This document summarizes VLAN trunking concepts including: 1. It describes the two main trunk encapsulation types, 802.1Q and ISL, and their differences in header size and VLAN number limits. 2. It provides an overview of access port and trunk port configuration and their roles in VLANs. Access ports are assigned to a single VLAN while trunks can carry multiple VLANs. 3. It explains the purpose and configuration of the VLAN Trunking Protocol (VTP) which allows switches to exchange VLAN information in a domain.

1. VLANS packetlife.net
Trunk Encapsulation Trunk Types
26 6 6 2 4 802.1Q ISL
ISL Dest Source Header Size 4 bytes 26 bytes
ISL Type FCS
Header MAC MAC
Trailer Size N/A 4 bytes
Dest Source Standard IEEE Cisco
Untagged Type
MAC MAC
Maximum VLANs 4094 1000
Dest Source
802.1Q 802.1Q Type VLAN Numbers
MAC MAC
6 6 4 2 0 Reserved 1004 fdnet
VLAN Creation 1 default 1005 trnet
Switch(config)# vlan 100 1002 fddi-default 1006-4094 Extended
Switch(config-vlan)# name Engineering 1003 tr 4095 Reserved
Access Port Configuration Terminology
Switch(config-if)# switchport mode access Trunking
Switch(config-if)# switchport nonegotiate Carrying multiple VLANs over the same
Switch(config-if)# switchport access vlan 100 physical connection
Switch(config-if)# switchport voice vlan 150
Native VLAN
Trunk Port Configuration By default, frames in this VLAN are untagged
when sent across a trunk
Switch(config-if)# switchport mode trunk Access VLAN
Switch(config-if)# switchport trunk encapsulation dot1q The VLAN to which an access port is assigned
Switch(config-if)# switchport trunk allowed vlan 10,20-30
Switch(config-if)# switchport trunk native vlan 10 Voice VLAN
If configured, enables minimal trunking to
SVI Configuration support voice traffic in addition to data traffic
on an access port
Switch(config)# interface vlan100
Switch(config-if)# ip address 192.168.100.1 255.255.255.0 Dynamic Trunking Protocol (DTP)
Can be used to automatically establish trunks
VLAN Trunking Protocol (VTP) between capable ports (insecure)
Domain Switched Virtual Interface (SVI)
Common to all switches participating in VTP A virtual interface which provides a routed
gateway into and out of a VLAN
Server Mode
Generates and propagates VTP advertisements to clients; Switch Port Modes
default mode on unconfigured switches
trunk
Client Mode Forms an unconditional trunk
Receives and forwards advertisements from servers; VLANs
dynamic desirable
cannot be manually configured on switches in client mode
Attempts to negotiate a trunk with the far end
Transparent Mode dynamic auto
Forwards advertisements but does not participate in VTP; Forms a trunk only if requested by the far end
VLANs must be configured manually
access
Pruning Will never form a trunk
VLANs not having any access ports on an end switch are
removed from the trunk to reduce flooded traffic Troubleshooting
VTP Configuration show vlan
show interface [status | switchport]
Switch(config)# vtp mode {server | client | transparent}
Switch(config)# vtp domain <name> show interface trunk
Switch(config)# vtp password <passsword>
Switch(config)# vtp version {1 | 2} show vtp status
Switch(config)# vtp pruning show vtp password
by Jeremy Stretch v2.0