Vlan Types

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Vlan Types

  1. 1. Types of VLANs/VLAN TypesNowadays, there is essentially one way of implementing VLANs - port-based VLANs.A port-based VLAN is associated with a port called an access VLAN.However in the network there are a number of terms for VLANs. Some terms definethe type of network traffic they carry and others define a specific function a VLANperforms. The following describes common VLAN terminology:Data VLANA data VLAN is a VLAN that is configured to carry only user-generated traffic. A VLANcould carry voice-based traffic or traffic used to manage the switch, but this trafficwould not be part of a data VLAN. It is common practice to separate voiceand management traffic from data traffic. The importance of separating user datafrom switch management control data and voice traffic is highlighted by the use of aspecial term used to identify VLANs that only carry user data - a "data VLAN". A dataVLAN is sometimes referred to as a user VLAN.Default VLANAll switch ports become a member of the default VLAN after the initial boot up of theswitch. Having all the switch ports participate in the default VLAN makes them allpart of the same broadcast domain. This allows any device connected to any switchport to communicate with other devices on other switch ports. The default VLANfor Cisco switches is VLAN 1.VLAN 1 has all the features of any VLAN, except that you cannot rename it and youcannot delete it. Layer 2 control traffic, such as CDP and spanning tree protocoltraffic, will always be associated with VLAN 1 - this cannot be changed. In the figure,VLAN 1 traffic is forwarded over the VLAN trunks connecting the S1, S2, and S3switches. It is a security best practice to change the default VLAN to a VLAN otherthan VLAN 1; this entails configuring all the ports on the switch to be associated witha default VLAN other than VLAN 1. VLAN trunks support the transmission of trafficfrom more than one VLAN. Although VLAN trunks are mentioned throughout thishttp://www.router-switch.com/
  2. 2. section, they are explained in the next section on VLAN trunking.Note:Some network administrators use the term "default VLAN" to mean a VLANother than VLAN 1 defined by the network administrator as the VLAN that all portsare assigned to when they are not in use. In this case, the only role that VLAN 1 playsis that of handling Layer 2 control traffic for the network.Native VLANA native VLAN is assigned to an 802.1Q trunk port. An 802.1Q trunk port supportstraffic coming from many VLANs (tagged traffic) as well as traffic that does not comefrom a VLAN (untagged traffic). The 802.1Q trunk port places untagged traffic on thenative VLAN. In the figure, the native VLAN is VLAN 99. Untagged traffic is generatedby a computer attached to a switch port that is configured with the native VLAN.Native VLANs are set out in the IEEE 802.1Q specification to maintain backwardcompatibility with untagged traffic common to legacy LAN scenarios. For ourpurposes, a native VLAN serves as a common identifier on opposing ends of a trunklink. It is a best practice to use a VLAN other than VLAN 1 as the native VLAN.Management VLANA management VLAN is any VLAN you configure toaccessthe management capabilities of a switch.VLAN 1 would serveasthe management VLAN if you did not proactively define a unique VLAN to serve asthe management VLAN. You assign the management VLAN an IP address and subnetmask. A switch can be managed via HTTP, Telnet, SSH, or SNMP. Sincetheout-of-the-box configuration of a Cisco switch has VLAN 1 as the default VLAN,you see that VLAN 1 would be a bad choice as the management VLAN; you wouldntwant an arbitrary user connecting to a switch to default to the management VLAN.Recall that you configured the management VLAN as VLAN 99 in the Basic SwitchConcepts and Configuration chapter.Voice VLANshttp://www.router-switch.com/
  3. 3. It is easy to appreciate why aseparate VLAN is needed to support Voice over IP (VoIP).Imagine you are receiving an emergency call and suddenly the quality of thetransmission degrades so much you cannot understand what the caller issaying. VoIPtraffic requires:Assured bandwidth to ensure voice qualityTransmission priority over other types of network trafficAbility to be routed around congested areas on the networkDelay of less than 150 milliseconds (ms) across the networkTo meet these requirements, the entire network has to be designed to support VoIP.The details of how to configure a network to support VoIP are beyond the scope ofthe course, but it is useful to summarize how a voice VLAN works between a switch,a Cisco IP phone, and a computer.In the figure, VLAN 150 is designed to carry voice traffic. The student computer PC5 isattached to the Cisco IP phone, and the phone is attached to switch S3. PC5 is inVLAN 20, which is used for student data. The F0/18 port on S3 is configured to be invoice mode so that it will tell the phone to tag voice frames with VLAN 150. Dataframes coming through theCisco IP phone from PC5 are left untagged. Data destinedfor PC5 coming from port F0/18 is tagged with VLAN 20 on the way to the phone,which strips the VLAN tag before the data is forwarded to PC5. Tagging refers to theaddition of bytes to a field in the data frame which is used by the switch to identifywhich VLAN the data frame should be sent to.A Cisco Phone is a Switchhttp://www.router-switch.com/
  4. 4. The Cisco IP Phone contains an integrated three-port 10/100 switch as shown in theFigure. The ports provide dedicated connections to these devices:Port 1 connects to the switch or other voice-over-IP (VoIP) device.Port 2 is an internal 10/100 interface that carries the IP phone traffic.Port 3 (access port) connects to a PC or other device.The figure shows one way to connect an IP Phone.The voice VLAN feature enables switch ports to carry IP voice traffic from an IP phone.When the switch is connected to an IP Phone, the switch sends messages thatinstruct the attached IP phone to send voice traffic tagged with the voice VLAN ID150. The traffic from the PC attached to the IP Phone passes through the IP phoneuntagged. When the switch port has been configured with a voice VLAN, the linkbetween the switch and the IP phone acts as a trunk to carry both the tagged voicetraffic and untagged data traffic.http://www.router-switch.com/
  5. 5. Sample ConfigurationThe figure shows sample output. A discussion of the Cisco IOS commands are beyondthe scope of this course, but you can see that the highlighted areas in the sampleoutput show the F0/18 interface configured with a VLAN configured for data (VLAN20) and a VLAN configured for voice (VLAN 150).More aboutVLAN TypesHow a Switch distinguishes between VLANs? This is done by associating the workstations to a specific VLAN using specified format. This is known as VLANmembership. Four prominent VLAN membership methods are by port, MAC address,protocol type, and subnet address. Each of these is discussed below:1. VLAN membership by Port:Here, you define which ports of a Switch belong to which VLAN. Any work stationconnected to a particular port will automatically be assigned that VLAN. For example,in a Switch with eight ports, ports 1-4 may be configured with VLAN 1, and ports 5-8may be configured with VLAN2.One of the disadvantages of this method is that it requires Switch portreconfiguration whenever a user (of course, with associated workstation) movesfrom one place to another. VLANs by port association operate at Layer 1 of the OSImodel.2. VLAN membership by MAC Address:Here, membership in a VLAN is based on the MAC address of the user workstation. ASwitch that participates in VLAN uses the MAC addresses to assign a VLAN to eachhttp://www.router-switch.com/
  6. 6. workstation. When a workstation moves to another place, the corresponding switchautomatically discovers the VLAN association based on the MAC address of theworkstation. Since the MAC address is normally inseparable from that of theworkstation, this method of VLAN membership is more amenable to workstationmoves.This type of VLAN works at Layer 2 of the OSI model.3. Membership by Protocol Type:Layer 2 header contains the protocol type field. You can use this information todecide on the VLAN association. For example, all IP traffic may be associated withVLAN 1 and all IPX traffic may be associated with VLAN 2.4. Membership by IP Subnet AddressIn this type of VLAN association, membership is based on the Layer 3 header. TheSwitch reads the Layer 3 IP address and associates a VLAN membership. Note thateven though the Switch accesses Layer 3 information, it still works at Layer 2 of OSImodel only. A VLAN Switch doesnt do any routing based on IP address.Examples:IP Subnet VLAN192.23.160.0 1192.23.161.0 2112.18.0.0 3IP Subnet addresses assignment to different VLANs.IP address based VLANs allow user moves. However, it is likely to take more time toforward a packet by a Switch because it has to read Layer 3 information. Hence thelatency rates may be relatively more using this type of VLAN membership.More Related Networking Tips:‘What Happens in the VLAN Stays in the VLAN?’How Private VLANs Work?How to Configure Private VLANs on Cisco 3560 Switches?VLAN Trunking Protocol (VTP) & VTP ModesTypes of NetworksVLAN Switch Port Modeshttp://www.router-switch.com/

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