D-Link DGS-3224TGR - Switch User Manual - Filtering
DGS-3224TGR Gigabit Ethernet Switch User’s Guide
If the Aging Time is too short however, many entries may be aged out too soon. This will result in a high
percentage of received packets whose source addresses cannot be found in the forwarding table, in
which case the switch will broadcast the packet to all ports, negating many of the benefits of having a
switch.
Static forwarding entries are not affected by the aging time.
Filtering
The switch uses a filtering database to segment the network and control communication between
segments. It can also filter packets off the network for intrusion control. Static filtering entries can
be made by MAC Address filtering.
Each port on the switch is a unique collision domain and the switch filters (discards) packets whose
destination lies on the same port as where it originated. This keeps local packets from disrupting
communications on other parts of the network.
For intrusion control, whenever a switch encounters a packet originating from or destined to a MAC
address entered into the filter table, the switch will discard the packet.
Some filtering is done automatically by the switch:
• Dynamic filtering – automatic learning and aging of MAC addresses and their location on the
network. Filtering occurs to keep local traffic confined to its segment.
• Filtering done by the Spanning Tree Protocol that can filter packets based on topology,
making sure that signal loops don’t occur.
• Filtering done for VLAN integrity. Packets from a member of a VLAN (VLAN 2, for example)
destined for a device on another VLAN (VLAN 3) will be filtered.
Spanning Tree
802.1w Rapid Spanning Tree
The DGS-3224TGR implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree
Protocol (RSTP) as defined by the IEE 802.1w specification and a version compatible with the IEEE
802.1d STP. RSTP can operate with legacy equipment implementing IEEE 802.1d, however the
advantages of using RSTP will be lost.
The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1d STP standard. RSTP
was developed in order to overcome some limitations of STP that impede the function of some recent
switching innovations, in particular, certain Layer 3 function that are increasingly handled by Ethernet
switches. The basic function and much of the terminology is the same as STP. Most of the settings
configured for STP are also used for RSTP. This section introduces some new Spanning Tree concepts
and illustrates the main differences between the two protocols.
Port Transition States
An essential difference between the two protocols is in the way ports transition to a forwarding state
and the in the way this transition relates to the role of the port (forwarding or not forwarding) in the
topology. RSTP combines the transition states disabled, blocking and listening used in 802.1d and
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