Port Trunking; Vlans & Broadcast Domains - D-Link DES-6000 User Manual

Modular Ethernet Switch User's Guide

Port Trunking

Port trunking is used to combine a number of ports together to make a single high-bandwidth data pipeline.
The participating parts are called members of a trunk group, with one port designated as the anchor of the
group. Since all members of the trunk group must be configured to operate in the same manner, all settings
changes made to the anchor port are applied to all members of the trunk group. Thus, when configuring the
ports in a trunk group, you only need to configure the anchor port.
The Switch supports up to 16 trunk groups. Each module on the switch supports up to two trunk groups
except gigabit modules, which don't support trunk groups. The Switch treats all ports in a trunk group as a
single port. As such, trunk ports will not be blocked by Spanning Tree.
Data transmitted to a specific host (destination address) will always be transmitted over the same port in a
trunk group. This allows packets in a data stream to arrive in the same order they were sent. A trunk
connection can be made with any other switch that maintains host-to-host data streams over a single trunk
port. Switches that use a load-balancing scheme that sends the packets of a host-to-host data stream over
multiple trunk ports cannot have a trunk connection with the Switch.
VLANs & Broadcast Domains
VLANs are a collection of users or switch ports grouped together in a secure, autonomous broadcast and
multicast domain. The main purpose of setting up VLANs on a network is to limit the range and effects of
broadcast packets, which can develop into broadcast storms and seriously impair network performance.
Three types of VLANs and broadcast domains are implemented on the switch: 802.1Q VLANs, port-based
VLANs, and MAC-based broadcast domains. Only one of the three types can be active on the switch at any
given time, however. Thus, you will need to choose the type of VLAN or broadcast domain you wish to setup
on your network and configure the switch accordingly. MAC-based broadcast domains and port-based VLANs
are limited to the switch and devices directly connected to it, while 802.1Q VLANs support IEEE 802.1Q
tagging, which enables them to span the entire network (assuming all switches on the network are IEEE
802.1Q-compliant).
All VLANs allow a network to be segmented in order to reduce the size of broadcast domains. All broadcast,
multicast, and unknown packets entering the switch on a particular VLAN or broadcast domain will only be
forwarded to the stations (MAC-based) or ports (802.1Q and Port-based) that are members of that VLAN or
broadcast domain. 802.1Q VLANs can also be setup to limit unicast packets to members of a particular VLAN,
thus providing a degree of security to your network.
Another benefit of 802.1Q and port-based VLANs is that you can change the network topology without
physically moving stations or changing cable connections. Stations can be 'moved' to another VLAN and thus
communicate with members and share resources on the new VLAN, simply by changing the port VLAN
settings from one VLAN (the sales VLAN, for example) to another VLAN (the marketing VLAN). This allows
VLANs to accommodate network moves, changes and additions with the utmost flexibility. MAC-based
broadcast domains, on the other hand, allow a station to be physically moved yet still belong to the same
broadcast domain without having to change configuration settings.
The untagging feature of IEEE 802.1Q VLANs allows VLANs to work with legacy switches and NICs 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.
Switch Management Concepts 25