SJA1105 Flush L2 Lookup Table (TCAM) for RSTP

[Ben5049]

Hi there! I am trying to implement the rapid spanning tree protocol (RSTP) on an SJA1105Q, however I have run into an issue. The protocol requires flushing all the MAC address forwarding rules learned on a particular port, but the SJA1105's TCAM has no mechanism for doing this as far as I can tell.

Has anyone implemented this or know how it could be done?

Thanks a lot!

[PavelL]

Hello @Ben5049 ,

Neither the SJA1105 nor the SJA1110 switches support STP, RSTP, or MSTP protocols natively. 

Instead, the SJA1110 supports advanced IEEE standards such as 802.1CB for frame replication and elimination, as well as other TSN-related features. However, it does not include native support for spanning tree protocols.

If you need to flush dynamically learned MAC address forwarding rules (e.g., in response to a topology change), one possible approach is to reset the switch from the host processor and reload a static configuration. This method is discussed in the following NXP Community thread:
https://community.nxp.com/t5/Other-NXP-Products/SJA1105PEL-clear-dynamically-learned-MAC-addresses-R...

For applications requiring STP/RSTP/MSTP, an external CPU running Linux with a software bridge and user-space daemon (e.g., mstpd) may be used to manage topology and MAC learning externally.

Bet regards,

Pavel

[Ben5049]

Thanks a lot for your response @PavelL. In the thread you linked the user mentioned using the MAXAGE setting to rapidly age out all learned entries. As I understand it, this wouldn't work because MAXAGE is set for an entrywhen it is learned and cannot be changed after. Is this correct? That leaves resetting the switch core or manually invalidating all entries?

I am using a microcontroller as the host so Linux software isn't really an option.

[PavelL · Accepted solution]

Hello @Ben5049 ,

Yes, it seems you are right. The SJA1105Q is an Automotive Ethernet switch, and protocols like STP/RSTP/MSTP are not typically part of standard automotive use cases.

MAC address learning is controlled by the L2 Lookup Parameters Table. Please refer to the UM11044 Software User Manual for SJA1105P/Q/R/S, chapter 5.2.7, for details on how learning and aging are configured. This table defines parameters such as aging time, learning mode, and address resolution behavior.

Importantly, this table can be dynamically updated at runtime — see chapter 6.1.3.8 in the same manual for details on dynamic configuration updates via the SPI interface.

Additionally, the AH1704 – SJA1105PQRS Application Hints document contains a dedicated section on address learning (chapter 7.5.4), which may help clarify the behavior and limitations of the learning mechanism.

Both documents are available in the Documentation section of the NXP website.

Best regards,

Pavel

[Ben5049]

Hi @PavelL,

I've read both of these documents and they are very helpful. I just wanted to check my understanding, especially since AH1704 briefly mentions RSTP (in the context of sending management frames).

Thanks a lot again for your help,

Ben