Pre-requisites: VLAN basics
A network could technically have multiple VLANs using only access ports. This design isn’t all that realistic though. It is inevitable that multiple VLANs will have to traverse a single port. When a port is configured to carry multiple VLANs, it is called a trunk port.
There are two methods to allow a trunk port to carry multiple VLANs: ISL and 802.1Q. This document intends to describe both.
ISL
Inter-switch Link (ISL) is Cisco’s proprietary method for port trunking. ISL has been around for quite some time but is being phased out in favor of 802.1Q (to be discussed later). When a frame is being prepared to be sent through an ISL enabled port, ISL encapsulates the entire Ethernet frame in an ISL header and ISL CRC footer. Because information is being prepended and appended to the frame, ISL’s tagging method is sometimes called double-tagging.
802.1Q
801.1Q (sometimes called ‘dot one q’) is a standards based method of VLAN trunking. While ISL encapsulates the Ethernet frame, 802.1Q simply adds some information in the Ethernet header (specifically, after the source address). The 802.1Q VLAN tag is comprised of the following elements:
* 2 byte protocol ID (always 0×8100)
* 3 bit user priority (CoS)
* 1 bit canonical indicator
* 12 bit VLAN id
It is in the 12-bit VLAN id where the switch records the VLAN which this frame is assigned.
802.1Q introduces the concept of ‘native VLAN’ which isn’t present in ISL. A native VLAN is the VLAN a switch assumes the frame is a member of if there is no 802.1Q header. By default, the native VLAN is 1. However, security vulnerabilities enabled by an obvious native VLAN make changing it a recommended task.
DTP
Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol which assists in the negotiation of trunking. DTP operates in three modes:
* dynamic desirable – Actively ask neighbor to become a trunk port. This port will also become a trunk if asked by a neighbor who is also dynamic desirable.
* dynamic auto – Will go into trunk mode if it is asked by a neighbor but will not ask neighbors to become trunks.
* nonegotiate – Port permanently becomes a trunk port. These ports will not send DTP packets nor will they pay attention to any incoming DTP packets. The neighbor switch must also be set in no negotiate.
802.1Q Operation
Because ISL is a deprecated technology, I am not going to go into details about how it operates. Instead, 802.1Q’s operational basics will be explained.
Once a trunk link has been brought up and negotiated (if negotiated that is) traffic is sent over the link. Lets say the native VLAN is 2. If traffic is assigned to VLAN 2, the switch will send the frame without a 802.1Q header tag. The receiving switch, assuming it is configured properly, will know it is for VLAN 2 because there is no VLAN header. On the other hand, if a frame includes VLAN information in its Ethernet header, the switches will read the designated VLAN.
A future post will demonstrate how to configure VLAN trunking and some things to watch out for.
