Tagging on VLAN 1

[glio]

I admit... I'm a total newbie on VLAN so please bear with me for the silly question I raised...

An NIC of my NW6.5 box is bound to VLAN 2, 3 and 4. What would happen if the switch port connecting to the NIC is also tagged with VLAN ID 2, 3 and 4? Can the workstations on different VLANs but connecting to the same switch communicate with the NW6.5 box?

 

[LawnBoy]

What you're saying is that your server is a member of VLANs 2, 3, and 4. For any device on your network to talk to the server, it must be a member of at least one of the VLANs 2, 3, or 4. (with the special exception of DHCP, see your other post).

 

[Belushi]

Just to clarrify why an "ip helper" type command is necessary because there is some conflicting info given here. The reason is because broadcasts are not forwarded between vlans by default and routers do not forward them by default. That is pretty much the main reason vlans exist in the first place...to segment broadcast domains.

DHCP uses broadcasts, therefore if the DHCP server is located on a different vlan than the client, then some type of command is needed to tell the router to forward the broadcasts. In the cisco world that command is "ip helper". It forwards broadcasts using 8 different udp based ports by default.

 

[glio]

Something regarding performance...

The DHCP server runs several virtual interface that connects to all VLANs (8 in total). There is no need for broadcast forwarding now and workstations on all VLANs can now obtain the IPs according to the VLANs and subnets they belong to...

Just wondering... in reality, is it a good practice? Is there anyone out there who runs a network in similar fashion?

 

[Belushi]

I haven't heard of anybody doing this before but I suppose there is no harm in it as long as performance isn't impacted. In your setup the actual NIC has to listen to all broadcasts on all 8 vlans it is connected too. Even though it is virtualized, so to speak, the actual NIC must be interrupted and listen to each broadcast on each vlan.

If performace isn't degraded due to this style of design, then go for it. There may be times in the future though when the DHCP broadcast may need to be forwarded because it isn't a part of the same LAN. For example, many people have DHCP servers that dish out IP addresses to people across WAN links.