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MSTP interaction with other protocols



I have some doubts regarding the interaction of a switch running MSTP (802.1 S) with other switches running legacy 802.1D and 802.1W.

Lets say S1 running MSTP is connected to S2 and S3 via ports 1 and 2 with S2 running 802.1W and S3 running 802.1D.

Now when port 1 on S1 receives a RSTP BPDU, i believe the MST will start a "PVST+" simuation  and send out a RsTP BPDU out that port. Correct?

Now when port 2 on S1 receives 802.1D BPDU, will it send the legacy 802.1D BPDU out that port to S3, and activate the timers like MAXAGE and FWDDELAY with the default values?

Basically, what i want to know is how MSTP interacts wilth all other bridging protocols. It would be great if someone could point to any Cisco white papaer/documentation on this.

Thanks in advace!!!

1 Reply 1

Reza Sharifi
Hall of Fame Master Hall of Fame Master
Hall of Fame Master


Have a look at the config guide for interpretability between these protocols:

Interoperability with 802.1D STP

A switch running both MSTP and RSTP supports a built-in protocol  migration mechanism that enables it to interoperate with legacy 802.1D  switches. If this switch receives a legacy 802.1D configuration BPDU (a  BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on  that port. An MST switch can also detect that a port is at the boundary  of a region when it receives a legacy BPDU, an MST BPDU (version 3)  associated with a different region, or an RST BPDU (version 2).

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