Allied Telesis 2.6.1 manual Learning Process

Layer 2 Switching 81

The default settings for the Ingress Rules are to Admit All Frames, and for Ingress Filtering to be OFF. This means that if no VLAN configuration has been done, all incoming frames pass on to the Learning Process, regardless of whether or not they are VLAN tagged. The parameters for each port’s Ingress Rules can be configured using the command:

SET SWITCH PORT={port-listALL} [ACCEPTABLE={VLANALL}] [INFILTERING={ONOFF}] [other-parameters...]

The ACCEPTABLE parameter sets the Acceptable Frame Types parameter, in the Ingress Rules, which controls reception of VLAN-tagged and VLAN- untagged frames on the port. If ALL is specified, then the Acceptable Frame Types parameter is set to Admit All Frames. If VLAN is specified, the parameter is set to Admit Only VLAN-tagged Frames, and any frame received that carries a null VLAN Identifier (VID) is discarded by the ingress rules. Untagged frames and priority-tagged frames carry a null VID. Untagged frames admitted according to the ACCEPTABLE parameter have the VID of the VLAN for which the port is untagged associated with them. The ACCEPTABLE parameter can only be set if the port is untagged for one VLAN. In this case, the default is ALL, admitting all tagged and untagged frames. If the port is tagged for all the VLANs to which it belongs, the ACCEPTABLE parameter is automatically set to VLAN, and cannot be changed to admit untagged frames.

The INFILTERING parameter enables or disables Ingress Filtering of frames admitted according to the ACCEPTABLE parameter, on the specified ports. Each port on the switch belongs to one or more VLANs. If INFILTERING is set to ON, Ingress Filtering is enabled: any frame received on a specified port is only admitted if the port belongs to the VLAN with which the frame is associated. Conversely, any frame received on the port is discarded if the port does not belong to the VLAN with which the frame is associated. Untagged frames admitted by the ACCEPTABLE parameter are admitted, since they have the numerical VLAN Identifier (VID) of the VLAN for which the port in an untagged member. If OFF is specified, Ingress Filtering is disabled, and no frames are discarded by this part of the Ingress Rules. The default setting is OFF.

To display the current Ingress Rules, use the command:

SHOW SWITCH PORT=port-list

The Learning Process

The Learning Process uses an adaptive learning algorithm, sometimes called backward learning, to discover the location of each station on the extended LAN.

All frames admitted by the Ingress Rules on any port are passed on to the Forwarding Process if they are for destinations within the same VLAN. Frames destined for other VLANs are passed to the layer three protocol, for instance IP. For every frame admitted, the frame’s source MAC address and numerical VLAN Identifier (VID) are compared with entries in the Forwarding Database for the VLAN (also known as a MAC address table, or a forwarding table) maintained by the switch. The Forwarding Database contains one entry for every unique station MAC address the switch knows in each VLAN.

Software Release 2.6.1 C613-02039-00 REV A