Assigning VLANs to a Port and Associated Issues

There are two ways to assign VLANs to ports on the Avaya Multiservice Switch. You configure the Port(s) to the VLAN desired for the individual port(s), or entire module. Assigning the VLAN this way enables the port(s) to receive information for the assigned VLAN and causes all untagged frames arriving on this port to be assigned to the specified VLAN. The port(s) will still assign incoming tagged packets to the VLAN indicated by the tag.

In the current release of software, a non-trunk port only supports a single VLAN per port. Multiple VLANs per port is NOT recommended and can have adverse effects on network performance.

The second way is to assign the VLAN to a port is by using the following CLI command in Enable/Configure mode:

(configure)# set VLAN <vlan-id or name> <options>

By selecting the VLAN name or VLAN ID, you can bind the selected VLAN to additional ports. When binding VLANs this way, the port is part of the flooding domain of the selected VLAN. This is an alternative to using the binding types "bind to all" and "bind to receive", that allows you to add a port(s) to a subset of VLANs in the switch.

Note: This configuration should only be used under special circumstances and with the assistance of customer support as undesirable results may occur (e.g. destination unknown unicast storms).

See Chapter 6:�Using VLANs, Hunt Groups, and VTP Snooping for a description of the parameters that will be displayed when viewing VLAN information with the Web Agent and the CLI command syntax.

Although this method of adding VLANs to a port could be thought to support the request for multiple VLANs per port (also referred to as overlapping VLANs), it is NOT recommended on clear (non trunked) ports due to the impact it has on the network. The impact is that destination unknown unicast packets are flooded on the VLAN in which the source host is located. This causes all ports assigned to this VLAN to receive the destination unknown unicast. If enough of these destination unknown unicast packets are being sent it could have a major impact on the network.

The following is an example of a problem with assigning more the one VLAN to a port.

• All PCs and Servers are connected to the Avaya Multiservice Switch 10/100TX switch ports. The following configuring of ports to VLANs are done with the Web Agent.
• PC1 is a member of VLAN1 and is connect to switch Port 1
• PC2 is a member of VLAN2 and is connected to switch Port 2
• SERVER1 is a member of VLAN4 and is connected to switch Port 4

With the set vlan CLI command, assign port 1, and port 2 to VLAN4 and also assign port 4 to VLAN1 and VLAN2. Then PC1 and PC2 could communicate with SERVER1 across VLANs without any noticeable problems. However, when PC1 transmits a packet to SERVER1, the following occurs:

• The packet is sent to the Switch
• The switch looks for the MAC address of SERVER1in the Address Forwarding Table for VLAN1 (because this is the actual port VLAN assigned to the port)
• The switch will be unable to find an entry for the MAC address of SERVER1
• The switch will flood this packet to all ports assigned to VLAN1 (regardless of how the VLAN was assigned to the port)

Note: The only exception to flooding a port is if 'Known Mode' is set to enable on the port, which stops the flooding of destination unknown unicast packets.

If you had many clients/servers on separate VLANs and they tried to communicate over multiple VLANs using this method, you would flood your network with undesired packets, thus slowing your overall network performance. Currently, the best solution is to use a L3 module to route between the VLANs. This would eliminate the broadcast of destination unknown unicast packets.