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DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
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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
Engineering
2
9, 10
Sales
5
1, 2, 3, 4
Port-based VLANs
Port-based VLANs limit traffic that flows into and out of switch ports. Thus, all devices connected to a port are
members of the VLAN(s) the port belongs to, whether there is a single computer directly connected to a switch, or an
entire department.
On port-based VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers. NICs send and receive
normal Ethernet packets. If the packet’s destination lies on the same segment, communications take place using
normal Ethernet protocols. Even though this is always the case, when the destination for a packet lies on another
switch port, VLAN considerations come into play to decide if the packet gets dropped by the Switch or delivered.
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.
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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