DES-3010F/DES-3010FL/DES-3010G/DES-3018/DES-3026 Fast Ethernet Switch Manual
If the packet is tagged with VLAN information, the ingress port will first determine if the ingress port itself is a member of
the tagged VLAN. If it is not, the packet will be dropped. If the ingress port is a member of the 802.1Q VLAN, the Switch
then determines if the destination port is a member of the 802.1Q VLAN. If it is not, the packet is dropped. If the
destination port is a member of the 802.1Q VLAN, the packet is forwarded and the destination port transmits it to its
attached network segment.
If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a VID (if the
port is a tagging port). The switch then determines if the destination port is a member of the same VLAN (has the same
VID) as the ingress port. If it does not, the packet is dropped. If it has the same VID, the packet is forwarded and the
destination port transmits it on its attached network segment.
This process is referred to as ingress filtering and is used to conserve bandwidth within the Switch by dropping packets that
are not on the same VLAN as the ingress port at the point of reception. This eliminates the subsequent processing of
packets that will just be dropped by the destination port.
Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the
Switch to the "default." As new VLANs are configured in Port-based mode, their respective member ports are removed
from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link must be through an
external router.
NOTE:
If no VLANs are configured on the Switch, then all packets will be
forwarded to any destination port. Packets with unknown source
addresses will be flooded to all ports. Broadcast and multicast packets will
also be flooded to all ports.
An example is presented below:
VLAN Name
VID
Switch Ports
System (default)
1
5, 6, 7, 8, 21, 22, 23, 24
Engineering
2
9, 10, 11, 12
Marketing
3
13, 14, 15, 16
Finance
4
17, 18, 19, 20
Sales
5
1, 2, 3, 4
Table 7- 1. VLAN Example - Assigned Ports
VLAN Segmentation
Take for example a packet that is transmitted by a machine on Port 1 that is a member of VLAN 2. If the destination lies on
another port (found through a normal forwarding table lookup), the Switch then looks to see if the other port (Port 10) is a
member of VLAN 2 (and can therefore receive VLAN 2 packets). If Port 10 is not a member of VLAN 2, then the packet
will be dropped by the Switch and will not reach its destination. If Port 10 is a member of VLAN 2, the packet will go
through. This selective forwarding feature based on VLAN criteria is how VLANs segment networks. The key point being
that Port 1 will only transmit on VLAN 2.
VLAN and Trunk Groups
The members of a trunk group have the same VLAN setting. Any VLAN setting on the members of a trunk group will
apply to the other member ports.
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