?– Because of the spanning trees, bridges almost always deliver frames in the same
order in which they were transmitted. Exceptions cannot occur in the older versions
of the Spanning Tree Protocols and are extremely rare in the newer versions,
even when a link or bridge is lost or regained in the network. Routers attempt to
keep packets in order, but make no guarantees. Duplicated or out-of-order packets
are common when the network topology changes.
Virtual LANs
Since the completion of IEEE Std 802.1Q-2005, Virtual LANs (VLANs) have been available
as a standard feature of bridges. VLANs enable a bridged network to offer up to
4094 separate instances of the MAC service, with each one connecting one or more
LANs. Since a VLAN bridge cannot transfer a frame from one VLAN to another, and
since the MAC address space of each VLAN can be configured to be independent of
every other VLAN, VLANs provide the basic service characteristic that customers of
Carrier Ethernet service require??”isolation from other customers. A VLAN is perfectly
equivalent to an EVC. As a consequence, the IEEE Std 802.1Q VLANs were offered to
customers by service providers as soon as they were standardized.
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