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VLAN Tagging

Trunk Links are designed to pass frames (packets) from all VLANs, allowing to connect multiple
switches together and independently configure each port to a specific VLAN. However, how
these packets run through the Trunk Links and network backbone, eventually finding their way
to the destination port without getting mixed or lost with the rest of the packets flowing through
the Trunk Links. This is process belongs to the world of VLAN Tagging!
VLAN Tagging
VLAN Tagging, also known as Frame Tagging, is a method developed by Cisco to help identify
packets travelling through trunk links. When an Ethernet frame traverses a trunk link, a special
VLAN tag is added to the frame and sent across the trunk link. As it arrives at the end of the
trunk link the tag is removed and the frame is sent to the correct access link port according to the
switch's table, so that the receiving end is unaware of any VLAN information. The diagram
below illustrates the process described above:

There are two 3500 series Catalyst switches and one Cisco 3745 router connected via the Trunk
Links. The Trunk Links allow frames from all VLANs to travel throughout the network
backbone and reach their destination regardless of the VLAN the frame belongs to. On the other
side, the workstations are connected directly to Access Links (ports configured for one VLAN
membership only), gaining access to the resources required by VLAN's members. Again, when

but encapsulating the Ethernet frame with a new 26 byte ISL header and adding an additional 4 byte frame check sequence (FCS) field at the end of frame. that is. a switch's uplink is always a Trunk Link and any normal port where you would usually connect a workstation. is an Access Link port! VLAN Tagging Protocol We're now familiar with the term 'Trunk Link' and its purpose. The protocol can be used in various equipments such as switch ports. Being a propriety protocol. . You may also be interested in knowing that ISL is what we call.. the VLAN Highway as we like to call it. InterSwitch Link (ISL) ISL is a Cisco propriety protocol used for FastEthernet and Gigabit Ethernet links only. ISL is capable of supporting up to 1000 VLANs and does not introduce any delays in data transfers between Trunk Links. meaning. You'll find more information on VLAN implementations on our last page of the VLAN topic. This is stressed because a lot of people think that it's the other way around. an 'external tagging process'. as illustrated below: Despite this extra overhead. This is because a port can be configured as an Access Link or Trunk Link (in the case where it's 100Mbits or faster).. What you might not have known though is that there is more than one method to 'tag' these frames as they run through the Trunk Links or . server interface cards to create a trunk to a server and much more. to allow frames from multiple VLANs to run across the network backbone. we are describing it based on the way it has been configured.we call a port 'Access Link' or 'Trunk Link'. ISL is available and supported naturally on Cisco products only. This means that the protocol does not alter the Ethernet frame as shown above in our previous diagram .placing the VLAN Tag inside the Ethernet frame. finding their way to their destination. router interfaces.

.In the above diagram we can see an ISL frame encapsulating an Ethernet II frame. IEEE 802. there are several more reasons for which most engineers prefer this method of tagging. a large amount indeed which is merely impossible to deplet in your local area network. what we call a 'giant' or 'jumbo' frame! Lastly.1q standard is of course an alternative to Cisco's ISL. the frame can end up being 1548 bytes long! For those who can't remember. the 802. As with all 'open standards' the IEEE 802.1q tagging method is by far the most popular and commonly used even in Cisco oriented network installations mainly for compatability with other equipment and future upgrades that might tend towards different vendors. The 802. and one that all vendors implement on their network equipment to ensure compatibility and seamless integration with the existing network infrastructure. we won't provide further details here.1q tagging method supports a whopping 4096 VLANs (as opposed to 1000 VLANs ISL supports). In addition to the compatability issue. and because of the addition of an ISL header and FCS field. making an ISL frame of 1548 bytes. This is the actual frame that runs through a trunk link between two Cisco devices when configured to use ISL as their trunk tagging protocol. This method allows us to optimise the root switch placement for each available VLAN while supporting neat features such as VLAN load balancing between multiple trunks. The encapsulation method mentioned above also happens to be the reason why only ISL-aware devices are able to read it.1q The 802. ISL uses Per VLAN Spanning Tree (PVST) which runs one instance of the Spanning Tree Protocol (STP) per VLAN. Ethernet's maximum frame size is 1518 bytes. These include:  Support of up to 4096 VLANs  Insertion of a 4-byte VLAN tag with no encapsulation  Smaller final frame sizes when compared with ISL Amazingly enough.1q standard was created by the IEEE group to address the problem breaking large networks into smaller and manageable ones through the use of VLANs. Since the ISL's header fields are covered on a separate page.

then only 1 instance of STP is maintained for all VLANs. which is certainly not something a network administrator would want.  Cisco always advises that disabling a STP instance on one 802. the switch maintains one instance of the Spanning Tree Protocol (STP) per VLAN. ensuring maximum compatability. And because not everything in this world is perfect. but this is not true.1q VLAN trunk without disabling it on the rest of the available VLANs. visit our protocol page where further details are given. This difference in size might also be interpreted by many that the IEEE 802. In the case of non-Cisco switches. is not a good idea because network loops might be created. otherwise network loops are likely to occur. Cisco recommends you use ISL tagging when in a Cisco native environment. while the maximum Ethernet II frame size now becomes 1522 bytes.1q trunk is the same for both ends of the trunk link. the maximum Ethernet frame is considerably smaller in size (by 26 bytes) when using the IEEE 802. This means that if you have 10 VLANs in your network. As you may have already concluded yourself.The 4-byte tag we mentioned is inserted within the existing Ethernet frame. there will also be 10 instances of STP running amongst the switches. right after the Source MAC Address as illustrated in the diagram below: Because of the extra 4-byte tag. In fact. but as outlined earlier. it does come with its restrictions:  In a Cisco powered network. LAN Emulation (LANE) . most network engineers and administrators believe that the IEEE802.1q tagging method is much faster than ISL.  It is imperative that the VLAN for an IEEE 802. no matter how good the 802. It's best to either disable or enable STP on all VLANs.1q approach is much safer. If you require more information on the tag's fields. the minimum Ethernet II frame size increases from 64 bytes to 68 bytes.1q tagging protocol might seem.1q tagging method rather than ISL.

the WAN network becomes totally transparent to the end users: Every LAN or native ATM host. IPX/SPX without modification. regardless of their location and distance. When implemented between two point-to-point links. . With this new technology (so to speak .LAN Emulation was introduced to solve the need of creating VLANs over WAN links. rather than physical location. maps MAC addresses to ATM addresses. emulating Layer 2 protocols (DataLink layer) and transporting higher layer protocols such as TCP/IP.0. allowing network managers to define workgroups based on logical function. but briefly covering it so we can grasp the concept. like the switch or router shown in the diagram. The LANE specification defines a LAN Emulation Configuration Server (LECS). LANE has been supported by Cisco since 1995 and Cisco's ISO release's actually been around since 1995!). Just keep in mind that we won't be looking at it in much depth. LANE is not very common and you will most probably never see it implemented in small to mid-sized networks. The LANE Client works with the LAN Emulation Server (LES) to handle all messages and packets flowing through the network. this is no reason to ignore it. ensuring that the end clients are not aware of the WAN network infrastructure and therefore making it transparent. we are now able to create VLANs between remote offices. The LAN Emulation Server with the help of the LANE Client. connects to the ATM network via a special software interface called 'LAN Emulation Client'. a service running inside an ATM switch or a physical server connected to the ATM switch. however. that resides within the ATM network and allows network administrators to control which LANs are combined to form VLANs.

providing a fully redundant network.10 network is created. This backbone interconnects all major network switches. all Ethernet VLANs are able to run over the FDDI network.10 SAID field. and as such. The diagram above shows two Catalyst switches connected to a FDDI backbone. a mapping between the Ethernet VLAN and 802. the switches have an Ethernet port belonging to VLAN 6. allowing the frame to transit trunk links as described: . and to 'connect' these ports we map each switch's Ethernet module with its FDDI module. Lastly.10 (FDDI) Tagging VLAN frames on Fiber Distributed Data Interface (FDDI) networks is quite common in large scale networks. This implementation is usually found on Cisco's high-end switch models such as the Catalyst 5000 series where special modules are installed inside the switches. At both ends. the diagram below shows the IEEE 802.10 frame. connecting them to an FDDI backbone. To provide further detail. The links between the switches and the backbone can either be Access type links (meaning one VLAN passes through them) or Trunk links (all VLANs are able to pass through them). When intalling the appropriate switch modules and with the use of the 802.802. The various modules available for the Cisco Catalyst switches allow the integration of Ethernet into the FDDI network. along with the SAID field in which the VLAN ID is inserted. the special FDDI modules mentioned above support both single VLANs (non-trunk) and multiple VLANs (trunk).

the original Ethernet II frame is converted to an Ethernet SNAP frame and then finally to an IEEE 802. The most important bit to remember here is the SAID field and its purpose. that's normal:) You'll be surprised to find out that the Cisco switch in the previous diagram must process the Ethernet II frame and convert it before placing it on the IEEE 802.10 backbone or trunk. During this stage. . This conversion is required to maintain compatibility and reliability between the two different topologies.It's okay if your impressed or seem confused with the structure of the above frame.10 frame.