18-9
Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
OL-13270-01
Chapter18 Configuring MSTP Understanding RSTP

Interoperability with IEEE 802.1D STP

A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate
with legacy IEEE 802.1D switches. If this switch receives a legacy IEEE 802.1D configuration BPDU
(a BPDU with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP
switch also can detect that a port is at the boundary of a region when it receiv es a legac y BPDU, an MSTP
BPDU (Version 3) associated with a different region, or an RSTP BPDU (Version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives
IEEE 802.1D BPDUs because it cannot detect whether the legacy switch has b een removed from the link
unless the legacy switch is the designated switch. A switch might also continue to assign a boundary role
to a port when the switch to which this switch is connected has joined the region. To restart the protocol
migration process (force the renegotiation with neighboring switches), use the clear spanning-tree
detected-protocols privileged EXEC command.
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are
RSTP BPDUs. Therefore, MSTP switches send either a Version 0 configuration and TCN BPDUs or
Version3 MSTP BPDUs on a boundary por t. A boundary port connects to a LAN, the designated switch
of which is either a single spanning-tree switch or a switch w ith a different MST con figurati on.
Understanding RSTP
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree.
Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the
default settings in the IEEE 802.1D spanning tree).
These sections describe how the RSTP works:
•Port Roles and the Active Topology, page18-9
•Rapid Convergence, page 18- 10
•Synchronization of Port Roles, page 18-11
•Bridge Protocol Data Unit Format and Processing, page 18-1 2
For configuration information, see the “Configuring MSTP Features” section on page 18-14.

Port Roles and the Active Topology

The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learn in g th e
active topology. The RSTP builds upon the IEEE 802. 1D STP to select the switch with the highest switch
priority (lowest numerical priority value) as the root switch as described in the “Spanning-Tree Topology
and BPDUs” section on page 17-3. Then the RSTP assigns one of these port rol es t o individual por ts:
•Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.
•Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch
is attached to the LAN is called the designated port.
•Alternate port—Offers an alternate path toward the root switch to that provided by the current root
port.