Understand what a trunk is. A trunk is a link between a switch and another device that allows the traffic from multiple VLANs to cross it. Special trunk protocols are used to carry the additional information needed, which can not be incorporated into the standard Ethernet frame. Two protocols—the Cisco proprietary ISL and the standards-based IEEE 802.1Q—have been developed for this purpose. When a packet crosses a trunk, it retains any ISL or dot1q information detailing what VLAN the packet belongs to.
Understand the difference between ISL and 802.1Q. ISL is a Cisco proprietary VLAN format, whereas 802.1Q is a standard. Network cards are made to support both types, which enables PCs and servers to receive and send VLAN-specific traffic. The big difference between the two is that ISL encapsulates the original packet in a new 30-byte frame, whereas 802.1Q just inserts a 4-byte additional field into the existing Ethernet frame.
Know the configuration differences between the different switches. The 4000 series uses the standard set commands, whereas the 2950 and 3550 series use IOS commands. The IOS-based switches configure VLAN and VTP configurations in VLAN database configuration mode, but assign interfaces to VLANs in interface configuration mode. The 4000 makes no such distinctions.
Understand when a VLAN should be used. VLANs are used to separate broadcast traffic into different groupings. If a switch has ports 1–10 in VLAN 1 and ports 11–20 in VLAN 2, a packet arriving from a device connected to port 5 can’t talk to a device connected to port 15 without some sort of routing engine participating. Know that VLANs can be used for security as well as to break up existing large broadcast domains.
Understand what VTP is and how it is used. VTP carries VLAN information between switches, which can be configured to be servers, clients, or transparent. VTP information is contained within a domain, and ensures that VLAN naming and numbering is consistent within a domain, as well as reducing configuration overhead.