# VLANs

> Markdown source of <https://docs.calebsargeant.com/networking/cisco/switching/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

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

### Deleting VLAN Database

``` text
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

``` text
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

1.  Verify current VTP status
2.  Configure VTP domain/password
3.  Configure VTP mode
4.  Set VTP version number
5.  Verify

``` text
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