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VLANs

Dell™ PowerConnect™ 5224 Systems User's Guide

  VLANs and Frame Tagging

  VLAN Configuration

  Automatic VLAN Registration

  VLAN Examples

VLANs and Frame Tagging

The PowerConnect 5224 switch supports IEEE 802.1Q-compliant virtual LANs (VLANs). This capability provides a highly efficient architecture for establishing VLANs within a network and for controlling broadcast/ multicast traffic between workgroups. Central to this capability is an explicit frame tagging approach for carrying VLAN information between interconnected network devices.

With frame tagging, a four-byte data tag field is attached to frames that cross the network. The tag identifies to which VLAN the frame belongs. The tag may be added to the frame by the end station itself or by a network device, such as a switch. In addition to VLAN information, the relative priority of the frame in the network can be specified by the tag.

VLANs provide greater network efficiency by reducing broadcast traffic, and they also allow you to make network changes without having to update IP addresses or IP subnets. VLANs inherently provide a high level of network security, since traffic must pass through a Layer 3 switch or a router to reach a different VLAN.

The PowerConnect 5224 switch supports the following VLAN features:

• Up to 255 VLANs based on the IEEE 802.1Q standard
  
  
• Distributed VLAN learning across multiple switches using explicit or implicit tagging and GVRP (GARP VLAN Registration Protocol)
  
  
• Port overlapping, allowing a port to participate in multiple VLANs
  
  
• End stations that can belong to multiple VLANs
  
  
• Passing traffic between VLAN-aware and VLAN-unaware devices
  
  
• Four-level priority tagging
  
  
• Link aggregation with VLANs
  
  

VLAN Configuration

By default, VLAN operation on the switch is enabled. Therefore, all frames are transferred internally through the switch with a VLAN tag. This tag may already be on the frame entering the switch, or added to the frame by the switch. VLAN information already existing on frames entering the switch is automatically handled by the switch. The switch learns VLAN information from tagged frames and appropriately switches frames out the proper ports based on this information. The configuration of VLANs for frames entering the switch without tags must be made by the user of the switch. This configuration can be made either through the web or console interface, or through Simple Network Management Protocol (SNMP).

Assigning Ports to VLANs

Before enabling VLANs for the switch, you must first assign each port to the VLAN groups in which it will participate. By default, all ports are assigned to VLAN 1 as untagged ports. You should add a tagged port (a port attached to a VLAN-aware device) if you want it to carry traffic for one or more VLANs and the device at the other end of the link also supports VLANs. Assign the port at the other end of the link to the same VLANs. However, if you want a port on this switch to participate in one or more VLANs and the device at the other end of the link does not support VLANs, you must add an untagged port (a port attached to a VLAN-unaware device).

Port-based VLANs are tied to specific ports. The switch's forwarding determination is based on the destination MAC address and its associated port. Therefore, to make valid forwarding and flooding decisions, the switch learns the relationship of the MAC address to its related port (and to the VLAN) at run-time.

VLAN Classification

Packets that the switch receives are treated in the following ways:

• When an untagged packet enters a port, the system automatically tags it with the port's default VLAN ID tag number. Each port has a default VLAN ID setting that is user configurable. The default setting is 1. You can change the default VLAN ID setting for each port from the VLAN Port Settings page.
  
  
• When a tagged packet enters a port, the default VLAN ID setting has no effect on the tag.
  
  
• The packet proceeds to the VLAN specified by its VLAN ID tag number.
  
  
• If the port in which the packet entered does not belong to the VLAN specified by the packet's VLAN ID tag, the system drops the packet.
  
  

NOTE: You can change port VLAN membership settings in the VLAN Membership page.

• If the port belongs to the VLAN specified by the packet's VLAN ID, the system can send the packet to other ports with the same VLAN ID.
  
  

• Packets leaving the switch are either tagged or untagged depending on that port's membership properties.
  
  
• In the VLAN Membership page, if a U is assigned to a port and VLAN, packets leaving the switch from that port and VLAN are untagged. If a T is assigned to a port and VLAN, packets leaving the switch from that port and VLAN are tagged with the respective ID for the VLAN to which that port belongs.
  
  

Port Overlapping

Port overlapping can be used to allow access to commonly shared network resources among different VLAN groups, such as file servers or printers. If you implement VLANs that do not overlap but still need to communicate, you must connect them using a router or Layer 3 switch.

Forwarding Tagged/Untagged Frames

Ports can be assigned to multiple tagged or untagged VLANs. Each port on the switch is, therefore, capable of passing tagged or untagged frames. To forward a frame from a VLAN-aware device to a VLAN-unaware device, the switch first determines where to forward the frame. The switch then strips off the VLAN tag. However, to forward a frame from a VLAN-unaware device to a VLAN-aware device, the switch first determines where to forward the frame. It then inserts a VLAN tag reflecting this port's default VID. The default port VLAN ID is 1, but it can be changed from the VLAN Port Settings page.

Automatic VLAN Registration

GARP VLAN Registration Protocol (GVRP) defines a way for switches to exchange VLAN information to automatically register VLAN members on ports across the network.

GVRP uses GVRP Bridge Protocol Data Units (GVRP BPDUs) to advertise static VLANs to other switches in the network. Any GVRP-enabled device receiving the advertisements can dynamically join the advertised VLAN. All GVRP-dynamically learned VLANs operate as tagged VLANs. A GVRP-enabled port only joins a VLAN when an advertisement for that VLAN is received on that specific port. A GVRP-enabled port forwards advertisements from other ports on the switch but does not join the advertised VLAN.

Hosts, such as computers and servers, can be connected to switch ports that are part of a statically configured VLAN. If GVRP is enabled on the switch, these VLANs are advertised to the rest of the network. If a host (or its network adapter) supports GVRP, it can directly indicate the VLAN groups that it is supposed to join. When the attached GVRP-enabled switch receives the VLAN advertisements, it automatically places the receiving port in the specified VLANs and then forwards the advertisements to all other ports. When the advertisements arrive at another GVRP-enabled switch, the switch places the receiving port in the specified VLANs, and passes the advertisements on to all other ports. As a result, VLAN requirements are spread throughout the network, which allows GVRP-compliant devices to be automatically configured for VLAN groups based solely on host requests.

The following figure shows how GVRP can propagate VLANs across a network.

VLAN Examples

The following four examples demonstrate typical web-interface VLAN configurations for the switch.

• Example 1 demonstrates a simple two-group VLAN setup.
  
  
• Example 2 demonstrates a more elaborate setup, illustrating all possible scenarios for a comprehensive understanding of tagged VLANs.
  
  
• Examples 3 and 4 show how GVRP can be used to automatically propagate VLANs across a network.
  
  

Example 1

Example 1 illustrates a simple two-group VLAN setup.

1. In the VLAN Membership page, select Add a new VLAN from the Show VLAN drop-down menu.
   
   
2. In the Name box, type New to represent the new VLAN.
   
   
3. In the VLAN ID box, type 2 for the new VLAN.
   
   
4. Click Apply Changes to create the new VLAN.
   
   

5. Select VLAN 2 from the Show VLAN drop-down menu.
   
   

Because there are no ports in the new VLAN, all the port and trunk toggle buttons are blank.

6. Click the toggle buttons in the Static row under the port/trunk numbers to select the desired port members of the new VLAN.
   
   
7. Click Apply Changes to confirm the settings.
   
   

8. To allow untagged packets to participate in the new VLAN, change the Port VLAN IDs for the relevant ports in the Port Settings page.
   
   
9. Click Apply Changes to save any changes. Click Refresh if you don't want to save the changes.
   
   

Example 2

Example 2 illustrates a more complicated setup and demonstrates several scenarios for configuring VLANs.

1. Set up four VLANs, as shown in following table.
   
   

All switch ports remain as members of the default VLAN (VLAN
ID 1).

VLAN ID	Name	Port Members (Tagged/Untagged)
2	Admin	1 (U), 2 (U), 10 (U)
5	Internal	1 (U), 4 (U), 5 (U)
10	Web	1 (T), 9 (T), 10 (T), 11 (U), 12 (U)
15	Collocation	1 (U), 2 (U), 14 (U)

2. Set up the Port VLAN IDs (PVIDs), as shown in the following figure:
   
   

The specific ports shown in the previous figure have the following PVID settings. The PVID settings for each port are configured in the Port Settings page.

Port 01: 2	Port 05: 5	Port 09: 1	Port 13: 1
Port 02: 2	Port 06: 1	Port 10: 2	Port 14: 15
Port 03: 1	Port 07: 1	Port 11: 10	Port 15: 1
Port 04: 5	Port 08: 1	Port 12: 10	Port 16: 1

The PVID of a port must be set to a VLAN ID of which the port is an untagged member.

NOTE: Port 9 cannot be removed from VLAN 1 because its PVID is set to VLAN 1.

3. Return to the VLAN Membership page and remove the ports configured in this example (1, 2, 4, 5, 10, 11, 12, 14) from VLAN 1.
   
   

The VLANs set up in the this example procedure produce the following results:

• If an untagged packet enters Port 4, the switch tags it with a VLAN tag value of 5. The packet can be forwarded to Port 5 and/or 1. As the packet leaves Port 5 and/or 1, it is stripped of its tag and becomes an untagged packet.
  
  
• If a tagged packet with a VLAN tag value 5 enters Port 4, the packet has access to Ports 5 and 1. If the packet leaves Port 5 and/or 1, it is stripped of its tag as it leaves the switch and becomes an untagged packet.
  
  
• If a tagged packet with a VLAN tag value 10 enters Port 9, it can be forwarded to Ports 1, 10, 11, and 12. If the packet leaves Port 1 or 10, it is tagged with a VLAN ID value of 10. If the packet leaves Port 11 or 12, it leaves as an untagged packet.
  
  
• If a tagged packet with a VLAN tag value 15 enters Port 9, it is forwarded to ports in VLAN 15, even though Port 9 is not a member of VLAN 15. The tagged packet enters Port 9 because the Ingress Filtering parameter for Port 9 is set to disabled (the default). If Ingress Filtering is disabled, a tagged packet is forwarded if its VLAN tag value matches a VLAN ID already configured on the switch, otherwise it is dropped.
  
  
• If a tagged packet with a VLAN tag value of 1 enters Port 10, it is dropped because Port 10 is not a member of VLAN 1 and its Ingress Filtering parameter is set to enabled.
  
  

Example 3

Example 3 illustrates how GVRP is implemented where VLANs configured in a core aggregation switch are automatically learned by wiring-closet switches.

1. Set up static VLANs in the core switch, as shown in the following table.
   
   

VLAN ID	Name	Port Members (Tagged/Forbidden)
11	Red	1 (T), 3 (T), 2 (F)
22	Green	2 (T), 3 (T), 1 (F)
33	Blue	1 (T), 2 (T), 3 (F)

2. From the core switch web interface, go to the VLAN/GVRP page and ensure that GVRP Status is set to enabled.
   
   
3. For the other three switches, ensure that GVRP is enabled globally in the VLAN/GVRP page, as well as on ports connecting to the core switch in the VLAN/Port Settings page.
   
   

With GVRP enabled on the core switch, GVRP advertisements for the three static VLANs are sent from all ports on the switch. When the three wiring-closet switches receive the advertisements, they dynamically create the VLANs and the receiving ports join these VLANs.

The clouds connected to the wiring closet switches in the previous figure represent other switches and end-users on that network segment. By setting one VLAN as forbidden on the connecting port, the core switch limits each network segment to only two of the VLANs. For example, users attached to Switch 3 have access to VLANs 11 and 22, but not to VLAN 33. End-user requests enable ports on Switch 3 to join VLAN 33, but these users do not have access to the rest of the network.

Example 4

Example 4 illustrates how GVRP is implemented where VLANs configured in wiring-closet switches are automatically recognized by a core aggregation switch.

1. Set up static VLANs in Switch 1, as shown in the following table:
   
   

VLAN ID	Name	Port Members (Tagged/Untagged)
11	Red	5 (T), (other ports as required)
33	Blue	5 (T), (other ports as required)

2. Set up static VLANs in Switch 2, as shown in the following table:
   
   

VLAN ID	Name	Port Members (Tagged/Untagged)
22	Green	2 (T), (other ports as required)
33	Blue	2 (T), (other ports as required)

3. Set up static VLANs in Switch 3, as shown in the following table:
   
   

VLAN ID	Name	Port Members (Tagged/Untagged)
11	Red	1 (T), (other ports as required)
22	Green	1 (T), (other ports as required)

4. For each of the three wiring-closet switches, ensure that GVRP is enabled globally in the VLAN/GVRP page.
   
   
5. For each of the three wiring-closet switches, ensure that GVRP is disabled for each port in the VLAN/Port Settings page.
   
   
6. For the core switch, ensure that GVRP is enabled globally in the VLAN/GVRP page, as well as on ports connecting to the wiring-closet switches in the VLAN/Port Settings page.
   
   

With GVRP enabled on the wiring closet switches, GVRP advertisements for the configured static VLANs are sent to the core switch. When the core switch receives the advertisements, it dynamically creates the VLANs and places the receiving ports in these VLANs.

The GVRP port settings on the wiring-closet switches need to be set to disabled. This setting prevents these switches from dynamically creating other VLANs, or adding port members to the existing static VLANs. The global GVRP switch setting still enables the static VLANs to be advertised to the rest of the network. For example, users attached to Switch 3 have access to VLANs 11 and 22, but not to VLAN 33. VLAN 33 cannot be created on Switch 3, even though advertisements are received on Port 1 from other switches in the network.

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