-lists
• Router-on-a-stick

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Routers and VLANs (continued)

• Enable inter-VLAN communication between VLAN 1
  and VLAN 2
• Router(config) interface e0.1
• Router(config-subif) ip address 164.106.1.1
  255.255.255.0
• Router(config-subif) encapsulation isl 1
• Router(config-if) exit
• Router(config) interface e0.2
• Router(config-subif) ip address 164.106.2.1
  255.255.255.0
• Router(config-subif) encapsulation isl 2

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Routers and VLANs (continued)
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Summary

• Ethernet (CSMA/CD) is a media access method that
  was developed in the 1960s
• Stations on an Ethernet LAN must listen to the
  network media before transmitting to ensure that
  no other station is currently transmitting
• If two stations transmit simultaneously on the
  same collision domain, a collision will occur
• The transmitting stations must be able to
  recognize the collision and ensure that other
  stations know about the collision by transmitting
  a jam signal
• Once the jam signal has cleared the network,
  other stations can begin transmitting, but the
  stations that caused the collision must wait for
  a random backoff period before attempting to
  transmit again

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Summary (continued)

• The delays caused by collisions on a network can
  seriously affect performance when collisions
  exceed 5 of the traffic on the collision domain
• One way to reduce the number of collisions on a
  network is to segment the network with a bridge,
  switch, or router
• Switches do the most to divide the collision
  domain and reduce traffic without dividing the
  broadcast domain
• This means that the LAN segment still appears to
  be a segment when it comes to broadcast and
  multicast traffic

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Summary (continued)

• Switches microsegment unicast traffic by routing
  frames directly from the incoming port to the
  destination port
• This means that packets sent between two hosts on
  a LAN segment do not interrupt communication of
  other hosts on the segment
• Switches are therefore able to increase the speed
  at which communications occur between multiple
  hosts on the segment
• Another way to increase the speed at which a LAN
  operates is to upgrade from Ethernet to Fast
  Ethernet
• This allows you to increase the speed at which
  frames are transferred on the wire, thereby
  increasing the performance of the network

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Summary (continued)

• To fully implement Fast Ethernet, you have to
  replace all the hubs, NICs, and any other network
  interfaces with interfaces that support Fast
  Ethernet
• Several Fast Ethernet devices allow for
  compatibility between Fast Ethernet and standard
  Ethernet, but to take full advantage of Fast
  Ethernet, all components must be upgraded
• Full duplex can also improve Ethernet performance
  over half-duplex operations because no collisions
  can occur on a full-duplex LAN
• Full duplex also allows frames to be sent and
  received simultaneously, which makes a 10-Mbps
  full-duplex connection seem like two 10-Mbps
  half-duplex connections

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Summary (continued)

• Full-duplex operations are only supported by
  devices designed for this type of communication
• This means that the half-duplex devices on a
  network will have to be completely replaced to
  take advantage of the speed offered by
  full-duplex operations
• The Spanning Tree Protocol (STP), which is
  enabled by default on most bridges and switches,
  allows administrators to create physical loops
  between bridges and switches without creating
  logical loops that would pose a problem for
  packet delivery
• Another way to increase the performance,
  flexibility, and security of a network is to
  implement VLANs via switches

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Summary (continued)

• VLANs are separate broadcast domains that are not
  limited by physical configurations, instead a
  VLAN is a logical broadcast domain implemented
  via one or more switches
• Performance benefits associated with VLANs are
  derived from limiting the amount of broadcast
  traffic that would naturally pass through a
  switch without filtration
• The enhanced flexibility to assign any port on
  any switch to a particular VLAN makes moving,
  adding, and changing network configurations
  easier
• VLAN information is communicated to switches
  using the VLAN trunking protocol (VTP)