Catalyst 4500 Series Switch Software Configuration Guide, 12.2(54)SG

About VLANs

A VLAN is a group of devices on one or more LANs that are configured to communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

Note VTP version 3 updates do not pass through promiscuous trunk ports.

VLANs define broadcast domains in a Layer 2 network. A broadcast domain is the set of all devices that receives broadcast frames originating from any device within the set. Broadcast domains are typically bounded by switches because switches do not forward broadcast frames. Layer 2 switches create broadcast domains based on the configuration of the switch. Switches are multiport bridges that allow you to create multiple broadcast domains. Each broadcast domain is like a distinct virtual bridge within a switch.

You can define one or many virtual bridges within a switch. Each virtual bridge you create in the switch defines a new broadcast domain (VLAN). Traffic cannot pass directly to another VLAN (between broadcast domains) within the switch or between two switches. To interconnect two different VLANs, you must use switches or Layer 3 switches. See the “About Layer 3 Interfaces” section for information on inter-VLAN routing on Catalyst 4500 series switches.

Figure 13-1 shows an example of three VLANs that create logically defined networks.

Figure 13-1 Sample VLANs

VLANs are often associated with IP subnetworks. For example, all of the end stations in a particular IP subnet belong to the same VLAN. Traffic between VLANs must be routed. You must assign LAN interface VLAN membership on an interface-by-interface basis (termed interface-based or static VLAN membership).

You can set the following parameters when you create a VLAN in the management domain:

• VLAN number
• VLAN name
• VLAN type
• VLAN state (active or suspended)
• Maximum transmission unit (MTU) for the VLAN
• Security Association Identifier (SAID)
• VLAN number to use when translating from one VLAN type to another

Note When the software translates from one VLAN type to another, it requires a different VLAN number for each media type.

VLAN Configuration Guidelines and Restrictions

Follow these guidelines and restrictions when creating and modifying VLANs in your network:

• Before creating a VLAN, put the Catalyst 4500 series switch in VTP server mode or VTP transparent mode. If the Catalyst 4500 series switch is a VTP server, you must define a VTP domain. For information on configuring VTP, see the “VLAN Trunking Protocol” section.
• You cannot use the end command in VLAN database mode.
• You cannot use Ctrl-Z to exit VLAN database mode.
• If a Catalyst 4948 switch running MSTP and configured with all possible VLANs (4094) is in the path of two HSRP peers with the timeout set below 500 ms, HSRP flaps.

Workarounds:

– Use fewer VLANs.

– Set the timers greater than 600 ms.

– Enter the no igmp snooping (globally) and access-list hardware capture mode VLAN commands.

VLAN Ranges

Note You must enable the extended system ID to use 4094 VLANs. See the “Understanding the Bridge ID” section.

With Cisco IOS Release 12.2(25)EWA and later, Catalyst 4500 series switches support 4096 VLANs in compliance with the IEEE 802.1Q standard. These VLANs are organized into three ranges: reserved, normal, and extended.

Some of these VLANs are propagated to other switches in the network when you use the VLAN Trunking Protocol (VTP). The extended-range VLANs are not propagated, so you must configure extended-range VLANs manually on each network device.

Table 13-1 describes the uses for VLAN ranges.

Configurable Normal-Range VLAN Parameters

Note Ethernet VLANs 1 and 1006 through 4094 use only default values.

You can configure the following parameters for VLANs 2 through 1001:

• VLAN name
• VLAN type
• VLAN state (active or suspended)
• SAID
• STP type for VLANs

VLAN Default Configuration

Table 13-2 shows the default VLAN configuration values.

Note Catalyst 4500 series switches do 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 by using VTP. The software reserves parameters for these media types, but they are not supported.

Configuring VLANs

Note Before you configure VLANs, you must use VLAN Trunking Protocol (VTP) to maintain global VLAN configuration information for your network. For complete information on VTP, see the “$paratext>” section.

Note VLANs support a number of parameters that are not discussed in detail in this section. For complete information, refer to the Cisco IOS Command Reference.

Note The VLAN configuration is stored in the vlan.dat file, which is stored in nonvolatile memory. You can cause inconsistency in the VLAN database if you manually delete the vlan.dat file. If you want to modify the VLAN configuration or VTP, use the commands described in the following sections and in the Cisco IOS Command Reference.

The following sections describe how to configure VLANs:

• Configuring VLANs in Global Configuration Mode
• Assigning a Layer 2 LAN Interface to a VLAN

Configuring VLANs in Global Configuration Mode

If the switch is in VTP server or transparent mode (see the “VLAN Trunking Protocol” section), you can configure VLANs in global and VLAN configuration modes. When you configure VLANs in global and config-vlan configuration modes, the VLAN configuration is saved in the vlan.dat files, not the running-config or startup-config files. To display the VLAN configuration, enter the show vlan command.

If the switch is in VLAN transparent mode, use the copy running-config startup-config command to save the VLAN configuration to the startup-config file. After you save the running configuration as the startup configuration, the show running-config and show startup-config commands display the VLAN configuration.

Note When the switch boots, if the VTP domain name and VTP mode in the startup-config and vlan.dat files do not match, the switch uses the configuration in the vlan.dat file.

You use the interface configuration command mode to define the port membership mode and add and remove ports from a VLAN. The results of these commands are written to the running-config file, and you can display the contents of the file by entering the show running-config command.

User-configured VLANs have unique IDs from 1 to 4094. To create a VLAN, enter the vlan command with an unused ID. To verify whether a particular ID is in use, enter the show vlan id ID command . To modify a VLAN, enter the vlan command for an existing VLAN.

See the “VLAN Default Configuration” section for the list of default parameters that are assigned when you create a VLAN. If you do not use the media keyword when specifying the VLAN type, the VLAN is an Ethernet VLAN.

To create a VLAN, perform this task:

When you create or modify an Ethernet VLAN, note the following:

• Because Layer 3 ports and some software features require internal VLANs allocated from 1006 and up, configure extended-range VLANs starting with 4094 and work downward.
• You can configure extended-range VLANs only in global configuration mode. You cannot configure extended-range VLANs in VLAN database mode.
• Layer 3 ports and some software features use extended-range VLANs. If the VLAN you are trying to create or modify is being used by a Layer 3 port or a software feature, the switch displays a message and does not modify the VLAN configuration.
• When you create VLANs with the VLAN configuration command, they are automatically added to the existing VTP domain; no action is required of the user.

This example shows how to create an Ethernet VLAN in global configuration mode and verify the configuration:

Assigning a Layer 2 LAN Interface to a VLAN

A VLAN created in a management domain remains unused until you assign one or more LAN interfaces to the VLAN.

Note Make sure you assign LAN interfaces to a VLAN of the proper type. Assign Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet interfaces to Ethernet-type VLANs.

To assign one or more LAN interfaces to a VLAN, complete the procedures in the “Configuring Ethernet Interfaces for Layer 2 Switching” section.

About VTP

VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs within a VTP domain. A VTP domain (also called a VLAN management domain) is made up of one or more network devices that share the same VTP domain name and that are interconnected with trunks. VTP minimizes misconfigurations and configuration inconsistencies that can result in a number of problems, such as duplicate VLAN names, incorrect VLAN-type specifications, and security violations.

Before you create VLANs, you must decide whether you want to use VTP in your network. With VTP, you can make configuration changes centrally on one or more network devices and have those changes automatically communicated to all the other network devices in the network. For details on configuring VLANs, see the “VLANs” section.

These sections describe how VTP works:

• Understanding the VTP Domain
• Understanding VTP Modes
• Understanding VTP Advertisements
• Understanding VTP Versions
• Understanding VTP Pruning

Understanding the VTP Domain

A VTP domain is made up of one or more interconnected network devices that share the same VTP domain name. A network device can be configured to be in only one VTP domain. You make global VLAN configuration changes for the domain using either the command-line interface (CLI) or Simple Network Management Protocol (SNMP).

By default, the Catalyst 4500 series switch is in VTP server mode and the domain is set to NULL until the switch receives an advertisement for a domain over a trunk link or you configure a management domain. You cannot create or modify VLANs on a VTP server until the management domain name is specified or learned.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch ignores advertisements with a different management domain name or an earlier configuration revision number.

If you configure the switch as VTP transparent, you can create and modify VLANs, but the changes affect only the individual switch.

When you make a change to the VLAN configuration on a VTP server, the change is propagated to all network devices in the VTP domain. VTP advertisements are transmitted out all Inter-Switch Link (ISL) and IEEE 802.1Q trunk connections.

VTP maps VLANs dynamically across multiple LAN types with unique names and internal index associations. Mapping eliminates unnecessary device administration for network administrators.

Understanding VTP Modes

You can configure a Catalyst 4500 series switch to operate in any one of these VTP modes:

• Server—In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters (such as VTP version and VTP pruning) for the entire VTP domain. VTP servers advertise their VLAN configuration to other network devices in the same VTP domain and synchronize their VLAN configuration with other network devices based on advertisements received over trunk links. VTP server is the default mode.

Note In VTP version 3, manipulation of VLANs can be done only to primary servers.

• Client—VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.
• Transparent—VTP transparent network devices do not participate in VTP. A VTP transparent network device does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements. However, in VTP version 2, transparent network devices do forward VTP advertisements that they receive on their trunking LAN interfaces.
• Off—In VTP off mode, a network device functions in the same manner as a VTP transparent device except that it does not forward VTP advertisements.

Note Catalyst 4500 series switches automatically change from VTP server mode to VTP client mode if the switch detects a failure while writing configuration to NVRAM. If this happens, the switch cannot be returned to VTP server mode until the NVRAM is functioning.

Understanding VTP Advertisements

Each network device in the VTP domain sends periodic advertisements out each trunking LAN interface to a reserved multicast address. VTP advertisements are received by neighboring network devices, which update their VTP and VLAN configurations as necessary.

The following global configuration information is distributed in VTP advertisements:

• VLAN IDs (ISL and 802.1Q)
• Emulated LAN names (for ATM LANE)
• 802.10 SAID values (FDDI)
• VTP domain name
• VTP configuration revision number
• VLAN configuration, including maximum transmission unit (MTU) size for each VLAN
• Frame format

Understanding VTP Versions

VTP Version 2

If you use VTP in your network, you must decide whether to use VTP version 2 or version 3.

Note Catalyst 4500 series switches do not support Token Ring or FDDI media. The switch does not forward FDDI, FDDI-Net, Token Ring Concentrator Relay Function (TrCRF), or Token Ring Bridge Relay Function (TrBRF) traffic, but it does propagate the VLAN configuration by using VTP.

VTP version 2 supports the following features, which are not supported in version 1:

• Token Ring support—Supports Token Ring LAN switching and VLANs (TrBRF and TrCRF).
• Unrecognized Type-Length-Value (TLV) support—A VTP server or client propagates configuration changes to its other trunks, even for TLVs it is not able to parse. The unrecognized TLV is saved in NVRAM.
• Version-dependent transparent mode—In VTP version 1 and version 2, a VTP transparent network device forwards VTP messages in transparent mode without checking the version.
• Consistency checks—In VTP version 2, VLAN consistency checks (such as VLAN names and values) are performed only when you enter new information through the CLI or SNMP. Consistency checks are not performed when new information is obtained from a VTP message or when information is read from NVRAM. If the digest on a received VTP message is correct, its information is accepted without consistency checks.

VTP Version 3

VTP version 3 supports the following features not supported in version 1 or version 2:

• Hidden password support—Supports the option of configuring the password as hidden or secret .

When the hidden keyword is specified, that password must be reentered if a takeover command is issued in the domain. The secret key generated from the password string is saved in the const_nvram:vlan.dat file. When configured with this option, the password does not appear in plain text in the configuration. Instead, the secret key associated with the password is saved in hexadecimal format in the running configuration. If the hidden keyword is not specified, the password is saved in clear text in the const_nvram:vlan.dat file as in VTP version 1 and VTP version 2.

When the secret keyword is specified, the password secret key can be directly configured.

• Extended VLAN database propagation support—In VTP version 2, VLAN configuration information is propagated only for VLANs numbered 1 to 1000. In VTP version 3, information also is propagated for extended-range VLANs (VLANs numbered 1006 to 4094).
• On Catalyst 4500 series switches running VTP version 1, VTP version 2, or VTP version 3, default VLANs 1 and 1002 to 1005 cannot be modified.

Note VTP pruning continues to apply only to VLANs numbered 1 to 1000.

• Propagation of any database in a domain—In addition to propagating VLAN database information, VTP can propagate Multiple Spanning Tree (MST) protocol database information.
• Disabling VTP—When VTP is disabled on a trunking port, it applies to all VTP instances on that port. When VTP is disabled globally, the setting applies to all the trunking ports in the system.
• In VTP version 1 and VTP version 2, the role of a VTP server is to back up the database to NVRAM and to allow the administrator to change database information. VTP version 3 introduces the roles of VTP primary server and VTP secondary server. A VTP primary server is used to update the database information. The updates sent out are honored by all the devices in the system. A VTP secondary server can only back up to its NVRAM the VTP configuration received by using updates from the VTP primary server.

The status of primary and secondary servers is a runtime status and is not a configurable option. By default, all devices are initiated as secondary servers. Primary server status is needed only when database updates are needed, and is obtained when the administrator issues a takeover message in the domain. See the “Starting a Takeover” section.

Primary server status is lost upon reload of the device, or when switchover or domain parameters change. Secondary servers back up the configuration and continue to propagate it. Because of that, you may have a working VTP domain without any primary servers.

Understanding VTP Pruning

VTP pruning enhances network bandwidth use by reducing unnecessary flooded traffic, such as broadcast, multicast, and unicast packets. VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate network devices. By default, VTP pruning is disabled.

For VTP pruning to be effective, all devices in the management domain must either support VTP pruning or, on devices that do not support VTP pruning, you must manually configure the VLANs allowed on trunks.

Figure 13-2 shows a switched network without VTP pruning enabled. Interface 1 on Switch 1 and Interface 2 on Switch 4 are assigned to the Red VLAN. A broadcast is sent from the host connected to Switch 1. Switch 1 floods the broadcast and every network device in the network receives it, even though Switches 3, 5, and 6 have no interfaces in the Red VLAN.

You can enable pruning globally on the Catalyst 4500 series switch (see the “Enabling VTP Pruning” section).

Figure 13-2 Flooding Traffic without VTP Pruning

Figure 13-3 shows the same switched network with VTP pruning enabled. The broadcast traffic from Switch 1 is not forwarded to Switches 3, 5, and 6 because traffic for the Red VLAN has been pruned on the links indicated (Interface 5 on Switch 2 and Interface 4 on Switch 4).

Figure 13-3 Flooding Traffic with VTP Pruning

Enabling VTP pruning on a VTP server enables pruning for the entire management domain. VTP pruning takes effect several seconds after you enable it. By default, VLANs 2 through 1000 are eligible for pruning. VTP pruning does not prune traffic from pruning-ineligible VLANs. VLAN 1 is always ineligible for pruning; traffic from VLAN 1 cannot be pruned.

To configure VTP pruning on a trunking LAN interface, use the switchport trunk pruning vlan command. VTP pruning operates when a LAN interface is trunking. You can set VLAN pruning eligibility regardless of whether VTP pruning is enabled or disabled for the VTP domain, whether any given VLAN exists, and regardless of whether the LAN interface is currently trunking.

VTP Configuration Guidelines and Restrictions

Follow these guidelines and restrictions when implementing VTP in your network:

• Supervisor engine redundancy does not support nondefault VLAN data file names or locations. Do not enter the vtp file file_name command on a switch that has a redundant supervisor engine.
• Before installing a redundant supervisor engine, enter the no vtp file command to return to the default configuration.
• When a VTP version 3 device on a trunk port receives messages from a VTP version 2 device, it sends a scaled-down version of the VLAN database on that particular trunk in a VTP version 2 format. A VTP version 3 device does not send out VTP version 2 formatted packets on a trunk port unless it first receives VTP version 2 packets on that trunk.
• Even when a VTP version 3 device detects a VTP version 2 device on a trunk port, it continues to send VTP version 3 packets in addition to VTP version 2 packets, to allow co-existence of two kinds of neighbors off the trunk.
• A VTP version 3 device does not accept configuration information from a VPT version 2 or version 1 device.
• Unlike in VPT version 2, when VTP is configured to be version 3, this does not configure all the version-3-capable devices in the domain to start behaving as VPT version 3 systems.
• When a VTP version 1 device, capable of version 2 or version 3, receives a VTP version 3 packet, the device is configured as a VTP version 2 device provided a VTP version 2 conflict does not exist.
• Devices that are only VTP version 1 capable cannot interoperate with VTP version 3 devices.
• In a Token Ring environment, you must enable VTP version 2 or version 3 for Token Ring VLAN switching to function properly.
• Two VPT version 3 regions can only communicate in transparent mode over a VTP version 1 or VTP version 2 region.
• All network devices in a VTP domain must run the same VTP version.
• You must configure a password on each network device in the management domain when VTP is in secure mode.

• A VTP version 2-capable network device can operate in the same VTP domain as a network device running VTP version 1 if VTP version 2 is disabled on the VTP version 2-capable network device (VTP version 2 is disabled by default).
• Do not enable VTP version 2 on a network device unless all of the network devices in the same VTP domain are version 2-capable. When you enable VTP version 2 on a server, all of the version 2-capable network devices in the domain enable VTP version 2.
• Enabling or disabling VTP pruning on a VTP server enables or disables VTP pruning for the entire management domain.
• Configuring VLANs as eligible for pruning on a Catalyst 4500 series switch affects pruning eligibility for those VLANs on that switch only, not on all network devices in the VTP domain.
• The VLAN database is saved in the NVRAM file in a format compliant with the VTP version running on the system. Since older images supporting only VTP version 2 do not recognize the VTP version 3 file format, the NVRAM VLAN database information is lost if the system is downgraded from a new image supporting VTP to one that does not.

VTP Default Configuration

Table 13-3 shows the default VTP configuration.

The default VTP mode for newly manufactured Catalyst 4500 supervisor engines, Catalyst 4900 series switches, and the Cisco ME 4924-10GE switch is transparent. Deleting vlan.dat or entering the
erase cat4000_flash: command, and resetting the switch changes the VTP mode to server.

Configuring VTP

These sections describe how to configure VTP:

• Configuring VTP Global Parameters
• Configuring the VTP Mode
• Starting a Takeover
• Displaying VTP Statistics
• Displaying VTP Devices in a Domain

Configuring VTP Global Parameters

These sections describe configuring the VTP global parameters:

• Configuring a VTP Password
• Enabling VTP Pruning
• Enabling the VTP Version Number

Note You can enter the VTP global parameters in either global configuration mode or in EXEC mode.

Configuring a VTP Password

To configure the VTP global parameters, use these commands:

This example shows one way to configure a VTP password in global configuration mode:

This example shows how to configure a VTP password in EXEC mode:

Note The password is not stored in the running-config file.

This example shows how to configure a hidden password:

This example shows how the password WATER is displayed when it is configured with the hidden keyword.

Enabling VTP Pruning

To enable VTP pruning in the management domain, perform this task:

This example shows one way to enable VTP pruning in the management domain:

This example shows how to enable VTP pruning in the management domain with any release:

This example shows how to verify the configuration:

For information about configuring prune eligibility, see the “Understanding VTP Pruning” section.

Enabling the VTP Version Number

VTP version 2 is disabled by default on VTP version-2-capable network devices. When you enable VTP
version 2 on a network device, every VTP version-2-capable network device in the VTP domain enables version 2.

Note In a Token Ring environment, you must enable VTP version 2 or VTP version 3 for Token Ring VLAN switching to function properly on devices that support Token Ring interfaces.

To enable the VTP version, perform this task:

This example shows one way to enable VTP version 2:

This example shows how to enable VTP version 2 with any release:

This example shows how to verify the configuration:

Configuring the VTP Mode

To configure the VTP mode, perform this task:

Note When VTP is disabled, you can enter VLAN configuration commands in configuration mode instead of the VLAN database mode and the VLAN configuration is stored in the startup configuration file.

This example shows how to configure the switch as a VTP server:

This example shows how to configure the switch as a VTP client:

This example shows how to disable VTP on the switch:

This example shows how to disable VTP on the switch and to disable VTP advertisement forwarding:

This example shows an example of the VTP configuration parameters when the device is running VTP version 1:

This example shows an example of the VTP configuration parameters when the device is running VTP version 2:

This example shows an example of the VTP configuration parameters when the device is running VTP version 3:

Starting a Takeover

This process applies to VTP version 3 only. To start a takeover, perform this task:

This example shows how to start a takeover and direct it to the vlan database:

Displaying VTP Statistics

To display VTP statistics, including VTP advertisements sent and received and VTP errors, perform this task:

This example shows how to display VTP statistics:

Displaying VTP Devices in a Domain

To display information for all the VTP devices in a domain, perform this task:

This example shows how to display information for VTP devices in a domain:

Switch#

About VMPS

These subsections describe what a VMPS server does and how it operates:

• Understanding the VMPS Server
• Security Modes for VMPS Server
• Fallback VLAN
• Illegal VMPS Client Requests

Understanding the VMPS Server

A VLAN Membership Policy Server (VMPS) provides a centralized server for selecting the VLAN for a port dynamically based on the MAC address of the device connected to the port. When the host moves from a port on one switch in the network to a port on another switch in the network, that switch dynamically assigns the new port to the proper VLAN for that host.

A Catalyst 4500 series switch running Cisco IOS software does not support the functionality of a VMPS. It can only function as a VLAN Query Protocol (VQP) client, which communicates with a VMPS through the VQP. For VMPS functionality, you need to use a Catalyst 4500 series switch (or Catalyst 6500 series switch) running Catalyst operating system (OS) software.

VMPS uses a UDP port to listen to VQP requests from clients, so, it is not necessary for VMPS clients to know if the VMPS resides on a local or remote device on the network. Upon receiving a valid request from a VMPS client, a VMPS server searches its database for an entry of a MAC-address to VLAN mapping.

In response to a request, the VMPS takes one of the following actions:

• If the assigned VLAN is restricted to a group of ports, the VMPS verifies the requesting port against this group and responds as follows:

– If the VLAN is allowed on the port, the VMPS sends the VLAN name to the client in response.

– If the VLAN is not allowed on the port and the VMPS is not in secure mode, the VMPS sends an “ access-denied” response.

– If the VLAN is not allowed on the port and the VMPS is in secure mode, the VMPS sends a “port-shutdown” response.

• If the VLAN in the database does not match the current VLAN on the port and active hosts exist on the port, the VMPS sends an “ access-denied” (open), a “fallback VLAN name” (open with fallback VLAN configured), a “ port-shutdown” (secure), or a “new VLAN name” (multiple) response, depending on the secure mode setting of the VMPS.

If the switch receives an “ access-denied” response from the VMPS, the switch continues to block traffic from the MAC address to or from the port. The switch continues to monitor the packets directed to the port and sends a query to the VMPS when it identifies a new address. If the switch receives a “port-shutdown” response from the VMPS, the switch disables the port. The port must be manually reenabled by using the CLI, Cisco Visual Switch Manager (CVSM), or SNMP.

You can also use an explicit entry in the configuration table to deny access to specific MAC addresses for security reasons. If you enter the none keyword for the VLAN name, the VMPS sends an “access-denied” or “ port-shutdown” response.

Security Modes for VMPS Server

VMPS operates in three different modes. The way a VMPS server responds to illegal requests depends on the mode in which the VMPS is configured:

• Open Mode
• Secure Mode
• Multiple Mode

Open Mode

If no VLAN is assigned to this port, VMPS verifies the requesting MAC address against this port:

• If the VLAN associated with this MAC address is allowed on the port, the VLAN name is returned to the client.
• If the VLAN associated with this MAC address is not allowed on the port, the host receives an “access denied” response.

If a VLAN is already assigned to this port, VMPS verifies the requesting MAC address against this port:

• If the VLAN associated with this MAC address in the database does not match the current VLAN assigned on the port, and a fallback VLAN name is configured, VMPS sends the fallback VLAN name to the client.
• If a VLAN associated with this MAC address in the database does not match the current VLAN assigned on the port, and a fallback VLAN name is not configured, the host receives an “access denied” response.

Secure Mode

If no VLAN is assigned to this port, VMPS verifies the requesting MAC address against this port:

• If the VLAN associated with this MAC address is allowed on the port, the VLAN name is returned to the client.
• If the VLAN associated with this MAC address is not allowed on the port, the port is shut down.

If a VLAN is already assigned to this port, VMPS verifies the requesting MAC address against this port:

• If a VLAN associated with this MAC address in the database does not match the current VLAN assigned on the port, the port is shutdown, even if a fallback VLAN name is configured.

Multiple Mode

Multiple hosts (MAC addresses) can be active on a dynamic port if they are all in the same VLAN. If the link fails on a dynamic port, the port returns to the unassigned state. Any hosts that come online through the port are checked again with VMPS before the port is assigned to a VLAN.

If multiple hosts connected to a dynamic port belong to different VLANs, the VLAN matching the MAC address in the last request is returned to the client provided that multiple mode is configured on the VMPS server.

Note Although Catalyst 4500 series and Catalyst 6500 series switches running Catalyst operating system software support VMPS in all three operation modes, the User Registration Tool (URT) supports open mode only.

Fallback VLAN

You can configure a fallback VLAN name on a VMPS server.

If no VLAN has been assigned to this port, VMPS compares the requesting MAC address to this port:

• If you connect a device with a MAC address that is not in the database, the VMPS sends the fallback VLAN name to the client.
• If you do not configure a fallback VLAN name and the MAC address does not exist in the database, the VMPS sends an “ access-denied” response.

If a VLAN is already assigned to this port, VMPS compares the requesting MAC address to this port:

• If the VMPS is in secure mode, it sends a “ port-shutdown” response, whether a fallback VLAN has been configured on the server.

Illegal VMPS Client Requests

Two examples of illegal VMPS client requests are as follows:

• When a MAC-address mapping is not present in the VMPS database and “no fall back” VLAN is configured on the VMPS.
• When a port is already assigned a VLAN (and the VMPS mode is not “multiple”) but a second VMPS client request is received on the VMPS for a different MAC-address.

Overview of VMPS Clients

The following subsections describe how to configure a switch as a VMPS client and configure its ports for dynamic VLAN membership.

The following topics are included:

• Understanding Dynamic VLAN Membership
• Default VMPS Client Configuration
• Configuring a Switch as a VMPS Client
• Administering and Monitoring the VMPS
• Troubleshooting Dynamic Port VLAN Membership

Understanding Dynamic VLAN Membership

When a port is configured as “dynamic,” it receives VLAN information based on the MAC-address that is on the port. The VLAN is not statically assigned to the port; it is dynamically acquired from the VMPS based on the MAC-address on the port.

A dynamic port can belong to one VLAN only. When the link becomes active, the switch does not forward traffic to or from this port until the port is assigned to a VLAN. The source MAC address from the first packet of a new host on the dynamic port is sent to the VMPS as part of the VQP request, which attempts to match the MAC address to a VLAN in the VMPS database. If there is a match, the VMPS sends the VLAN number for that port. If there is no match, the VMPS either denies the request or shuts down the port (depending on the VMPS security mode setting). See the “About VMPS” section for a complete description of possible VMPS responses.

Multiple hosts (MAC addresses) can be active on a dynamic port if all are in the same VLAN. If the link goes down on a dynamic port, the port returns to the unassigned state and does not belong to a VLAN. Any hosts that come online through the port are checked again with the VMPS before the port is assigned to a VLAN.

For this operation to work, the client device must be able to reach the VMPS. A VMPS client sends VQP requests as UDP packets, trying a certain number of times before giving up. For details on how to set the retry interval, refer to section “Configuring the Retry Interval” section.

The VMPS client also periodically reconfirms the VLAN membership. For details on how to set the reconfirm frequency, refer to section “Administering and Monitoring the VMPS” section.

A maximum of 50 hosts are supported on a given port at any given time. Once this maximum is exceeded, the port is shut down, irrespective of the operating mode of the VMPS server.

Note The VMPS shuts down a dynamic port if more than 50 hosts are active on that port.

Default VMPS Client Configuration

Table 13-4 shows the default VMPS and dynamic port configuration on client switches.

Configuring a Switch as a VMPS Client

This section contains the following topics:

• Configuring the IP Address of the VMPS Server
• Configuring Dynamic Access Ports on a VMPS Client
• Reconfirming VLAN Memberships
• Configuring Reconfirmation Interval
• Reconfirming VLAN Memberships

Configuring the IP Address of the VMPS Server

To configure a Catalyst 4500 series switch as a VMPS client, you must enter the IP address or hostname of the switch acting as the VMPS.

To define the primary and secondary VMPS on a Catalyst 4500 series switch, perform this task:

This example shows how to define the primary and secondary VMPS devices:

Note You can configure up to four VMPS servers using this CLI on the VMPS client.

Configuring Dynamic Access Ports on a VMPS Client

To configure a dynamic access port on a VMPS client switch, perform this task:

This example shows how to configure a dynamic access port and to verify the entry:

Voice Ports

If a VVID (voice VLAN ID) is configured on a dynamic access port, the port can belong to both an access VLAN and a voice VLAN. Consequently, an access port configured for connecting an IP phone can have separate VLANs for the following:

• Data traffic to and from the PC that is connected to the switch through the access port of the IP phone (access VLAN)
• Voice traffic to and from the IP phone (voice VLAN)

Reconfirming VLAN Memberships

To confirm the dynamic port VLAN membership assignments that the switch has received from the VMPS, perform this task:

Configuring Reconfirmation Interval

VMPS clients periodically reconfirm the VLAN membership information received from the VMPS. You can set the number of minutes the VMPS client waits before reconfirming the VLAN-to-MAC-address assignments.

To configure the reconfirmation interval, perform this task:

This example shows how to change the reconfirmation interval to 60 minutes and verify the change:

Configuring the Retry Interval

You can set the number of times that the VMPS client attempts to contact the VMPS before querying the next server.

To configure the retry interval, perform this task:

This example shows how to change the retry count to 5 and to verify the change:

Administering and Monitoring the VMPS

You can display the following information about the VMPS with the show vmps command:

The following example shows how to display VMPS information:

Note Refer to the Cisco IOS Command Reference for details on VMPS statistics.

Troubleshooting Dynamic Port VLAN Membership

VMPS errdisables a dynamic port under the following conditions:

• The VMPS is in secure mode, and it does not allow the host to connect to the port. The VMPS errdisables the port to prevent the host from connecting to the network.
• More than 50 active hosts reside on a dynamic port.

For information on how to display the status of interfaces in error-disabled state, refer to
Chapter7, “Checking Port Status and Connectivity” To recover an errdisabled port, use the
errdisable recovery cause vmps global configuration command.

Dynamic Port VLAN Membership Configuration Example

Figure 13-4 shows a network with a VMPS servers and VMPS client switches with dynamic ports. In this example, these assumptions apply:

• The VMPS server and the VMPS client are separate switches.
• The Catalyst 4000 family Switch 1 (running Catalyst Operating System) is the primary VMPS server.
• The Catalyst 6000 family Switch 3 (running Catalyst Operating System) and the URT are secondary VMPS servers.
• End stations are connected to these clients:

– Catalyst 4500 series XL Switch 2 (running Catalyst Cisco IOS)

– Catalyst 4500 series XL Switch 9 (running Catalyst Cisco IOS)

• The database configuration file is called Bldg-G.db and is stored on the TFTP server with the IP address 172.20.22.7.

Figure 13-4 Dynamic Port VLAN Membership Configuration

Two topologies are possible. Figure 13-5 illustrates a topology with one end station attached directly to a Catalyst 4500 series switch operating as a VMPS client. Figure 13-6 illustrates a topology with an end station attached to a Cisco IP phone, which is attached to a Catalyst 4500 series switch.

Figure 13-5 Topology with an End Station Attached Directly to a Catalyst 4500 Series Switch Operating as a VMPS Client

Figure 13-6 Topology with an End Station Attached to a Cisco IP Phone that is Attached to a Catalyst 4500 Series Switch

In the following procedure, the Catalyst 4000 and Catalyst 6000 series switches (running Catalyst operating system) are the VMPS servers. Use this procedure to configure the Catalyst 4500 series switch clients in the network:

Step 1 Configure the VMPS server addresses on Switch 2, the client switch.

a. Starting from privileged EXEC mode, enter global configuration mode:

b. Enter the primary VMPS server IP address:

c. Enter the secondary VMPS server IP addresses:

d. To verify your entry of the VMPS IP addresses, return to privileged EXEC mode:

e. Display VMPS information configured for the switch:

Step 2 Configure port Fa0/1 on Switch 2 as a dynamic port.

a. Return to global configuration mode:

b. Enter interface configuration mode:

c. Configure the VLAN membership mode for static-access ports:

d. Assign the port dynamic VLAN membership:

e. Return to privileged EXEC mode:

Step 3 Connect End Station 2 on port Fa2/1. When End Station 2 sends a packet, Switch 2 sends a query to the primary VMPS server, Switch 1. Switch 1 responds with the VLAN ID for port Fa2/1. If spanning-tree PortFast mode is enabled on Fa2/1, port Fa2/1 connects immediately and begins forwarding.

Step 4 Set the VMPS reconfirmation period to 60 minutes. The reconfirmation period is the number of minutes the switch waits before reconfirming the VLAN to MAC address assignments.

Step 5 Confirm the entry from privileged EXEC mode:

Step 6 Repeat Steps 1 and 2 to configure the VMPS server addresses, and assign dynamic ports on each VMPS client switch.

VMPS Database Configuration File Example

This example shows a sample VMPS database configuration file as it appears on a VMPS server. A VMPS database configuration file is an ASCII text file that is stored on a TFTP server accessible to the switch that functions as the VMPS server.