DES-3226S Layer 2 Fast Ethernet Switch User’s Guide
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Data transmitted to a specific host (destination address) will always be transmitted over the same port in a link aggregation
group. This allows packets in a data stream to arrive in the same order they were sent. A aggregated link connection can be
made with any other Switch that maintains host-to-host data streams over a single link aggregate port. Switches that use a load-
balancing scheme that sends the packets of a host-to-host data stream over multiple link aggregation ports cannot have an
aggregated connection with the DES-3226S Switch.
VLANs
A VLAN is a collection of end nodes grouped by logic rather than physical location. End nodes that frequently communicate
with each other are assigned to the same VLAN, regardless of where they are located physically on the network. Logically, a
VLAN can be equated to a broadcast domain, because broadcast packets are forwarded only to members of the VLAN on
which the broadcast was initiated.
VLAN Implementation
The following rules are important to keep in mind when setting up VLANs on your network:
1. No matter what basis is used to uniquely identify end nodes and assign these nodes VLAN membership, packets
cannot
cross VLANs without a network device performing a routing function between the VLANs.
2. The DES-3226 supports only IEEE 802.1Q VLANs. The port untagging function can be used to remove the 802.1Q
tag from packet headers to maintain compatibility with devices that are tag unaware.
3. The Switch’s default is to assign all ports to a single 802.1Q VLAN named DEFAULT_VLAN.
4. The DEFAULT_VLAN has a VID = 1.
5. The DES-3226 supports Asymmetric VLANs. The member ports of VLANs can be overlapped. See the section on
Asymmetric VLANs below.
IEEE 802.1Q VLANs
Some relevant terms:
Tagging
- The act of putting 802.1Q VLAN information into the header of a packet.
Untagging
- The act of stripping 802.1Q VLAN information out of the packet header.
Ingress port
- A port on a Switch where packets are flowing into the Switch and VLAN decisions must be made.
Egress port
- A port on a Switch where packets are flowing out of the Switch, either to another Switch or to an end station,
and tagging decisions must be made.
IEEE 802.1Q (tagged) VLANs are implemented on the DES-3226S Switch. 802.1Q VLANs require tagging, which enables
the VLANs to span an entire network (assuming all Switches on the network are IEEE 802.1Q-compliant).
Any port can be configured as either
tagging
or
untagging
. The
untagging
feature of IEEE 802.1Q VLANs allow VLANs to
work with legacy Switches that don’t recognize VLAN tags in packet headers. The
tagging
feature allows VLANs to span
multiple 802.1Q VLAN compliant Switches through a single physical connection and allows Spanning Tree to be enabled on
all ports and work normally.
802.1Q VLAN Packet Forwarding
Packet forwarding decisions are made based upon the following three types of rules:
•
Ingress rules – rules relevant to the classification of received frames belonging to a VLAN.
•
Forwarding rules between ports – decides filter or forward the packet
•
Egress rules – determines if the packet must be sent tagged or untagged.
Summary of Contents for DES-3226S
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