VLANs¶

Configuration and Verification¶

VLAN Foundations¶

Logically groups users
Segments broadcast domains
Subnet correlation
Access control
Quality of service

Local VLANs¶

Local VLANs do not extend beyond the distribution layer
Local VLAN traffic routed to other destinations
Should be created around physical boundries

Configuring VLANs¶

conf t
vlan 200
name MARKETTING
int r f0/0-10
switchport mode access
switchport access vlan 100

Deleting VLAN Database¶

wr e
del flash:vlan.dat

In-Depth Trunking¶

Trunking¶

Trunking (aka tagging) passes multi-vlan information between switches
Places VLAN information into each frame
Layer 2 feature

The Two Tagging Flavours¶

Inter-Switch Link (ISL)
- CISCO proprietary
- Encapsulates the entire frame
- Being phased out
802.1Q
- Open standard/industry standard
- Inserts tag into frame rather than encapsulation

Digging Deeper Into the Trunk¶

ISL
- 26 byte header (Junk, VLAN, Junk)
- Ethernet Frame
- 4 byte CRC
802.1Q
- Destination MAC
- Source MAC
- 4 byte tag (3 bit PRI, VLAN)
- Ethernet Frame
- FCS

Negotiating Trunking¶

Switches can auto-negotiate trunk connections using Dynamic Trunking Protocol (DTP)
Can be confusing...
Five different modes:
- Access
- Trunk
- Dynamic Auto
- Dynamic desirable
- Non-negotiate

Configuring Trunk Ports¶

int fa0/15
switchport trunk encap dot1q
switchport mode trunk
! must do this everywhere:
switchport nonegotiate
switchport trunk native vlan 10
switchport trunk allowed vlan x y z

VLAN Trunking Protocol¶

VTP Modes¶

Server (default)
- Power to change vlan information
- Sends and receives vtp updates
- Saves vlan configuration
Client
- Cannot change vlan information
- Sends and receives vtp updates
- Does not save vlan configuration
Transparent
- Power to change vlan information
- Forwards (passes through) vtp updates - vtp v2
- Does not listen to vtp advertisements
- Saves vlan configuration

VTP Pruning¶

Keeps unnecessary broadcast traffic from crossing trunk links
Only works on vtp servers

Configuring VTP¶

Verify current VTP status
Configure VTP domain/password
Configure VTP mode
Set VTP version number
Verify

sh vtp status
vtp domain calebsargeant.com
vtp mode
vtp version 2

Common VLAN problems¶

Native VLAN mismatch
Trunk negotiation issues
- Auto-to-auto does not become trunk
- If possible, avoid DTP (trunk nonegotiate)
VTP updates not applying
- Verify vtp domain/password
- Verify vtp version
- Verify trunk links
- Delete flash:lvlan.dat and reboot