Amer Networks SS3GR24i - Manual (Page 32)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
forwarded only to members of the VLAN on which the broadcast
was initiated.
VLANs in Layer 2
In normal 802.1Q VLAN implementation, packets cannot cross
VLANs in a Switch that is limited to Layer 2 functions. If a
member of one VLAN wants to connect to another VLAN, the link
must be through an external router.
Layer 3-Based VLANs
Layer 3-based VLANs use network-layer addresses (subnet
address for TCP/IP) to determine VLAN membership. These
VLANs are based on layer 3 information, however this does not
constitute a ‘routing’ function.
The SS3GR24i/SS3GR48i Switches allow an IP subnet to be
configured for each 802.1Q VLAN that exists on the Switch . That
is, a VLAN can be associated or attached to an IP subnet. This
represents an improvement in performance since it bypasses any
routing functions, packets transferred between subnets are
reduced to a “hardware” decision.
Even though a Switch inspects a packet’s IP address to determine
VLAN membership, no route calculation is performed, the RIP
protocol is not employed, and packets traversing the Switch are
bridged using the Spanning Tree algorithm.
A Switch that implements layer 3 (or ‘subnet’) VLANs without
performing any routing function between these VLANs is referred
to as performing ‘IP Switching’.
Planning VLAN Layout
VLANs on the SS3GR24i/SS3GR48i Switches have considerably
more functionality and are more complex than on a traditional
layer 2 Switch, and must therefore be laid out and configured with
more forethought. 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, VLANs that are assigned an IP
address and subnet mask enables IP routing between them.
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