Advanced Topics

4-2

Once a stable network topology has been established, all bridges listen for Hello
BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge. If a bridge
does not get a Hello BPDU after a predefined interval (Maximum Age), the bridge
assumes that the link to the Root Bridge is down. This bridge will then initiate
negotiations with other bridges to reconfigure the network to reestablish a valid
network topology.
The following figure gives an illustration of how the Spanning Tree Algorithm assigns
bridging device ports.
Virtual LANs
Switches do not inherently support broadcast domains, whi ch can lead to broadcast
storms in large networks that handle a lot of IPX or NetBEUI traffic. In conventional
networks with routers, broadcast traffic is split up into physically separate domains to
confine broadcast traffic to the
originating group and provide a much cleaner network
environment
. This switch creates segregated broadcast domains based on easily
configurable VLANs, these VLANs are then linked, as required, using a router or
Layer 3 switch.
An IEEE 802.1Q VLAN is a group of ports that can be located anywhere in the
network, but communicate as though they belong to the same physical segment.
VLANs help to simplify network management by allowing you to move devices to a
new VLAN without having to change any physical connections. VLAN s can be easily
organized to reflect departmental groups (such as Marketing or R&D), usage groups
(such as e-mail), or multicast groups (used for multimedia applicat ions such as video
conferencing).
VLANs provide greater network efficiency by reducing broadcast traffic, but also
allow you to make network changes without having to update IP addresses or IP
subnets. VLANs inherently provide a high level of network security, since traffic must
pass through a Layer 3 switch or a router to reach a different VLAN.