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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
2. Develop an IP addressing scheme. This involves allocating a block of IP addresses to each
network segment. Each network interface is then assigned a network address and a subnet
mask. See Chapter 5, Switch Management Concepts section titled IP Addressing and
Subnetting for more information.
3. Determine which network resources must be shared by the interfaces. Shared resources may be
connected directly to the Layer 3 switch, if need be. Static routes to each of the shared
resources should be determined.
4. Determine how each interface will communicate with the WAN or Internet. Again, static routes
should be determined and default gateways identified.
5. Develop a security scheme. Some interfaces on the network need more security or should be
isolated from the other interfaces. IP or MAC filtering can be used. Also, one or more VLANs on
the Layer 3 switch can be configured without an IP interface – in which case, these VLANs will
function as a layer 2 VLAN and would require an external router to connect to the rest of the
network.
6. Develop a policy scheme. Some interfaces will have a greater need for multicasting bandwidth,
for example. A policy is a mechanism to alter the normal packet forwarding in a network device,
and can be used to intelligently allocate bandwidth to time-critical applications such as the
integration of voice, video, and data on the network.
7. Develop a redundancy scheme. Planning redundant links and routes to network critical
resources can save valuable time in case of a link or device failure. The DES-3326 Spanning
Tree function can be used to block the redundant link until it is needed.
VLAN Layout
VLANs on the DES-3326 have rather more functions than on a traditional layer 2 switch, and must
therefore be laid-out and configured with a bit more care. Layer 3 VLANs (VLANs with an IP interface
assigned to them) could be thought of as network links – not just as a collection of associated end
users. Further, Layer 3 VLANs are assigned an IP network address and subnet mask to enable IP
routing between them.
Layer 3 VLANs must be configured on the switch before they can be assigned IP interfaces. Further, the
static VLAN configuration is specified on a per port basis. On the DES-3326, a VLAN can consist of
end-nodes – just like a traditional layer 2 switch, but a VLAN can also consist of one or more layer 2
switches – each of which is connected to multiple end-nodes or network resources.
So, a Layer 3 VLAN, consisting of 4 ports, could be connected to 4 layer 2 switches. If these layer 2
switches each have 24 ports, then the Layer 3 VLAN would contain 4 x 24 = 96 end nodes. Assigning an
IP interface to the Layer 3 VLAN would allow wire-speed IP routing from the WAN to each end node and
between end nodes.
So, the IP interfaces for a network must be determined first, and the VLANs configured on the switch to
accommodate the IP interfaces. Finally, the IP interfaces can be assigned to the VLANs.
Assigning IP Network Addresses and Subnet Masks to VLANs
The DES-3326 allows the assignment of IP interfaces to individual VLANs. Any VLAN configured on the
switch that is not assigned an IP interface, will behave as a layer 2 VLAN and will not be capable of IP
routing – even if the switch is in IP Routing mode.
Developing an IP addressing scheme is a complex subject, but it is sufficient here to mention that the
total number of anticipated end nodes – for each Layer 3 VLAN – must be accommodated with a unique
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