| Trunk links using trunking protocols enable multiple VLANs to travel from one switch port to another switch port, from one switch port to a router, from router to router, or, in some cases, a server using a NIC card that supports a trunking protocol. Trunk links are point-to-point high-speed links, from 100Mbps to 10Gbps, configured to carry multiple VLANs.  | The trunk protocols, frame tagging, and additional headers must be stripped from the frames before they are sent out the Access layer switch to the end user (you learn more about frame tagging in "IEEE 802.1Q," later in this chapter). The process of adding and removing trunking information must remain transparent to the end users' interface, because they do not understand the trunking process. |
Four different methods or protocols enable you to track VLAN frames as they traverse a trunk link: -
IEEE 802.10 -
IEEE 802.1Q -
Inter-Switch Link (ISL) -
LAN Emulation (LANE) The following sections discuss these methods. IEEE 802.10 The IEEE 802.10 standard is used to send multiple VLAN sessions over a Fiber Distributed Data Interface (FDDI) physical link. This standard uses a clear header , which is added to VLAN frames traversing an FDDI trunk. A clear header contains three fields: a Security Association Identifier (SAID), a Link Service Access Point (LSAP), and the Management Defined Field (MDF). The SAID field in the frame header is the field used to identify which VLAN the port is a member of. This protocol is proprietary to Cisco devices and is used primarily to transport VLAN information over FDDI backbones between Cisco routers and switches. IEEE 802.1Q The IEEE 802.1Q standard is referred to as the "Standard for Virtual Bridged Local Area Networks." This standard was agreed upon by members of the IEEE as a method of frame tagging , the process of inserting into a frame a field that is used to identify the frame's VLAN membership over a trunk link. This process works as follows : -
As a frame enters the switch fabric through a VLAN port, the data is tagged with the VLAN information of the port the frame arrived on. Just as in ISL, the tag remains in the frame as it is forwarded from switch to switch, and is removed prior to exiting the access link to the destination interface. Unlike ISL, however, which uses an external tagging process, 802.1Q uses an internal tagging process, by modifying the existing Ethernet frame itself. -
IEEE 802.1Q changes the frame header with a standard VLAN format, which allows multiple-vendor VLAN implementations . This enables a Bay Networks device or a 3Com device to pass VLAN traffic to a Cisco device and vice versa. -
When the frame is passed to an Access layer device to be sent directly to the destination interface, the frame is stripped of the tagging information, making the whole process transparent to the destination and sending device. Inter-Switch Link (ISL) Cisco created the ISL protocol and chose to keep ISL proprietary in nature to Cisco devices. ISL is a way of explicitly tagging VLAN information onto an Ethernet frame that is traversing the network through trunk links. This tagging information enables VLANs to be multiplexed over a trunk link through an external encapsulation method. By running ISL, you can interconnect multiple switches and still maintain VLAN information as traffic travels between switches on trunk links. The ISL process works like this: -
Each frame is tagged as it enters a trunk link on the switch. The original frame is not altered ; it is encapsulated within a new 26-byte ISL header and a 4-byte frame check sequence (FCS) at the end of the frame. -
After the frame is tagged with the appropriate VLAN information, the frame can go through all Cisco devices in the network without being reencapsulated. -
The ISL encapsulation is removed from the frame if the frame is set to exit out of a nontrunked link. The ISL header, shown in Figure 9.1, is entered into the frame. Figure 9.1. Here you see the placement of ISL header information in an ISL packet.  The ISL header contains the following: -
Destination address (DA) A 40-bit multicast address set to 01-00-0c-00-00. This address signals to the receiver that this packet is in ISL format. -
Type Indicates the media type that the frame is supporting. The possible options are 0000 for Ethernet, 0001 for token ring, 0010 for FDDI, and 0011 for Asynchronous Transfer Mode (ATM). -
User field A 4-bit field used to identify one of four possible priorities of the frame: XX00 for normal, XX01 for priority 1, XX02 for priority 2, and XX11 for the highest priority. -
Source MAC address (SA) Shows the sending switch port's IEEE 802.3 MAC address. Some receiving devices ignore the SA field. -
LEN field This 16-bit field shows the length of the packet, in bytes, minus the excluded fields. The excluded fields are the CRC, DA, Type, User, and SA fields, as well as the LEN field itself. The total of the excluded fields is 18 bytes. Therefore, the LEN field contains the total packet size minus 18 bytes from the excluded fields. -
AAAA03 Indicates an 802.2 Logical Link Control (LLC) header. -
High bit of source address (HSA) The 3-byte manufacturer's portion of the SA field or vendor field of the source port's MAC address. -
15-bit descriptor Used to distinguish the frame from other VLANs or colors; 10 bits are used to indicate the source port. -
Bridge Protocol Data Units (BPDU) bit Used to indicate Spanning Tree Protocol (STP) or Cisco Discovery Protocol (CDP) topology information. -
Index Used to indicate the port address as the frame exits the switch. This 16-bit index value can be set to any value and can be used for diagnostic purposes only. -
Reserve Used by FDDI and token ring. In token ring, the Frame Control (FC) and token ring AC bits (AC) fields are placed in the header. For FDDI, the FC is placed in the field. For Ethernet, the field contains all zeros. LANE LAN Emulation (LANE) is an IEEE standard for transporting VLANs over ATM networks. This process uses no encapsulation or frame tagging. ATM and LANE are beyond the scope of this book and thus are not covered in great detail. |
