13.2.3 GSRP Switching Control

When switching on GSRP switch, you cannot immediately resume end-to-end communication by simply controlling the switching of forwarding and blocking for the frame. However, that is not enough to immediately resume end-to-end communication, because the MAC address entries in the MAC address tables in the neighboring switches are still registered for the previous master GSRP switch. To immediately resume communication, the MAC address table entries on the neighboring switches need to be cleared when the GSRP switches change.

GSRP supports the following methods for clearing the MAC address table entries in the neighboring switches.

<Structure of this section>

(1) Transmitting GSRP Flush request Frames

When the GSRP backup switch takes over as the master switch, the backup switch sends a control frame called a GSRP Flush request frame to the neighboring switches to request the clearing of the MAC address table entries. A switch that can receive this GSRP Flush request frame and clear the internal MAC address table is GSRP aware. The Switch is GSRP-aware unless specified otherwise in the configuration. GSRP-aware switches flood GSRP Flush request frames. A switch that does not support GSRP Flush request frames is GSRP unaware. If the surrounding switches are GSRP unaware, you must use the "(2) Port reset function". The following figure provides an overview of clearing MAC address table entries by using GSRP Flush request frames.

Figure 13-3: Outline of Switching Control by GSRP Flush request Frame

Switch B takes over from Switch A. Switch B sends a GSRP Flush request frame to Switch C.
Switch C receives the GSRP Flush request frame, and clears the internal MAC address table.
As a result, Switch C floods a MAC address request on the port to which the PC is connected until the MAC address of the PC is learned from the frames sent from the PC.

The frames sent from the PC are forwarded to the destination via the master Switch (Switch B).
When a frame returns to the PC as a response, Switch C learns the MAC address of the PC.

Thereafter, Switch C forwards the frames from the PC only to Switch B.

(2) Port reset function

Port resetting temporarily disconnects the link between a GSRP switch and a neighboring switch. Use this function for neighboring switches that are GSRP-unaware. This function is useful because, when the switches detect a link disconnection on the port, switches clear the MAC address entries learned via a port from their MAC address tables.

The following figure provides an overview of clearing MAC address table entries by using port resetting.

Figure 13-4: Overview of switching using the port reset function

Switch B takes over from Switch A. Switch A uses port resetting to disconnect the link with the GSRP-unaware LAN switch.
The GSRP-unaware LAN switch clears the MAC address table for the port link that went down.
As a result, the GSRP-unaware LAN switch floods a MAC address request on the port to which the PC is connected until the MAC address of the PC is learned from the frames sent from the PC.

The frames sent from the PC are forwarded to the destination via the master Switch (Switch B).
When a frame returns to the PC as a response, the GSRP-unaware LAN switch learns the MAC address of the PC.

Thereafter, the GSRP-unaware LAN switch forwards the frames from the PC only to Switch B.