www.firewall.cx Open in urlscan Pro
162.254.144.167  Public Scan

Form analysis
2 forms found in the DOM

POST /networking-topics/vlan-networks.html

<form id="searchbox-40" class="searchbox" action="/networking-topics/vlan-networks.html" method="post" role="search">
  <input type="text" value="" name="searchword" placeholder="search..." autocomplete="off">
  <ul class="results" style="display: none;"></ul>
  <button type="reset" value="Reset"></button>
  <input type="hidden" name="task" value="search">
  <input type="hidden" name="option" value="com_search">
  <input type="hidden" name="Itemid" value="195">
</form>

Name: formAcymailing44431 — POST /networking-topics/vlan-networks.html

<form id="formAcymailing44431" action="/networking-topics/vlan-networks.html" onsubmit="return submitacymailingform('optin','formAcymailing44431')" method="post" name="formAcymailing44431">
  <div class="acymailing_module_form">
    <div class="acymailing_introtext"><strong> Notify me of new articles</strong></div>
    <table class="acymailing_form">
      <tbody>
        <tr>
          <td class="acyfield_name acy_requiredField">
            <input id="user_name_formAcymailing44431" style="width:80%" onfocus="if(this.value == 'Name') this.value = '';" onblur="if(this.value=='') this.value='Name';" type="text" class="inputbox required" name="user[name]" value="Name"
              title="Name">
          </td>
        </tr>
        <tr>
          <td class="acyfield_email acy_requiredField">
            <input id="user_email_formAcymailing44431" style="width:80%" onfocus="if(this.value == 'E-mail') this.value = '';" onblur="if(this.value=='') this.value='E-mail';" type="text" class="inputbox required" name="user[email]" value="E-mail"
              title="E-mail">
          </td>
        </tr>
        <tr>
          <td class="acysubbuttons">
            <input class="button subbutton btn btn-primary" type="submit" value="Subscribe" name="Submit"
              onclick="try{ return submitacymailingform('optin','formAcymailing44431'); }catch(err){alert('The form could not be submitted '+err);return false;}">
          </td>
        </tr>
      </tbody>
    </table>
    <input type="hidden" name="ajax" value="0">
    <input type="hidden" name="acy_source" value="module_139">
    <input type="hidden" name="ctrl" value="sub">
    <input type="hidden" name="task" value="notask">
    <input type="hidden" name="redirect" value="https%3A%2F%2Fwww.firewall.cx%2Fnetworking-topics%2Fvlan-networks%2F219-vlan-tagging.html">
    <input type="hidden" name="redirectunsub" value="https%3A%2F%2Fwww.firewall.cx%2Fnetworking-topics%2Fvlan-networks%2F219-vlan-tagging.html">
    <input type="hidden" name="option" value="com_acymailing">
    <input type="hidden" name="hiddenlists" value="2">
    <input type="hidden" name="acyformname" value="formAcymailing44431">
  </div>
</form>

Text Content

 * Firewall.cx Team
 * News
 * Alternative Menu
 * Recommended Sites
 * Forum
 * Contact Us - Feedback

 * Home
 * Networking
    * Network Fundamentals
    * Network Cabling
    * OSI Model
    * Ethernet
      * Ethernet Frame Formats
      * Fast Ethernet
    * Network Protocols
      * TCP
      * IP Protocol
      * Subnetting
      * ICMP
      * Domain Name System (DNS)
      * Supernetting & CIDR
      * Spanning Tree Protocol (STP)
      * Netflow
    * Routing
      * Routing Protocols
      * OSPF Routing Protocol
    * Network Address Translation
    * VLAN Networks
      * Designing VLANs
      * Virtual Trunk Protocol (Cisco VTP)
    * Firewalls
      * Palo Alto Firewalls
    * WAN Technologies

 * Cisco
    * Cisco Routers
    * Cisco Switches
    * Cisco Data Center
    * Cisco VoIP/CCME - CallManager
    * Cisco Firewalls
    * Cisco Wireless
    * Cisco Services & Technologies
    * Cisco Authors & CCIE Interviews
    * Cisco Data Center User Group

 * Microsoft
    * Windows XP, Vista, 7
    * Windows 8 & Windows 8.1
    * Windows 2000 Server
    * Windows 2003 Server
    * Windows 2012 Server
    * Windows 2016 Server

 * Linux
    * Introduction To Linux
    * Linux Administration
    * System and Network Services
    * OpenMosix- Linux Supercomputer

 * More Content
    * Hot Product Reviews
    * Security Articles
    * Network Protocol Analyzers
    * Web Application Security Scanners
    * ManageEngine
      * OpManager - Network Monitoring & Management
    * SASE & SD-WAN Networks
    * Security Service Edge (SSE)
    * IP PBX - Unified Comms
    * Virtualization & VM Backup
    * GFI Network Security
      * GFI LanGuard: Network Security Scanner
      * GFI WebMonitor: Web Security & Monitoring
    * Other Articles
    * Whitepapers
    * IT Books - Cert. Guide Reviews
    * FCX Related
      * Free Cisco Lab
    * Cisco Password Decoder
    * Network Tools
    * E-mail Security White Papers
    * Cloud-Based Solutions

 * VPN
    * VPN News
    * Best VPN Reviews
    * VPN Guides & Articles

 * Downloads


Thursday, 16 February 2023
HomeNetworkingVLAN NetworksVLAN Tagging - Understanding VLANs Ethernet Frames


HOT DOWNLOADS


 * AUTOMATIC PATCHING: O/S +750 APPS
   
   Free Download


 * FREE HYPER-V & VMWARE BACKUP
   
   Get 2 VMs for FREE, forever!


 * DEAL WITH BANDWIDTH SPIKES
   
   Free Download


 * FREE HYPER-V & VMWARE BACKUP
   
   Get 2 VMs for FREE, forever!


 * AUTOMATIC PATCHING: O/S +750 APPS
   
   Free Download


 * FREE NETWORK-SERVER MONITORING
   
   Manage your Network!


 * DEAL WITH BANDWIDTH SPIKES
   
   Free Download


 * FREE HYPER-V & VMWARE BACKUP
   
   Get 2 VMs for FREE, forever!


 * AUTOMATIC PATCHING: O/S +750 APPS
   
   Free Download





VLAN TAGGING - UNDERSTANDING VLANS ETHERNET FRAMES

Written by Administrator. Posted in VLAN Networks

We mentioned that Trunk Links are designed to pass frames (packets) from all
VLANs, allowing us to connect multiple switches together and independently
configure each port to a specific VLAN. However, we haven't explained 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:



Here we see 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 we call a port 'Access Link' or 'Trunk Link', we are describing it
based on the way it has been configured. This is because a port can be
configured as an Access Link or Trunk Link (in the case where it's 100Mbits or
faster).

This is stressed because a lot of people think that it's the other way around,
meaning, 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 Configuration, InterVLAN routing,Trunk Link configuration for Cisco Layer 3
switches (3550, 3560 series, 3750 series, 4500 series and 6500 series switches)
is covered extensively at the following article: Basic & Advanced Catalyst Layer
3 Switch Configuration: Creating VLANs, InterVLAN Routing (SVI), VLAN Security –
VLAN Hopping, VTP Configuration, Trunk Links, NTP. IOS License Requirements for
SVI Routing.

 

VLAN TAGGING PROTOCOL

We're now familiar with the term 'Trunk Link' and its purpose, that is, to allow
frames from multiple VLANs to run across the network backbone, finding their way
to their destination. 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 ...
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. The protocol can be used in various equipments such as switch ports,
router interfaces, server interface cards to create a trunk to a server and much
more. You'll find more information on VLAN implementations on our last page of
the VLAN topic.

Being a propriety protocol, ISL is available and supported naturally on Cisco
products only:) You may also be interested in knowing that ISL is what we call,
an 'external tagging process'. 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, 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, as illustrated below:



Despite this extra overhead, ISL is capable of supporting up to 1000 VLANs and
does not introduce any delays in data transfers between Trunk Links.

In the above diagram we can see an ISL frame encapsulating an Ethernet II frame.
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.

The encapsulation method mentioned above also happens to be the reason why only
ISL-aware devices are able to read it, and because of the addition of an ISL
header and FCS field, the frame can end up being 1548 bytes long! For those who
can't remember, Ethernet's maximum frame size is 1518 bytes, making an ISL frame
of 1548 bytes, what we call a 'giant' or 'jumbo' frame!

Lastly, ISL uses Per VLAN Spanning Tree (PVST) which runs one instance of the
Spanning Tree Protocol (STP) per VLAN. 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.

Since the ISL's header fields are covered on a separate page, we won't provide
further details here.

 

IEEE 802.1Q

The 802.1q standard was created by the IEEE group to address the problem
breaking large networks into smaller and manageable ones through the use of
VLANs. The 802.1q standard is of course an alternative to Cisco's ISL, and one
that all vendors implement on their network equipment to ensure compatibility
and seamless integration with the existing network infrastructure.

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.

In addition to the compatability issue, there are several more reasons for which
most engineers prefer this method of tagging. 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, the 802.1q tagging method supports a whopping 4096 VLANs (as
opposed to 1000 VLANs ISL supports), a large amount indeed which is merely
impossible to deplet in your local area network.

The 4-byte tag we mentioned is inserted within the existing Ethernet frame,
right after the Source MAC Address as illustrated in the diagram below:



Because of the extra 4-byte tag, the minimum Ethernet II frame size increases
from 64 bytes to 68 bytes, while the maximum Ethernet II frame size now becomes
1522 bytes. If you require more information on the tag's fields, visit our
protocol page where further details are given.

As you may have already concluded yourself, the maximum Ethernet frame is
considerably smaller in size (by 26 bytes) when using the IEEE 802.1q tagging
method rather than ISL. This difference in size might also be interpreted by
many that the IEEE 802.1q tagging method is much faster than ISL, but this is
not true. In fact, Cisco recommends you use ISL tagging when in a Cisco native
environment, but as outlined earlier, most network engineers and administrators
believe that the IEEE802.1q approach is much safer, ensuring maximum
compatability.

And because not everything in this world is perfect, no matter how good the
802.1q tagging protocol might seem, it does come with its restrictions:

 * In a Cisco powered network, the switch maintains one instance of the Spanning
   Tree Protocol (STP) per VLAN. This means that if you have 10 VLANs in your
   network, there will also be 10 instances of STP running amongst the switches.
   In the case of non-Cisco switches, then only 1 instance of STP is maintained
   for all VLANs, which is certainly not something a network administrator would
   want.
 * It is imperative that the VLAN for an IEEE 802.1q trunk is the same for both
   ends of the trunk link, otherwise network loops are likely to occur.
 * Cisco always advises that disabling a STP instance on one 802.1q VLAN trunk
   without disabling it on the rest of the available VLANs, is not a good idea
   because network loops might be created. It's best to either disable or enable
   STP on all VLANs.

 

LAN EMULATION (LANE)

LAN Emulation was introduced to solve the need of creating VLANs over WAN links,
allowing network managers to define workgroups based on logical function, rather
than physical location. With this new technology (so to speak - it's actually
been around since 1995!), we are now able to create VLANs between remote
offices, regardless of their location and distance.

LANE is not very common and you will most probably never see it implemented in
small to mid-sized networks, however, this is no reason to ignore it. Just keep
in mind that we won't be looking at it in much depth, but briefly covering it so
we can grasp the concept.

LANE has been supported by Cisco since 1995 and Cisco's ISO release 11.0. When
implemented between two point-to-point links, the WAN network becomes totally
transparent to the end users:



Every LAN or native ATM host, like the switch or router shown in the diagram,
connects to the ATM network via a special software interface called 'LAN
Emulation Client'. The LANE Client works with the LAN Emulation Server (LES) to
handle all messages and packets flowing through the network, ensuring that the
end clients are not aware of the WAN network infrastructure and therefore making
it transparent.

The LANE specification defines a LAN Emulation Configuration Server (LECS), a
service running inside an ATM switch or a physical server connected to the ATM
switch, that resides within the ATM network and allows network administrators to
control which LANs are combined to form VLANs.

The LAN Emulation Server with the help of the LANE Client, maps MAC addresses to
ATM addresses, emulating Layer 2 protocols (DataLink layer) and transporting
higher layer protocols such as TCP/IP, IPX/SPX without modification.

 

802.10 (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, connecting them to an FDDI backbone. This
backbone interconnects all major network switches, providing a fully redundant
network.

The various modules available for the Cisco Catalyst switches allow the
integration of Ethernet into the FDDI network. When intalling the appropriate
switch modules and with the use of the 802.10 SAID field, a mapping between the
Ethernet VLAN and 802.10 network is created, and as such, all Ethernet VLANs are
able to run over the FDDI network.



The diagram above shows two Catalyst switches connected to a FDDI backbone. 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). At both ends, the switches have an Ethernet port belonging
to VLAN 6, and to 'connect' these ports we map each switch's Ethernet module
with its FDDI module.

Lastly, the special FDDI modules mentioned above support both single VLANs
(non-trunk) and multiple VLANs (trunk).

To provide further detail, the diagram below shows the IEEE 802.10 frame, along
with the SAID field in which the VLAN ID is inserted, allowing the frame to
transit trunk links as described:



It's okay if your impressed or seem confused with the structure of the above
frame, that's normal:) You'll be suprised 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, the original Ethernet II frame is converted to an Ethernet
SNAP frame and then finally to an IEEE 802.10 frame. This conversion is required
to maintain compatability and reliability between the two different topologies.
The most important bit to remember here is the SAID field and its purpose.

 

SUMMARY

This page introduced four popular VLAN tagging methods, providing you with the
frame structure and general details of each tagging method. Out of all, the IEEE
802.1q and ISL tagging methods are the most popular, so make sure you understand
them quite well.

Previous - VLANs - Access & Trunk
Links                                                                       Next
- InterSwitch Link (ISL) Protocol Analysis

or                          

Back to VLAN Networks Section


ARTICLES TO READ NEXT:

123


 * VLANs - IEEE 802.1q Trunk Link Protocol Analysis
   
 * VLAN Security - Making the Most of VLANs
   
 * The VLAN Concept - Introduction to VLANs
   
 * VLAN InterSwitch Link (ISL) Protocol Analysis
   
 * VLAN Tagging - Understanding VLANs Ethernet Frames
   
 * InterVLAN Routing - Routing between VLAN Networks
   
 * VLANs - Access & Trunk Links
   




SECURE SD-WAN


SECURITY SERVICE EDGE (SSE)




FREE HYPER-V & VMWARE BACKUP


RECOMMENDED DOWNLOADS

 * Network Management - Monitor & Alert
 * Free Hyper-V & VMware Backup
 * SD-WAN Networks & Security
 * Bandwidth Monitor
 * Patch Manager Plus


BANDWIDTH MONITOR


NETWORK AND SERVER MONITORING

 * 
 * 
 * 
 * 

Join Us:


SECURE SD-WAN


FREE PATCHMANAGER




CISCO PRESS REVIEW PARTNER

Notify me of new articles




NETWORKING MENU

 * Network Fundamentals
 * Network Cabling
 * OSI Model
 * Ethernet
 * Network Protocols
 * Routing
 * Network Address Translation
 * VLAN Networks
   * Designing VLANs
   * Virtual Trunk Protocol (Cisco VTP)
 * Firewalls
 * WAN Technologies


POPULAR CISCO ARTICLES

DMVPN Configuration
Cisco IP SLA
VLAN Security
4507R-E Installation
CallManager Express Intro
Secure CME - SRTP & TLS
Cisco Password Crack
Site-to-Site VPN


POPULAR LINUX ARTICLES

Linux Init & RunLevels
Linux Groups & Users
Linux Performance Monitoring
Linux Vim Editor
Linux Samba
Linux DHCP Server
Linux Bind DNS
Linux File & Folder Permissions
Linux OpenMosix
Linux Network Config


RSS SUBSCRIPTION

Subscribe to Firewall.cx RSS Feed by Email


CCENT/CCNA

 * Router Basics
 * Subnetting
 * OSI Model
 * IP Protocol


CISCO ROUTERS

 * SSL WebVPN
 * Securing Routers
 * Policy Based Routing
 * Router on-a-Stick


VPN SECURITY

 * Understand DMVPN
 * GRE/IPSec Configuration
 * Site-to-Site IPSec VPN
 * IPSec Modes


CISCO HELP

 * VPN Client Windows 8
 * VPN Client Windows 7
 * CCP Display Problem
 * Cisco Support App.


WINDOWS 2012

 * New Features
 * Licensing
 * Hyper-V / VDI
 * Install Hyper-V


LINUX

 * File Permissions
 * Webmin
 * Groups - Users
 * Samba Setup

 * Firewall.cx Team
 * News
 * Alternative Menu
 * Recommended Sites
 * Forum
 * Contact Us - Feedback

© Copyright 2000-2022 Firewall.cx - All Rights Reserved
Information and images contained on this site is copyrighted material.

Firewall.cx - Cisco Networking, VPN - IPSec, Security, Cisco Switching, Cisco
Routers, Cisco VoIP - CallManager Express, Windows Server, Virtualization,
Hyper-V, Web Security, Linux Administration