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730 Configuring the Spanning Tree Protocol
RSTP-PV region and the MSTP region, the RSTP-PV switch sends VLAN1
BPDUs in IEEE standard format, so they can be interpreted by the MSTP
peers. Similarly, the RSTP-PV switch processes incoming MSTP BPDUs as
though they were BPDUs for the VLAN 1 RSTP-PV instance.
If the RSTP-PV switch ports connected to the MSTP switches are configured
with a native VLAN, the RSTP-PV switches are able to detect IEEE standard
format BPDUs arriving from peer switches, incorporate them into the
common spanning tree that operates in the native VLAN (VLAN 1), and
transmit untagged STP or RSTP packets to the STP/RSTP peers, in addition
to the SSTP format BPDUs.
SSTP BPDUs Flooding Across MST (CST) Regions
In addition to the IEEE standard RSTP or STP BPDUs that the RSTP-PV
switch sends to the MSTP (or RSTP or STP) region, the switch sends SSTP
format BPDUs for VLAN 1 untagged. The MSTP switch does not interpret
the SSTP BPDUs as standard BPDUs because they do not use the standard
destination MAC address, so it makes no spanning tree decisions based on
them. Instead, it floods the SSTP BPDUs over all ports in the corresponding
VLAN. These SSTP BPDUs may be multicast over the MSTP region to other
RSTP-PV switches, which use them to maintain the VLAN 1 spanning tree
topology across the MSTP (non-RSTP-PV) switches.
The RSTP-PV switches also send SSTP format BPDUs for the other (non-
VLAN 1) RSTP-PV instances into the MSTP region, tagged with the VID of
their associated VLANs. These SSTP packets are also be multicast by the
switches in the MSTP region, and will reach any other RSTP-PV regions
connected to the MSTP region. The switches in the remote RSTP-PV regions
receive and process them as normal RSTP-PV BPDUs. Thus, RSTP-PV
instances are transparently expanded across the MSTP region and their
spanning trees span the MSTP region. For RSTP-PV, the MSTP region is
treated as a single hub.
Interoperability with RSTP
In Figure 22-7:
• SW1 and SW2 are Dell Networking switches running RSTP-PV with
default bridge priority 32768.
• SW3 is a Dell Networking switch running RSTP with default bridge
priority 32768.