Routing Between VLANs

Routing Between VLANs
The main reason for the creation of a VLAN is to keep traffic within local workgroups. We have
already mentioned in this book that you cannot communicate between VLANs without a router
(layer 3 device), so understanding the configuration of VLANs and understanding routing need
to go hand in hand in order to understand the full process of inter-VLAN communications.
Route processors provide the communication that hosts need between VLANs. However, if
you are using local VLANs (see Chapter 14, “VLANs, Trunks, and VTP” for a thorough explanation),
a good rule of thumb is to design your networks so at least 80 percent of the users’ traffic
does not cross over into another VLAN. Therefore, you should design the network so that
the users have access to local servers and other needed resources to prevent excessive packets
from crossing the route processor.

Cisco recommends that VLANs should be configured one for one with IP subnet designs. This
means that you need to create a subnet design for your network, taking into account the needs of the
various VLANs. If you are using variable-length subnet masking (VLSM), this is pretty straightforward,
but if for some reason you are constrained to a single subnet mask, you may need to select
the mask first and then design your VLANs around the subnet design. For example, if you have engineering,
marketing, sales, and support departments, you will typically—not always, but typically—
create a subnet for each department, making sure you have room for growth. You would then create
a VLAN for each department. In Chapter 14, we discussed the differences between local and endto-
end VLANs. Regardless of the type of VLAN you configure, each of these types would be associated
with a subnet.
The route processor managing the inter-VLAN routing would have multiple interfaces
(real or virtual), and each would have an IP address in the subnet associated with the interface
VLAN. Each device within a VLAN would have a default gateway of the IP address of the
inter-VLAN device connected to its VLAN. The inter-VLAN device would then route any
packets with a destination not on the local network.
Before configuring routing between your VLANs, you need to understand the type of data
sharing that is needed. By understanding the user and business needs, you can design the network
with load balancing and/or redundant links if needed.
When configuring routing, you can choose from three options:

Multiple links

A single trunk link

An internal or external route processor