VLANs: Segmentation, Routing, and Configuration

VLAN Segmentation

• Focuses on the purpose of VLANs in switched networks
• Explains how switches forward frames based on VLAN configurations in multi-switch environments

VLAN Implementations

• Details configuring a switch port to be assigned to a VLAN based on requirements
• Explains configuring a trunk port on a LAN switch
• Focuses on troubleshooting VLAN and trunk configurations

Inter-VLAN Routing Using Routers

• Describes the options for configuring Inter-VLAN routing
• Explains configuring Router-on-a-Stick Inter-VLAN routing

Broadcast Domains

• Without VLANs, each LAN often constitutes a single broadcast domain potentially leading to network congestion
• Separate broadcast domains can be achieved in a traditional setup using three separate switches

VLANs as Logical Switches

• Each logical VLAN is like a separate physical switch
• Each VLAN is a separate broadcast domain
• Each VLAN contains its own MAC address table
• A computer in one VLAN cannot directly send frames to computers in another VLAN

VLAN Definitions and Characteristics

• Allows administrators to segment networks based on factors like function, project team, or application, irrespective of physical location
• Enables implementation of finely-grained access and security policies
• Acts as a logical partition within a Layer 2 network
• Supports the coexistence of multiple partitions (VLANs)
• Each constitutes its own broadcast domain, typically with its IP network
• VLANs are mutually isolated; traffic passes through a router
• Layer 2 network partitioning occurs within a Layer 2 device, usually a switch
• Hosts within a VLAN are generally unaware of the VLAN

Benefits of VLANs

• Improved Security
• Reduced Cost
• Better Performance
• Smaller Broadcast Domains
• IT Efficiency
• Management Efficiency
• Simpler Project and Application Management

Types of VLANs

• Data VLAN: Carries user-generated traffic
• Default VLAN: Initially includes all of a switch's ports until reconfiguration
• Management VLAN: facilitates access to switch management capabilities

VLAN Trunks in Multi-Switch Environments

• Links are configured between switches to transmit traffic from specified VLANs
• Network functions depend on the presence of VLAN trunks

VLAN Trunks

• Functions as a point-to-point link that carries multiple VLANs
• Typically established between switches to facilitate communication between devices in the same VLAN connected to different switches
• A trunk is not associated with specific VLANs
• Neither are the ports used to establish the connection used to establish the trunk link
• Cisco IOS supports IEEE 802.1q, a popular VLAN trunk protocol.

VLAN Spanning Multiple Switches

• VLANs can span multiple switches, creating a cohesive, logical network segment across a physical infrastructure

VLAN Tagging

• Tagging involves adding an additional header to a LAN frame
• Used to identify which VLAN the frame belongs to
• Cisco refers to tagging as "TRUNKING"
• Trunks are responsible for carrying traffic for multiple VLANs

Controlling Broadcast Domains with VLANs

• Limits broadcast frames
• VLAN works as a broadcast domain.
• Transmits broadcast frames in VLAN
• Controls broadcast frames
• Unicast and multicast frames are forwarded within the originating VLAN.

Tagging Ethernet Frames for VLAN Identification

• Process involves adding a VLAN identification header to a frame
• Properly transmits multiple VLAN frames through a trunk link
• Switches tag frames to identify the VLAN to which they belong
• Different tagging protocols include ISL and IEEE 802.1Q
• Protocol defines structure of the tagging header
• Switches add VLAN tags before placing them into trunk links and removes the tags before forwarding frames through non-trunk ports
• Properly frames can transverse any number of switches via trunk links

VLAN Trunking Types

• ISL (Inter-Switch Link): Cisco Proprietary
• IEEE 802.1Q

ISL (Inter-Switch Link)

• A full Ethernet frame is encapsulated with ISL
• VLAN ID indicates which VLAN it is
• CISCO proprietary

IEEE 802.1Q

• Insertion of the IEEE 802.1Q tag by the switch before sending the frame
• Indicates VLAN (12 bit)

IEEE 802.1q Configuration Commands

• interface FastEthernet0/1
• switchport mode trunk
• switchport trunk allowed vlan 10,20,30,99
• end

Trunk Default State Commands

• configure terminal
• interface interface id
• no switchport trunk allowed vlan
• no switchport trunk native vlan
• end

Troubleshoot VLANs and Trunks

• VLANs must be allowed in the trunk before their frames can be transmitted across the link
• The command switchport trunk allowed vlan is used to specify the VLANs allowed in a trunk link.
• The show interfaces trunk command is used to ensure the correct VLANs are permitted in a trunk.

Routing Between VLANs

• Layer 2 switches cannot forward traffic between VLANs without a router
• The method for forwarding network traffic from one VLAN to another using a router

Legacy Inter-VLAN Routing

• Actual routers were used to route traffic between VLANs
• Each VLANs connected to a physical router interface.
• Packets arrive at router get routed and leave through router
• Routing was achieved using VLANs with ip address to connect to router interfaces.
• Routing required many interafaces

Passing traffic between VLANs

• Each VLAN needs different IP Subnets
• Does not send data framses though VLAN
• Needs MAC addres for each VLAN tables

Passing Traffic between VLANs cont.

• VLANs each need a IP subnet
• Needs a router with 3 LAN ports
• Creates a waste of resources

Router-on-a-Stick Inter-VLAN Routing

• Only requires one physcial interface on the router
• Physcial Interface configures a 802.1Q to understand VLAN tags
• Logical sub interfaces are created from VLANs
• Sub Interface needs to be configrured to route from VLAN
• VLANs configured to become subnet interface in a default route

Subinterfaces configuration

• Subinterfaces on a router can be divided by a single physical network in multiple logical interfac
• Each physical interface can have 65, 535 logical interfaces

Router on a Stick Confiuration

• VLAN trunking allows as single physical interface to route though multiple VLANs
• A physical interface connects to a trunk link on the adjacent switch.
• Sub interfaces are created in each unique VLAN
• Configures a IP sub network to tag frames in a VLAN

Switch configuration

• trunk command
• Subinterfaces enable to configure

Default VLANS

• Assigned in VLAN 1

VLANs Range Support

• Cisco Cataylst 2960 and 3560 series supports over 4000 VLANs
  • Normal 1 to 1005 VLANs
  • Extended Range 1006 to 4096 VLANs

Creating a VLAN Commands

• configure terminal
• vlan vlan-id
• name vlan-name
• end

Assigning Ports

• interface F0/18
• switchport mode access
• switchport access vlan 20
• end

Changing VLAN Commands

• int F0/18
• access
• no switchport

Deleting VLANs Commands

• cof t no vlan end

Assigning Information commands

• show vlan brief id name summary