VLANs are isolated from one another. To communicate between VLANs, you
must use interVLAN routing.

You configure a trunk port (e.g.: 802.1Q) on the switch and connect it
to a router that has been configured with sub interfaces (e.g.: one
sub-interface configured as 192.168.142.1, the other 192.168.143.1).
These addresses would be the default gateway addresses of their
respective VLANs.

Lets say your VLAN IDs were 142 and 143, and you wanted to access a
server on VLAN 143, from a host on VLAN 142.

The host determines that the server is not on the same network by
applying the network mask to both addresses, and doing a comparison. The
host forwards a packet to it's default gateway (192.168.142.1). The
switch tags the packet with VLAN ID 142 (e.g.: 4 byte 802.1Q header) and
forwards it over the trunk to the router.

The router receives the packet, strips of the VLAN tag, looks at its
routing table and determines that it does have a route to the server via
sub-interface 192.168.143.1. The packet is tagged by the router with
VLAN ID 143, and forwarded over the trunk. The switch strips off the
VLAN tag, and forwards the packet to the server.

When the server responds, it uses its mask comparison to conclude that
the host is on another network, and that it therefore needs the
assistance of its default gateway which is at 192.168.143.1. It sends a
response packet to the host. This packet will be tagged by the switch
with VLAN ID 143, and forwarded to the router. The router will strip off
the tag, do a route lookup, re-tag the packet with VLAN ID 142 and
forward it out the trunk (via sub-interface 192.168.142.1). The switch
will remove the tag and forward it to the host.

Note that a "single" packet being sent between the host and server, has
to traverse the trunk "twice".

Host to router, then router to server.

The penalty for using VLANs, is the increased utilization of the
physical link between the switch and the router (i.e. the trunk).


Best Regards,
News Reader

Paul.k.Mcdowell@gmail.com wrote:
> I have two VLANs configured on one switch and I would like to get them
> communicating with eachother without having to use a router. I believe
> I need some form of trunking perhaps, but I'm not quite sure how to
> get it working (its been years since I had to do this last time). My
> switch is a cisco 3500 series inline power switch. config:
>
> Network A: 192.168.143.0 /24
> Network B: 192.168.142.0 /24
>
>
> switch port fa0/12 has CAT5 going to the .142 network switch
> Switch ports fa0/01 - fa0/11 are CAT5 going to .143 VOIP server
> systems
>
> The reason I need it to communicate between the two is so that my VOIP
> system can be accessed from the other network for management. Any help
> is appreciated.