Chapter 6
Local Area Network (LAN)
Layer 2 Switching and Virtual LANs (VLANs)
LOGO
Objectives
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Bridges
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802.3 LAN Development: Today’s LANs
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Devices Function at Layers
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Factors that Impact Network Performance
Network traffic (congestion).
Multitasking desktop operating systems
(Windows, UNIX, and Mac) allow
simultaneous network transactions.
Faster desktop operating systems
(Windows, UNIX, and Mac) can initiate
faster network activity.
Increased number of client/server
applications using shared network data.
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Network Congestion
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Half-Duplex Ethernet Design
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LAN Segmentation
Segmentation allows network congestion to
be significantly reduced within each
segment.
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LAN Segmentation with Bridges
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LAN Segmentation with Routers
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LAN Segmentation with Switches
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Ethernet Technologies
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Types of Ethernet
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Parameters for 10 Mbps Ethernet Operation
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Ethernet Frame
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Manchester Encoding Examples
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10BASE5 Architecture Example
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10BASE2 Network Design Limits
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10BASE-T Modular Jack Pinouts
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10BASE-T Repeated Network Design Limits
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Parameters for 100-Mbps Ethernet Operation
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Ethernet Frame
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MLT-3 Encoding Example
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100BASE-TX Modular Jack Pinout
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NRZI Encoding Examples
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100BASE-FX Pinout
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Example of Architecture Configuration
and Cable Distances
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Types of Ethernet
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Parameters for Gigabit Ethernet Operation
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Ethernet Frame
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Outbound (Tx) 1000Base-T Signal
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Actual 1000Base-T Signal Transmission
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Benefits of Gigabit Ethernet on Fiber
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Gigabit Ethernet Layers
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1000BASE-SX and LX
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Gigabit Ethernet Media Comparison
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Gigabit Ethernet Architecture
Maximum 1000BASE-SX Cable Distances
Maximum 1000BASE-LX Cable Distances
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Parameters for 10-Gbps Ethernet
Operation
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10GBASE LX-4 Signal Multiplexing
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10-Gigabit Ethernet Implementations
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Introducing Basic
Layer 2 Switching and
Bridging Functions
© 2004 Cisco Systems, Inc. All rights reserved.
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ICND v2.2—1-42
Ethernet Switches and Bridges
 Address learning
 Forwarding the filtering decisions
 Loop avoidance
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Transmitting Modes
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MAC Address Table
• The initial MAC address table is empty.
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Learning Addresses
• Station A sends a frame to station C.
• The switch caches the MAC address of station A to port E0
by learning the source address of data frames.
• The frame from station A to station C is flooded out to all
ports except port E0 (unknown unicasts are flooded).
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Learning Addresses (Cont.)
• Station D sends a frame to station C.
• The switch caches the MAC address of station D to port E3 by
learning the source address of data frames.
• The frame from station D to station C is flooded out to all ports
except port E3 (unknown unicasts are flooded).
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Filtering Frames
• Station A sends a frame to station C.
• The destination is known; the frame is not flooded.
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Filtering Frames (Cont.)
• Station A sends a frame to station B.
• The switch has the address for station B in the MAC
address table.
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Broadcast and Multicast Frames
• Station D sends a broadcast or multicast frame.
• Broadcast and multicast frames are flooded to all ports
other than the originating port.
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Transmitting Frames
Cut-Through
• Switch checks destination
address and immediately
begins forwarding frame
Store and Forward
• Complete frame is received and
checked before forwarding
Fragment-Free
• Switch checks the first 64 bytes,
then immediately
begins forwarding frame
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Transmitting Modes
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CONTINUE NEXT WEEK
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Identifying Problems
That Occur in Redundant
Switched Topologies
© 2004 Cisco Systems, Inc. All rights reserved.
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Redundant Topology
 Redundant topology eliminates single points of failure.
 Redundant topology causes broadcast storms, multiple frame
copies, and MAC address table instability problems.
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Broadcast Storms
• Host X sends a broadcast.
• Switches continue to propagate broadcast traffic
over and over.
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Multiple Frame Copies
• Host X sends a unicast frame to router Y.
• The MAC address of router Y has not been learned by
either switch.
• Router Y will receive two copies of the same frame.
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MAC Database Instability
•
•
•
•
•
Host X sends a unicast frame to router Y.
The MAC address of router Y has not been learned by either switch.
Switches A and B learn the MAC address of host X on port 0.
The frame to router Y is flooded.
Switches A and B incorrectly learn the MAC address of host X on port 1.
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Introducing Spanning
Tree Protocol
© 2004 Cisco Systems, Inc. All rights reserved.
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ICND v2.2—1-59
Spanning Tree Protocol
• Provides a loop-free redundant network topology by
placing certain ports in the blocking state
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Spanning Tree Operation
•
•
•
•
One root bridge per network
One root port per nonroot bridge
One designated port per segment
Nondesignated ports are unused
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Spanning Tree Protocol
Root Bridge Selection
• BPDU = Bridge Protocol Data Unit
(default = sent every two seconds)
• Root bridge = bridge with the lowest bridge ID
• Bridge ID =
In this example, which switch has the lowest bridge ID?
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Spanning Tree Port States (Cont.)
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Spanning Tree Operation
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Spanning Tree Path Cost
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The Active Topology After Spanning Tree Is Complete
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Spanning Tree Port States
• Spanning tree transits each port through
several different states:
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Spanning Tree Recalculation
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Spanning Tree Convergence
• Convergence occurs when all the switch and
bridge ports have transitioned to either the
forwarding or the blocking state.
• When the network topology changes,
switches and bridges must recompute STP,
which disrupts user traffic.
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Rapid Spanning-Tree Protocol
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Rapid Transition to Forwarding
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Per VLAN Spanning Tree +
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Introducing VLAN
Operations
© 2004 Cisco Systems, Inc. All rights reserved.
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VLAN Overview
• Segmentation
• Flexibility
• Security
VLAN = Broadcast Domain = Logical Network (Subnet)
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VLAN Operation
• Each logical VLAN is like a separate physical bridge.
• VLANs can span across multiple switches.
• Trunks carry traffic for multiple VLANs.
• Trunks use special encapsulation to distinguish between
different VLANs.
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VLAN Membership Modes
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802.1Q Trunking
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Importance of Native VLANs
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802.1Q Frame
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ISL Tagging
ISL trunks enable VLANs across a backbone.
 Performed with ASIC
 Not intrusive to client
stations; ISL header not
seen by client
 Effective between
switches, and between
routers and switches
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ISL Encapsulation
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Q&A
Q&A
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