Configuring the Spanning Tree Protocol 733
• The MSTP domain contains the root bridge for ALL VLANs. This implies
that the CIST Root Bridge ID is configured to be better than any RSTP-
PV STP root Bridge ID. If there is only one MSTP region connected to the
RSTP-PV domain, then all boundary ports on the virtual-bridge will be
unblocked and used by RSTP-PV. This is the only supported topology, as
the administrator can manipulate uplink costs on the RSTP-PV side and
obtain optimal traffic engineering results. In Figure 22-8, VLANs 2 and 3
have their STP costs configured to select different uplinks connected to
the MSTP region's boundary ports. Since the CIST Root is inside the
MSTP region, both boundary ports are non-blocking designated and the
load balancing scheme operates as expected.
Figure 22-8. MSTP and RSTP-PV Interoperability
MSTP Region
CIST Root
PVST+ Bridge
Boundary
Port 1
Boundary
Port 2
VLAN 3 STPVLAN 2 STP
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