LOGO Local Area Network (LAN) Layer 2 Switching and Virtual LANs (VLANs) Local Area Network (LAN) Layer 2 Switching and Virtual LANs (VLANs) Chapter 6

Objectives 2

Bridges 3

LAN Development: Today’s LANs 4

Devices Function at Layers 5

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. 6

Network Congestion 7

Half-Duplex Ethernet Design 8

LAN Segmentation 9 Segmentation allows network congestion to be significantly reduced within each segment.

LAN Segmentation with Bridges 10

LAN Segmentation with Routers 11

LAN Segmentation with Switches 12

Ethernet Technologies 13

Types of Ethernet 14

Parameters for 10 Mbps Ethernet Operation 15

Ethernet Frame 16

Manchester Encoding Examples 17

10BASE5 Architecture Example 18

10BASE2 Network Design Limits 19

10BASE-T Modular Jack Pinouts 20

10BASE-T Repeated Network Design Limits 21

Parameters for 100-Mbps Ethernet Operation 22

Ethernet Frame 23

MLT-3 Encoding Example 24

100BASE-TX Modular Jack Pinout 25

NRZI Encoding Examples 26

100BASE-FX Pinout 27

Example of Architecture Configuration and Cable Distances 28

Types of Ethernet 29

Parameters for Gigabit Ethernet Operation 30

Ethernet Frame 31

Outbound (Tx) 1000Base-T Signal 32

Actual 1000Base-T Signal Transmission 33

Benefits of Gigabit Ethernet on Fiber 34

Gigabit Ethernet Layers 35

1000BASE-SX and LX 36

Gigabit Ethernet Media Comparison 37

Gigabit Ethernet Architecture 38 Maximum 1000BASE-SX Cable Distances Maximum 1000BASE-LX Cable Distances

Parameters for 10-Gbps Ethernet Operation 39

10GBASE LX-4 Signal Multiplexing 40

10-Gigabit Ethernet Implementations 41

42 © 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—1-42 Introducing Basic Layer 2 Switching and Bridging Functions

Outline  Overview  Functions of Ethernet Switches and Bridges  Frame Transmission Modes  How Switches and Bridges Learn Source MAC Addresses  How Switches and Bridges Forward and Filter Frames  Summary 43

Ethernet Switches and Bridges 44  Address learning  Forwarding the filtering decisions  Loop avoidance

Transmitting Modes 45

MAC Address Table 46 The initial MAC address table is empty.

Learning Addresses 47 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).

Learning Addresses (Cont.) 48 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).

Filtering Frames 49 Station A sends a frame to station C. The destination is known; the frame is not flooded.

Filtering Frames (Cont.) 50 Station A sends a frame to station B. The switch has the address for station B in the MAC address table.

Broadcast and Multicast Frames 51 Station D sends a broadcast or multicast frame. Broadcast and multicast frames are flooded to all ports other than the originating port.

52 Cut-Through Switch checks destination address and immediately begins forwarding frame Fragment-Free Switch checks the first 64 bytes, then immediately begins forwarding frame Store and Forward Complete frame is received and checked before forwarding Transmitting Frames

Transmitting Modes 53

Summary  Ethernet switches and bridges increase the available bandwidth of a network by creating dedicated network segments and interconnecting the segments.  Switches and bridges use one of three operating modes to transmit frames: store and forward, cut- through, and fragment-free.  Switches and bridges maintain a MAC address table to store address-to-port mappings so that they can determine the locations of connected devices.  When a frame arrives with a known destination address, the frame is forwarded only on the specific port connected to the destination station. 54

CONTINUE NEXT WEEK 55

56 © 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—1-56 Identifying Problems That Occur in Redundant Switched Topologies

Outline  Overview  Redundant Switched and Bridged Topologies  Broadcast Storms  Multiple Frame Transmissions  MAC Database Instability  Summary 57

Redundant Topology  Redundant topology eliminates single points of failure.  Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems. 58

59 Host X sends a broadcast. Switches continue to propagate broadcast traffic over and over. Broadcast Storms

60 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. Multiple Frame Copies

61 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. MAC Database Instability

Summary  Bridged and switched networks are commonly designed with redundant links and devices, which can introduce problems, such as broadcast storms, multiple frame transmission, and MAC database instability.  A broadcast storm is created when each switch on a redundant network floods broadcast frames endlessly.  Multiple frame transmissions occur when multiple copies of the same frame arrive at the intended host, potentially causing problems with the receiving protocol.  MAC database instability occurs when multiple copies of a frame arrive on different ports of a switch. 62

63 © 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—1-63 Introducing Spanning Tree Protocol

Outline  Overview  Spanning Tree Protocol  Spanning Tree Operation  Root Bridge Selection  Spanning Tree Port States  Spanning Tree Path Costs  Spanning Tree Recalculation  Rapid Spanning Tree Protocol  Summary 64

Spanning Tree Protocol 65 Provides a loop-free redundant network topology by placing certain ports in the blocking state

66 One root bridge per network One root port per nonroot bridge One designated port per segment Nondesignated ports are unused Spanning Tree Operation

67 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? Spanning Tree Protocol Root Bridge Selection

68 Spanning Tree Port States (Cont.)

69 Spanning Tree Operation

70 Spanning Tree Path Cost

71

72

Spanning Tree Port States 73 Spanning tree transits each port through several different states:

Spanning Tree Recalculation 74

Spanning Tree Convergence 75 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.

Rapid Spanning-Tree Protocol 76

77 Rapid Transition to Forwarding

Per VLAN Spanning Tree + 78

Summary  STP is a bridge-to-bridge protocol used to maintain a loop- free network.  To maintain a loop-free network topology, STP establishes a root bridge, a root port, and designated ports.  With STP, the root bridge has the lowest BID, which is made up of the bridge priority and the MAC address.  When STP is enabled, every bridge in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, the ports then stabilize to the forwarding or blocking state.  If the network topology changes, STP maintains connectivity by transitioning some blocked ports to the forwarding state.  RSTP significantly speeds the recalculation of the spanning tree when the network topology changes. 79

80 © 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—2-80 Introducing VLAN Operations

Outline  Overview  VLANs Defined  VLAN Operation  VLAN Membership Modes  802.1Q Trunking  Inter-Switch Link Protocol and Encapsulation  VLAN Trunking Protocol Features  VTP Modes  VTP Operations  VTP Pruning  Summary 81

VLAN Overview 82 VLAN = Broadcast Domain = Logical Network (Subnet) Segmentation Flexibility Security

83 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. VLAN Operation

VLAN Membership Modes 84

Q Trunking 85

Importance of Native VLANs 86

Q Frame 87

ISL Tagging  Performed with ASIC  Not intrusive to client stations; ISL header not seen by client  Effective between switches, and between routers and switches 88 ISL trunks enable VLANs across a backbone.

ISL Encapsulation 89

LOGO LAN Design Guide

LAN Segmentation 91 Segmentation allows network congestion to be significantly reduced within each segment.

Hierarchical Design Model: Access Layer 92

Access Layer 93

Functions of the Access Layer 94

Distribution Layer 95

Distribution Layer  In a switched network, the distribution layer includes several functions such as the following:  Aggregation of the wiring-closet connections  Broadcast/multicast domain definition  VLAN routing  Any media transitions that need to occur  Security 96

Core Layer 97 The core layer is a high-speed switching backbone. The core layer should be designed to switch packets as fast as possible.

Summary  A VLAN permits a group of users to share a common broadcast domain regardless of their physical location in the internetwork. VLANs improve performance and security in switched networks.  In a network, a Catalyst switch operates in a network like a traditional bridge. Each VLAN configured on the switch implements address learning, forwarding and filtering decisions, and loop avoidance mechanisms.  Ports belonging to a VLAN are configured with a membership mode that determines to which VLAN the ports belong. Catalyst switches support two VLAN membership modes: static and dynamic.  The IEEE 802.1Q protocol is used to transport frames for multiple VLANs between switches and routers and for defining VLAN topologies. 98

Summary (Cont.)  ISL is a Cisco proprietary protocol to transport multiple VLANs between switches and routers. ISL provides VLAN tagging capabilities while maintaining full wire-speed performance.  VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the additions, deletions, and name changes of VLANs across networks.  VTP operates in one of three modes: server, client, or transparent. The default VTP mode is server mode, but VLANs are not propagated over the network until a management domain name is specified or learned.  VTP advertisements are sent throughout the management domain every 5 minutes or when there is a change. The configuration revision number that is included in each advertisement identifies the most current information.  VTP pruning uses VLAN advertisements to determine when a trunk connection is flooding traffic needlessly. 99

Q & A 100