Effects of VLANs on other switch features

Spanning Tree operation with VLANs

Depending on the spanning tree option configured on the switch, the spanning tree feature may operate as:

  • A single instance across all ports on the switch regardless of VLAN assignments

  • Multiple instances per-VLAN

For single-instance operation, if redundant physical links exist between the switch and another 802.1Q device, all but one link will be blocked, even if the redundant links are in separate VLANs. In this case you can use port trunking to prevent Spanning Tree from unnecessarily blocking ports (and to improve overall network performance). For multiple-instance operation, physically redundant links belonging to different VLANs can remain open. For more information, see Multiple Instance Spanning Tree Operation.


[NOTE: ]

NOTE: Spanning Tree operates differently in different devices. For example, in the (obsolete, non-802.1Q) HP Switch 2000 and the HP Switch 800T, Spanning Tree operates per-VLAN, allowing redundant physical links as long as they are in separate VLANs.


Spanning Tree operates differently in different devices

IP interfaces

There is a one-to-one relationship between a VLAN and an IP network interface. Since the VLAN is defined by a group of ports, the state (up/down) of those ports determines the state of the IP network interface associated with that VLAN. When a port-based VLAN or an IPv4 or IPv6 protocol-based VLAN comes up because one or more of its ports is up, the IP interface for that VLAN is also activated. Likewise, when a VLAN is deactivated because all of its ports are down, the corresponding IP interface is also deactivated.

VLAN MAC address

The switches have one unique MAC address for all of their VLAN interfaces. You can send an 802.2 test packet to this MAC address to verify connectivity to the switch and you can assign an IP address to the VLAN interface. When you Ping that address, ARP will resolve the IP address to this single MAC address.

In a topology where a switch has multiple VLANs and must be connected to a device having a single forwarding database, such as the Switch 4000M, some cabling restrictions apply. For more on this topic, see Multiple VLAN considerations.

Port trunks

When assigning a port trunk to a VLAN, all ports in the trunk are automatically assigned to the same VLAN. Do not split trunk members across multiple VLANs. A port trunk is tagged, untagged, or excluded from a VLAN the same way as individual, untrunked ports.

Port monitoring

If you designate a port on the switch for network monitoring, the port will appear in the Port VLAN Assignment screen and can be configured as a member of any VLAN. For information on how broadcast, multicast and unicast packets are tagged inside and outside of the VLAN to which the monitor port is assigned, see the section titled "VLAN-Related Problems" in the "Troubleshooting" appendix of the Management and Configuration Guide for your switch.

Jumbo packet support

Jumbo packet support is enabled per-VLAN and applies to all ports belonging to the VLAN. For more information, see the chapter titled "Port Traffic Controls" in the Management and Configuration Guide for your switch.

VLAN restrictions

  • A port must be a member of at least one VLAN. In the factory default configuration, all ports are assigned to the default VLAN (DEFAULT_VLAN; VID=1).

  • A port can be a member of one untagged, port-based VLAN. All other port-based VLAN assignments for that port must be tagged. The "Untagged" designation enables VLAN operation with non 802.1Q-compliant devices.

  • A port can be an untagged member of one protocol-based VLAN of each protocol type. When assigning a port to multiple, protocol-based VLANs sharing of the same type, note that the port can be an untagged member of only one such VLAN.

  • With routing enabled on the switch, the switch can route traffic between:

    • Multiple, port-based VLANs

    • A port-based VLAN and an IPv4 protocol-based VLAN

    • A port-based VLAN and an IPv6 protocol-based VLAN

    • An IPv4 protocol-based VLAN and an IPv6 protocol VLAN

    Other, routable, protocol-based VLANs must use an external router to move traffic between VLANs. With routing disabled, all routing between VLANs must be through an external router.

  • Before deleting a static VLAN, t first re-assign all ports in the VLAN to another VLAN. You can use the no vlan <vid> command to delete a static VLAN. For more information, see Creating a new static VLAN (port-based or protocol-based) (CLI) .

  • Protocol-based VLANs, port-based VLANs and LLDP radio port VLANs cannot run concurrently with RPVST+.