DES-3226L Layer 2
Fast Ethernet Switch User’s Guide
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 Switch. 802.1Q VLANs require
tagging, which enables them to span the entire network (assuming all switches on the network
are IEEE 802.1Q-compliant).
VLANs allow a network to be segmented in order to reduce the size of broadcast domains. All
packets entering a VLAN will only be forwarded to the stations (over IEEE 802.1Q enabled
switches) that are members of that VLAN, and this includes broadcast, multicast and unicast
packets from unknown sources.
VLANs can also provide a level of security to your network. IEEE 802.1Q VLANs will only
deliver packets between stations that are members of the VLAN.
Any port can be configured as either
tagging
or
untagging
. The
untagging
feature of IEEE
802.1Q VLANs allows 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-compliant
switches through a single physical connection and allows Spanning Tree to be enabled on all
ports and work normally.
The IEEE 802.1Q standard restricts the forwarding of untagged packets to the VLAN the
receiving port is a member of.
The main characteristics of IEEE 802.1Q are as follows:
•
Assigns packets to VLANs by filtering.
•
Assumes the presence of a single global spanning tree.
•
Uses an explicit tagging scheme with one-level tagging.
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 whether to filter or forward the packet.
•
Egress rules – determines if the packet must be sent tagged or untagged.
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