Chapter 3 - VLANs

VLANs

Logical grouping of devices or usersConfiguration done at switch via softwareNot standardized – proprietary software from vendor

VLANs

Logically segment the physical LAN infrastructure into different subnets (or broadcast domains for Ethernet)

Differences Between Traditional Switched LAN and VLANs

VLANs work at Layer 2 and Layer 3 of OSICommunications between VLANs is done by routersVLANs provide a method of controlling network broadcasts

Administrators assign users to VLANsVLANs increase network security – defines who can communicate with whomGroup switch ports and their connected users into logically defined workgroups

Transport of VLANs Across the Backbone

Ability to transport VLAN information between interconnected switches and routers that reside on the backbone– Remove physical boundaries between users– Increase configuration flexibility – users move– Provide mechanism for interoperability between

backbone components

VLAN transportation

Backbone commonly acts as collection point for large volumes of trafficCarries end user information and ID between switches, routers and directly attached servers

Routers in the VLAN

Traditionally provide firewalls, broadcast management etc.Provide connected routes between different VLANsCost effectively integrate external routers into switching architecture by using one or more high speed backbone connection like:– Fast Ethernet, or ATM connection

• Increasing the throughput between switches and routers

Frame Use in the VLAN

Switches core component of VLAN communicationEach switch makes forwarding and filtering decisions based on the frame– Based on VLAN metrics

Approaches for logically grouping users into distinct VLANs:– Frame filtering– Frame tagging (identification)

Frame Filtering

Frame Tagging

Uniquely assigns a VLAN ID to each frameVLAN IDs assigned by switch administratorChosen by IEEE for its scalabilityGaining recognition as the standard trunking mechanismIEEE 802.1q states that Frame Tagging is the way to implement VLANs

Frame Tagging Continued

Places a unique identifier in the header of each frame as it is forwarded throughout the networkWhen the frame exits the network backbone – switch removes the identifier before the frame is transmitted to its targetFrame identification functions at Layer 2 and requires little administrative overhead

Ports, VLANs and Broadcasts

VLANs make up a switched network – logically segmentedPorts assigned to the same VLAN share broadcastsThree VLAN implementation– Port-centric– Static– Dynamic

Port-Centric

All nodes connected to ports in the same VLAN are assigned same VLAN ID VLAN Membership by port make administrator’s job easier and more efficient because:

• Users assigned by port• VLANs easily administered• Increased security• Packets do not LEAK into other domains

Port-Centric VLANs

Static VLANs

Ports on switch that is statically assigned to a VLANRequire administrator to make changesSecureEasy to configureStraightforward to monitorWorks well in which moves are controlled and managed

STATIC VLANs

Dynamic VLANs

Ports on switch automatically determine their VLAN assignmentsBased on MAC addresses, logical addressing or protocol type of data packetLess administration with in the wiring closet when a user moves or new one addedCentralized notification when an unrecognized user is added to the networkMore administration is required to initially set up database within the VLAN management software

Dynamic VLANs

VLAN Additions, Moves and Changes

Companies continually reorganizing – These moves/changes are network manager’s biggest

headaches and one of the largest expenses related to managing a network

VLANs provide effective measures for controlling changes and reducing costsUsers in a VLAN can share the same network address space i.e. IP subnetVLANs require less rewiring, configuration and debugging

Movement of Users

VLANs Help Control Broadcast Activity

Most effective measures is to properly segment with firewalls that help prevent problems on segment from damaging other parts of the networkFirewall segmentation provides reliability and minimizes overhead broadcast trafficIf no routers are placed between switches, broadcasts (layer 2) are sent to every switched port – referred to as a FLAT network(one broadcast domain across the whole network)Flat Network– Provides low latency & high throughput– Easy to administer

VLANs Controlling Broadcast Activity

FLAT Network – Disadvantages– Increases vulnerability to broadcast traffic across all switches,

ports, backbone links and usersVLANs effectively extend firewalls from routers to the switch fabric and protecting against potentially dangerous broadcast problemsCreating firewalls– Assign switch ports or users to specific VLAN groups both

within single switches and across multiple connected switches

VLANs and Broadcast Activity

How do VLANs Improve Network Security

Restrict number of users in a VLAN groupPrevent another user from joining without first receiving approval from the VLAN network management applicationConfigure all unused ports to a default low-service VLAN

Tightening Network Security

VLANS Save Money

Connect existing HUBS to switchesEach hub segment connected to a switch can be assigned only ONE VLANStations that share a hub segment are in the same VLANIf a station need to be assigned a new VLAN that station must move to the new hub with the appropriate VLAN

Using Existing Hubs

Summary

Switch is designed to physically segment a LAN into individual domainsA typical LAN is configured according to the physical infrastructure it connectsLANs that use LAN switching devices - VLAN technology is cost effective and an efficient way of grouping network users into virtual workgroups regardless of their physical placement

Summary Continued

VLANs work at Layer 2 and Layer 3 of the OSIVLAN architecture must allow transportation of VLAN information between interconnected switches and routers on the corporate backboneMost common approach for logically grouping users into distinct VLANS are frame filtering and frame tagging

Summary Continued

Types of VLANS– Port-centric– Static– Dynamic

VLANs provide benefits– Reduce administration costs – moves, additions changes– Controlled broadcast activity– Workgroup and network security– Save money by using existing hubs