Extending Switched Networks with VLANs

© 2002, Cisco Systems, Inc. All rights reserved.
Extending Switched Networks with VLANs
Module 4
Objectives
Upon completing this module, you will be able to:
Use Cisco IOS commands to configure VLANs, VTP, IEEE 802.1Q trunking, and ISL trunking, given a functioning access layer switch
Execute an add, move, or change on an access layer switch, given a new network requirement
Use show commands to identify anomalies in VLAN, VTP, ISL trunking, and spanning-tree operations, given an operational access layer switch
Use debug commands to identify events and anomalies in VLAN, VTP, ISL trunking, and spanning-tree operations, given an operational access layer switch
© 2002, Cisco Systems, Inc. All rights reserved.
4
VLAN Operation Overview
Objectives
Upon completing this lesson, you will be able to:
Describe the features and operation of a VLAN, VTP, IEEE 802.1Q trunking, and ISL trunking
A VLAN = A Broadcast Domain = Logical Network (Subnet)
VLAN Overview
Segmentation

Flexibility

Security
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
802.1Q Trunking
Importance of Native VLANs
802.1Q Frame
Per-VLAN Spanning Tree
Performed with ASIC
Not intrusive to client stations; ISL header not seen by client
Effective between switches, and between routers and switches
ISL trunks enable VLANs across a backbone.
ISL Tagging
ISL Encapsulation
A messaging system that advertises VLAN configuration information
Maintains VLAN configuration consistency throughout a common administrative domain
Sends advertisements on trunk ports only
VTP Protocol Features
Forwards
advertisements
Synchronizes
Not saved in
NVRAM
Creates VLANs
Modifies VLANs
Deletes VLANs
Sends/forwards
advertisements
Synchronizes
Saved in NVRAM
Creates VLANs
Modifies VLANs
Deletes VLANs
Forwards
advertisements
Does not
synchronize
Saved in NVRAM
VTP Modes
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
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
Summary
A VLAN permits a group of users to share a common broadcast domain regardless of their physical location in the internetwork. VLAN improve performance and security in switched networks.
A Catalyst switch operates in a network like a traditional bridge. Each VLAN configured on the switch implements address learning, forwarding/filtering decisions, and loop avoidance mechanisms.
Ports belonging to a VLAN are configured with a membership mode that determines to which VLAN they 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.
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 protocol used to distribute and synchronize identifying information about VLANs configured throughout a switched network. VTP allows switched network solutions to scale to large sizes by reducing the manual configuration required on each switch in the network.
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.