VLAN operating notes

Disabled overlapping subnet configuration

Previous software versions allowed configuration of VLAN IP addresses in overlapping subnets, which can cause incorrect routing of packets and result in IP communication failure. As of software version xx.15.09, overlapping subnet configurations are no longer allowed. An overlapping subnet is determined by the configuration order. The subnet that is configured first is valid, but any subsequent IP addresses that overlap are not allowed.

When the switch is booted into software version xx.15.09 or later and the configuration file includes overlapping subnets, the following occurs:

The event log provides an error message in the format:

ip: VLANx : IP initialization failed for vlan x.

For a multinetted VLAN (multiple IP addresses assigned to the VLAN), only the IP addresses that are overlapping subnets are removed. The other IP addresses on the VLAN are retained and function correctly. The error message can be misleading; the IP addresses on the VLAN that are not overlapping are initialized correctly.
The output of the show ip command correctly indicates that the overlapping IP address does not exist on the VLANs that have error messages in the event log.
The output of the show running-config command incorrectly indicates that the overlapping IP address is configured. For example, in the following output, the IP address shown in VLAN6 is not actually configured on the VLAN; it has been removed.
An IP address that is not actually configured on the VLAN
```
switch(config)#: show running-config

.
.
.
  vlan 5
     name “VLAN5”
     ip address 11.22.33.1 255.0.0.0
     exit
  vlan 6
     name “VLAN6”
     ip address 11.23.34.1 255.255.255.0
     exit
```

The information is retained in the config file to allow you to boot up the switch and have it function as it did when it was configured with earlier software that allows overlapping subnets. This occurs because the overlapping IP address has been removed and is not visible to the switch. To resolve this:

If you attempt to remove the overlapping subnet from the VLAN, the switch displays an error message similar to:

The IP address <ip address> is not configured on this VLAN

This occurs because the overlapping IP address has been removed and is not visible to the switch. To resolve this:

Enter the show ip command to determine which addresses are visible to the switch.
Remove the erroneous IP addresses from the config file by entering the no ip address command to remove all the IP addresses from the specific VLAN. Be sure to document the other valid IP addresses on that VLAN so they can be restored after removing the erroneous IP addresses from the config file.

If you go back to a software version before xx.15.09 before removing the overlapping IP address, the prior software version enables the overlapping IP subnet.

DHCP/Bootp

If you are using DHCP/Bootp to acquire the switch's configuration, packet time-to-live and TimeP information, designates the VLAN on which DHCP is configured as the Primary VLAN.


	NOTE: In the factory-default configuration, the DEFAULT_VLAN is the Primary VLAN.

Per-VLAN features

IGMP and some other features operate on a per VLAN basis. This means you must configure such features separately for each VLAN in which you want them to operate.

Default VLAN

You can rename the default VLAN, but you cannot change its VID (1) or delete it from the switch.

VLAN port assignments

Any ports not specifically removed from the default VLAN remain in the DEFAULT_VLAN, regardless of other port assignments. Also, a port must always be a tagged or untagged member of at least one port-based VLAN.

Voice-Over-IP (VoIP)

VoIP operates only over static, port-based VLANs.

Multiple VLAN types configured on the same port

A port can simultaneously belong to both port-based and protocol-based VLANs.

Protocol Capacity

A protocol-based VLAN can include up to four protocol types. In protocol VLANs using the IPv4 protocol, to support normal IP network operation ARP must be one of these protocol types (to support normal IP network operation). Otherwise, IP traffic on the VLAN is disabled.

If you configure an IPv4 protocol VLAN that does not include the ARP VLAN protocol, the switch displays the following message which indicates a protocol VLAN configured with IPv4 but not ARP:

switch(config)#: vlan 97 protocol ipv4

IPv4 assigned without ARP, this may result in undeliverable IP packets.

Deleting Static VLANs

A VLAN can be deleted even if there are currently ports belonging to it. The ports are moved to the default VLAN.

Adding or Deleting VLANs

Changing the number of VLANs supported on the switch, requires a reboot.


	NOTE: From the CLI, you must perform a `write memory` command before rebooting. Other VLAN configuration changes are dynamic.

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