DES-3550 Layer 2
Fast Ethernet Switch User’s Guide
61
A weighted round robin system is employed on the Switch to determine the rate at which the
queues are emptied of packets. The ratio used for clearing the queues is 4:1. This means that
the highest priority queue, Queue 1, will clear 4 packets for every 1 packet cleared from
Queue 0.
Remember, the priority queue settings on the Switch are for all ports, and all devices
connected to the Switch will be affected. This priority queuing system will be especially
beneficial if your network employs switches with the capability of assigning priority tags.
VLANs
A Virtual Local Area Network (VLAN) is a network topology configured according to a
logical scheme rather than the physical layout. VLANs can be used to combine any collection
of LAN segments into an autonomous user group that appears as a single LAN. VLANs also
logically segment the network into different broadcast domains so that packets are forwarded
only between ports within the VLAN. Typically, a VLAN corresponds to a particular subnet,
although not necessarily.
VLANs can enhance performance by conserving bandwidth, and improve security by limiting
traffic to specific domains.
A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes
that frequently communicate with each other are assigned to the same VLAN, regardless of
where they are physically on the network. Logically, a VLAN can be equated to a broadcast
domain, because broadcast packets are forwarded to only members of the VLAN on which the
broadcast was initiated.
Notes About VLANs on the DES-3550
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.
The DES-3550 supports IEEE 802.1Q VLANs and Port-Based 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.
The Switch’s default is to assign all ports to a single 802.1Q VLAN named “default.”
The “default” VLAN has a VID = 1.
The member ports of Port-based VLANs may overlap, if desired.
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.
