The Forwarding Process; The Egress Rules - Allied Telesis SwitchBlade x908 Series Software Reference Manual

Switching Introduction

The Forwarding Process

After a VID is assigned to a frame using the ingress rules, the switch forwards it to the
destination MAC address specified in the frame. To do this the switch must learn which MAC
addresses are available on each port for each VLAN. When the destination MAC address is not
found, the switch floods the frame on all ports that are members of the VLAN except the port
on which the frame was received.
The forwarding database (also known as the MAC Address table) determines the egress port on
which the destination MAC address has been learned. MAC addresses are learned dynamically
as part of the layer two switching process.
The forwarding database is ordered according to MAC address and VLAN identifier. This
means a MAC address can appear more than once in the forwarding database having been
learned on the same port but for different VLANs. This could occur if the IP address of an end
station is changed thereby moving the end station to a different IP subnet-based VLAN while
still connected to the same switch port. When the forwarding database ageing process is
enabled, old entries in the forwarding database are deleted after a user-configurable period.
If the destination address is found, the switch discards the frame when the port is not in the
STP forwarding or disabled state if the destination address is on the same port as the source
address, or if there is a static filter entry for the destination address set to discard (see "Layer 2
Filtering" on page 14.10). Otherwise, the frame is forwarded on the indicated port.
Forwarding occurs only when the port on which the frame was received is in the Spanning Tree
forwarding or disabled state. The destination address is then looked up in the forwarding
database for the VLAN.

The Egress Rules

After the forwarding process has determined from which ports and transmission queues to
forward a frame, the egress rules for each port determine whether the outgoing frame is
VLAN-tagged with its numerical VLAN identifier (VID).
A port must belong to a VLAN at all times unless the port has been set as the mirror port for
the switch.
A port can transmit VLAN-tagged frames for any VLAN to which the port belongs. A port can
transmit untagged frames for any VLAN for which the port is configured, e.g. IP subnet-based
or protocol-based, unless prevented by the port-based VLAN egress rules. A port that belongs
to a port-based VLAN can transmit untagged packets for only one VLAN. For more
information about VLANs and VLAN tagging, see "Quality of Service" on page 14.18.
For more information on port tagging see the following commands:
switchport mode access command on page 17.9
switchport mode trunk command on page 17.11.
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
C613-50007-01 REV B AlliedWare Plus Operating System - Software Version 5.3.1 14.9