第9章：局域网交换与VLAN.ppt

LAN Switching and VLAN
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Switching
The Spanning-Tree Protocol
VLAN
Introduction of VLAN
VLAN Arch九分。
Layer 3 Switching
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Layer 4 Switching
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Multilayer Switching
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Switching
The Spanning-Tree Protocol
VLAN
Introduction of VLAN
VLAN Architecture
VLAN Implementation
Routing Between VLANs
Table of Contents
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Loops may occur in a network for a variety of reasons.
Usually loops in networks are the result of a deliberate attempt to provide redundancy.
Can also occur by configuration error
Two primary reasons loops can be absolutely disastrous in a bridged network:
broadcast loops
bridge-table corruption
Bridging Loops
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Redundancy Creates Loops
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L2 Loops
Broadcasts and Layer 2 loops can be a dangerous combination.
Ethernet frames have no TTL field
After an Ethernet frame starts to loop, it will probably continue until someone shuts off one of the switches or breaks a link
The switches will flip flop the bridging table entry for Host A (creating extremely high CPU utilization).
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L2 Loops - Flooded unicast frames
Where’s Host B? FLOOD
Where’s Host B? FLOOD
Where’s Host B? FLOOD
Where’s Host B? FLOOD
Uh oh.
Removed from the network
And the floods continue
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Overview of STP
Elements of the Spanning Tree Protocol
Main function: allow redundant paths in a switched/bridged network without incurring latency from the effects of loops.
STP prevents loops by calculating a stable spanning-tree network topology
Spanning-tree frames (called bridge protocol data units--BPDUs) are used to determine the spanning-tree topology
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Spanning Tree always uses the same four-step decision sequence:
Lowest root BID (Bridge Identification)
Lowest path cost to root bridge
Lowest sender BID
Lowest port ID
STP Decision Sequence
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BPDU