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Extending Switched Networks withVirtual LANs

Introducing VLAN Operations

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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

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VLAN = Broadcast Domain = Logical Network (Subnet)

VLAN Overview

Segmentation

Flexibility

Security

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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

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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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Per VLAN Spanning Tree +

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Performed with ASIC

Not intrusive to client stations;ISL header not seen by client

Effective between switches, andbetween routers and switches

ISL trunks enable VLANs across a backbone.

ISL Tagging

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ISL Encapsulation

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Has a messaging system that advertises VLAN configurationinformation

Maintains VLAN configuration consistency throughout a commonadministrative domain

Sends advertisements on trunk ports only

VTP Protocol Features

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Cannot create,change, or deleteVLANs

Forwardsadvertisements

Synchronizes

Not saved inNVRAM

Creates VLANs Modifies VLANs

Deletes VLANs

Sends and forwardsadvertisements

Synchronizes

Saved in NVRAM

Creates local VLANs only

Modifies local VLANs only

Deletes local VLANs only

Sends and forwardsadvertisements

Does notsynchronize

Saved in NVRAM

VTP Modes

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VTP advertisements are sent as multicast frames.

VTP servers and clients are synchronized to the latest revision number.

VTP advertisements are sent every 5 minutes or when there is a change.

VTP Operation

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Increases available bandwidth by reducing unnecessary flooded traffic

Example: Station A sends broadcast, and broadcast is flooded only

toward any switch with ports assigned to the red VLAN

VTP Pruning

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Summary

A VLAN permits a group of users to share a common broadcastdomain regardless of their physical location in theinternetwork. VLANs improve performance and security inswitched networks.

In a network, a Catalyst switch operates in a network like a

traditional bridge. Each VLAN configured on the switchimplements address learning, forwarding and filteringdecisions, and loop avoidance mechanisms.

Ports belonging to a VLAN are configured with a membershipmode that determines to which VLAN the ports belong. Catalyst

switches support two VLAN membership modes: static anddynamic.

The IEEE 802.1Q protocol is used to transport frames formultiple VLANs between switches and routers and for definingVLAN topologies.

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Summary (Cont.)

ISL is a Cisco proprietary protocol to transport multiple VLANsbetween switches and routers. ISL provides VLAN taggingcapabilities while maintaining full wire-speed performance.

VTP is a Layer 2 messaging protocol that maintains VLANconfiguration 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 VLANsare not propagated over the network until a managementdomain name is specified or learned.

VTP advertisements are sent throughout the managementdomain every 5 minutes or when there is a change. Theconfiguration revision number that is included in eachadvertisement identifies the most current information.

VTP pruning uses VLAN advertisements to determine when a

trunk connection is flooding traffic needlessly.

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