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Prerequisites for VLANs
The following are prerequisites and considerations for configuring VLANs:
Before you create VLANs, you must decide whether to use VLAN Trunking Protocol (VTP) to maintain global VLAN configuration for your network.
If you plan to configure many VLANs on the switch and to not enable routing, you can set the Switch Database Management (SDM) feature to the VLAN template, which configures system resources to support the maximum number of unicast MAC addresses.
Switches running the LAN Base feature set support only static routing on SVIs.
A VLAN should be present in the switch to be able to add it to the VLAN group.
Restrictions for VLANs
The following are restrictions for VLANs:
The switch supports per-VLAN spanning-tree plus (PVST+) or rapid PVST+ with a maximum of 128 spanning-tree instances. One spanning-tree instance is allowed per VLAN.
The switch supports IEEE 802.1Q trunking methods for sending VLAN traffic over Ethernet ports.
Configuring an interface VLAN router's MAC address is not supported. The interface VLAN already has an MAC address assigned by default.
Private VLANs are not supported on the switch.
You cannot have a switch stack containing a mix of Catalyst 3850 and Catalyst 3650 switches.
Information About VLANs
A VLAN is a switched network that is logically segmented by function, project team, or application, without regard to the physical locations of the users. VLANs have the same attributes as physical LANs, but you can group end stations even if they are not physically located on the same LAN segment. Any switch port can belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and flooded only to end stations in the VLAN. Each VLAN is considered a logical network, and packets destined for stations that do not belong to the VLAN must be forwarded through a router or a switch supporting fallback bridging. In a switch stack, VLANs can be formed with ports across the stack. Because a VLAN is considered a separate logical network, it contains its own bridge Management Information Base (MIB) information and can support its own implementation of spanning tree.
Figure 1. VLANs as Logically Defined Networks
VLANs are often associated with IP subnetworks. For example, all the end stations in a particular IP subnet belong to the same VLAN. Interface VLAN membership on the switch is assigned manually on an interface-by-interface basis. When you assign switch interfaces to VLANs by using this method, it is known as interface-based, or static, VLAN membership.
Traffic between VLANs must be routed.
The switch can route traffic between VLANs by using switch virtual interfaces (SVIs). An SVI must be explicitly configured and assigned an IP address to route traffic between VLANs.
The switch supports VLANs in VTP client, server, and transparent modes. VLANs are identified by a number from 1 to 4094. VLAN 1 is the default VLAN and is created during system initialization. VLAN IDs 1002 through 1005 are reserved for Token Ring and FDDI VLANs. All of the VLANs except 1002 to 1005 are available for user configuration.
There are 3 VTP versions: VTP version 1, version 2, and version 3. All VTP versions support both normal and extended range VLANs, but only with VTP version 3, does the switch propagate extended range VLAN configuration information. When extended range VLANs are created in VTP versions 1 and 2, their configuration information is not propagated. Even the local VTP database entries on the switch are not updated, but the extended range VLANs configuration information is created and stored in the running configuration file.
You configure a port to belong to a VLAN by assigning a membership mode that specifies the kind of traffic the port carries and the number of VLANs to which it can belong.
When a port belongs to a VLAN, the switch learns and manages the addresses associated with the port on a per-VLAN basis.
Table 1 Port Membership Modes and Characteristics
VLAN Membership Characteristics
A static-access port can belong to one VLAN and is manually assigned to that VLAN.
VTP is not required. If you do not want VTP to globally propagate information, set the VTP mode to transparent. To participate in VTP, there must be at least one trunk port on the switchor the switch stack connected to a trunk port of a second switch or switch stack.
A trunk port is a member of all VLANs by default, including extended-range VLANs, but membership can be limited by configuring the allowed-VLAN list. You can also modify the pruning-eligible list to block flooded traffic to VLANs on trunk ports that are included in the list.
VTP is recommended but not required. VTP maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs on a network-wide basis. VTP exchanges VLAN configuration messages with other switches over trunk links.
A voice VLAN port is an access port attached to a Cisco IP Phone, configured to use one VLAN for voice traffic and another VLAN for data traffic from a device attached to the phone.
VTP is not required; it has no effect on a voice VLAN.
Configurations for VLAN IDs 1 to 1005 are written to the vlan.dat file (VLAN database), and you can display them by entering the show vlan privileged EXEC command. The vlan.dat file is stored in flash memory. If the VTP mode is transparent, they are also saved in the switch running configuration file.
In a switch stack, the whole stack uses the same vlan.dat file and running configuration. On some switches, the vlan.dat file is stored in flash memory on the active switch.
You use the interface configuration mode to define the port membership mode and to add and remove ports from VLANs. The results of these commands are written to the running-configuration file, and you can display the file by entering the show running-config privileged EXEC command.
When you save VLAN and VTP information (including extended-range VLAN configuration information) in the startup configuration file and reboot the switch, the switch configuration is selected as follows:
If the VTP mode is transparent in the startup configuration, and the VLAN database and the VTP domain name from the VLAN database matches that in the startup configuration file, the VLAN database is ignored (cleared), and the VTP and VLAN configurations in the startup configuration file are used. The VLAN database revision number remains unchanged in the VLAN database.
If the VTP mode or domain name in the startup configuration does not match the VLAN database, the domain name and VTP mode and configuration for the VLAN IDs 1 to 1005 use the VLAN database information.
In VTP versions 1 and 2, if VTP mode is server, the domain name and VLAN configuration for VLAN IDs 1 to 1005 use the VLAN database information. VTP version 3 also supports VLANs 1006 to 4094.
Normal-Range VLAN Configuration Guidelines
Normal-range VLANs are VLANs with IDs from 1 to 1005.
Follow these guidelines when creating and modifying normal-range VLANs in your network:
Normal-range VLANs are identified with a number between 1 and 1001. VLAN numbers 1002 through 1005 are reserved for Token Ring and FDDI VLANs.
VLAN configurations for VLANs 1 to 1005 are always saved in the VLAN database. If the VTP mode is transparent, VTP and VLAN configurations are also saved in the switch running configuration file.
If the switch is in VTP server or VTP transparent mode, you can add, modify or remove configurations for VLANs 2 to 1001 in the VLAN database. (VLAN IDs 1 and 1002 to 1005 are automatically created and cannot be removed.)
Extended-range VLANs created in VTP transparent mode are not saved in the VLAN database and are not propagated. VTP version 3 supports extended range VLAN (VLANs 1006 to 4094) database propagation in VTP server mode.
Before you can create a VLAN, the switch must be in VTP server mode or VTP transparent mode. If the switch is a VTP server, you must define a VTP domain or VTP will not function.
The switch does not support Token Ring or FDDI media. The switch does not forward FDDI, FDDI-Net, TrCRF, or TrBRF traffic, but it does propagate the VLAN configuration through VTP.
The switch supports 128 spanning tree instances. If a switch has more active VLANs than supported spanning-tree instances, spanning tree can be enabled on 128 VLANs and is disabled on the remaining VLANs. If you have already used all available spanning-tree instances on a switch, adding another VLAN anywhere in the VTP domain creates a VLAN on that switch that is not running spanning-tree. If you have the default allowed list on the trunk ports of that switch (which is to allow all VLANs), the new VLAN is carried on all trunk ports. Depending on the topology of the network, this could create a loop in the new VLAN that would not be broken, particularly if there are several adjacent switches that all have run out of spanning-tree instances. You can prevent this possibility by setting allowed lists on the trunk ports of switches that have used up their allocation of spanning-tree instances. If the number of VLANs on the switch exceeds the number of supported spanning-tree instances, we recommend that you configure the IEEE 802.1s Multiple STP (MSTP) on your switch to map multiple VLANs to a single spanning-tree instance.
When a switch in a stack learns a new VLAN or deletes or modifies an existing VLAN (either through VTP over network ports or through the CLI), the VLAN information is communicated to all stack members.
When a switch joins a stack or when stacks merge, VTP information (the vlan.dat file) on the new switches will be consistent with the active switch.
Extended-range VLANs are VLANs with IDs from 1006 to 4094.
Follow these guidelines when creating extended-range VLANs:
VLAN IDs in the extended range are not saved in the VLAN database and are not recognized by VTP unless the switch is running VTP version 3.
You cannot include extended-range VLANs in the pruning eligible range.
For VTP version 1 or 2, you can set the VTP mode to transparent in global configuration mode. You should save this configuration to the startup configuration so that the switch boots up in VTP transparent mode. Otherwise, you lose the extended-range VLAN configuration if the switch resets. If you create extended-range VLANs in VTP version 3, you cannot convert to VTP version 1 or 2.
. When the maximum number of spanning-tree instances are on the switch, spanning tree is disabled on any newly created VLANs. If the number of VLANs on the switch exceeds the maximum number of spanning-tree instances, we recommend that you configure the IEEE 802.1s Multiple STP (MSTP) on your switch to map multiple VLANs to a single spanning-tree instance.
In a switch stack, the whole stack uses the same running configuration and saved configuration, and extended-range VLAN information is shared across the stack.
With VTP version 1 and 2, if the switch is in VTP transparent mode, you can assign VLAN IDs greater than 1006, but they are not added to the VLAN database.
The switch supports only Ethernet interfaces. Because FDDI and Token Ring VLANs are not locally supported, you only configure FDDI and Token Ring media-specific characteristics for VTP global advertisements to other switches.
Although the switch does not support Token Ring connections, a remote device with Token Ring connections could be managed from one of the supported switches. Switches running VTP Version 2 advertise information about these Token Ring VLANs:
Enters a VLAN ID, and enters VLAN configuration mode. Enter a new VLAN ID to create a VLAN, or enter an existing VLAN ID to modify that VLAN.
The available VLAN ID range for this command is 1 to 4094.
Additional vlan command options include:
access-map—Creates VLAN access-maps or enters the vlan access map command mode.
configuration—Enters the vlan feature configuration mode.
dot1q—Configures VLAN dot1q tag native parameters.
filter—Applies a VLAN filter map to a VLAN list.
group—Creates a VLAN group.
Switch(config-vlan)# name test20
(Optional) Enters a name for the VLAN. If no name is entered for the VLAN, the default is to append the vlan-id value with leading zeros to the word VLAN. For example, VLAN0004 is a default VLAN name for VLAN 4.
The following additional VLAN configuration command options are available:
are—Sets the maximum number of All Router Explorer (ARE) hops for the VLAN.
backupcrf—Enables or disables the backup concentrator relay function (CRF) mode for the VLAN.
bridge—Sets the value of the bridge number for the FDDI net or Token Ring net type VLANs.
exit—Applies changes, bumps the revision number, and exits.
media—Sets the media type of the VLAN.
no—Negates the command or default.
parent—Sets the value of the ID for the parent VLAN for FDDI or Token Ring type VLANs.
remote-span—Configures a remote SPAN VLAN.
ring—Sets the ring number value for FDDI or Token Ring type VLANs.
said—Sets the IEEE 802.10 SAID value.
shutdown—Shuts down the VLAN switching.
state—Sets the operational VLAN state to active or suspended.
ste—Sets the maximum number of Spanning Tree Explorer (STE) hops for the VLAN.
stp—Sets the Spanning Tree characteristics of the VLAN.
When you delete a VLAN from a switch that is in VTP server mode, the VLAN is removed from the VLAN database for all switches in the VTP domain. When you delete a VLAN from a switch that is in VTP transparent mode, the VLAN is deleted only on that specific switchor a switch stack.
You cannot delete the default VLANs for the different media types: Ethernet VLAN 1 and FDDI or Token Ring VLANs 1002 to 1005.
When you delete a VLAN, any ports assigned to that VLAN become inactive. They remain associated with the VLAN (and thus inactive) until you assign them to a new VLAN.
Extended-range VLANs enable service providers to extend their infrastructure to a greater number of customers. The extended-range VLAN IDs are allowed for any switchport commands that allow VLAN IDs.
With VTP version 1 or 2, extended-range VLAN configurations are not stored in the VLAN database, but because VTP mode is transparent, they are stored in the switch running configuration file, and you can save the configuration in the startup configuration file. Extended-range VLANs created in VTP version 3 are stored in the VLAN database.
You can change only the MTU size and the remote SPAN configuration state on extended-range VLANs; all other characteristics must remain at the default state.
Enters the interface configuration mode for the selected VLAN.
Switch(config-if)# ip mtu 1024Switch(config-if)#
(Optional) Modifies the VLAN by changing the MTU size. You can configure the MTU size between 68 to 1500 bytes.
Although all VLAN commands appear in the CLI help, only the ip mtumtu-size and remote-span commands are supported for extended-range VLANs.
Returns to privileged EXEC mode.
show vlan idvlan-id
Switch# show vlan id 2000
Verifies that the VLAN has been created.
copy running-config startup config
Switch# copy running-config startup-config
Saves your entries in the switch startup configuration file. To save an extended-range VLAN configuration, you need to save the VTP transparent mode configuration and the extended-range VLAN configuration in the switch startup configuration file. Otherwise, if the switch resets, it will default to VTP server mode, and the extended-range VLAN IDs will not be saved.
This step is not required for VTP version 3 because VLANs are saved in the VLAN database.
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