Contemporary Control Systems Compact Managed Switches Software Manual for Web Browser - Manual (Page 23)

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4.4.4.5 Core Switches and Edge Switches
A core switch is connected only to devices that are VLAN aware — thus, all frames
received by a core switch should already contain 802.1Q VLAN tags. An edge switch
adds VLAN tags to frames sent by non-VLAN aware devices and it removes VLAN tags
from frames destined for non-VLAN aware devices. Figure 18 illustrates how core and
edge switches differ in their placements within a typical network.
Figure 18 — Core Switches and Edge Switches
To function in VLAN mode, the switch requires VLAN tags. When it performs as a Port
VLAN switch (connected only to non-VLAN aware devices that do not use VLAN tags),
a default tag will be applied to the untagged frames entering the switch. When these
frames leave the switch, the tags should be removed. The switch can act as either a
core or an edge switch on a port-by-port basis — and this functionality allows the switch
to isolate non-VLAN aware devices by tags and impart added security.
Figure 19 shows how an edge switch
adds a VLAN tag to an untagged frame from PC1
so it can be used by the core switch and thus all of the VLAN, but the edge switch also
removes the tag from the frame destined for the VLAN-unaware PC2.
Figure 19 — Edge Switches Add or Drop VLAN Tags as Needed