Ieee 802.1W Rstp - Cisco Catalyst 4500 Series Configuration Manual

Chapter 20 Configuring STP and MST
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IEEE 802.1w RSTP

RSTP, specified in 802.1w, supersedes STP specified in 802.1D, but remains compatible with STP. You
configure RSTP when you configure the MST feature. For more information, see the
section on page
RSTP provides the structure on which the MST operates, significantly reducing the time to reconfigure
the active topology of a network when its physical topology or configuration parameters change. RSTP
selects one switch as the root of a spanning-tree-connected active topology and assigns port roles to
individual ports of the switch, depending on whether that port is part of the active topology.
RSTP provides rapid connectivity following the failure of a switch, switch port, or a LAN. A new root
port and the designated port on the other side of the bridge transition to the forwarding state through an
explicit handshake between them. RSTP allows switch port configuration so the ports can transition to
forwarding directly when the switch reinitializes.
RSTP provides backward compatibility with 802.1D bridges as follows:
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MST establishes and maintains additional spanning trees within each MST region. These spanning
trees are termed MST instances (MSTIs). The IST is numbered 0, and the MSTIs are numbered 1,
2, 3, and so on. Any MSTI is local to the MST region and is independent of MSTIs in another region,
even if the MST regions are interconnected.
MST instances combine with the IST at the boundary of MST regions to become the CST as follows:
– Spanning tree information for an MSTI is contained in an MSTP record (M-record).
M-records are always encapsulated within MST bridge protocol data units (BPDUs). The
original spanning trees computed by MSTP are called M-trees, which are active only within the
MST region. M-trees merge with the IST at the boundary of the MST region and form the CST.
MST provides interoperability with PVST+ by generating PVST+ BPDUs for the non-CST VLANs.
MST supports some of the PVST+ extensions in MSTP as follows:
– UplinkFast and BackboneFast are not available in MST mode; they are part of RSTP.
– PortFast is supported.
– BPDU filter and BPDU guard are supported in MST mode.
– Loop guard and root guard are supported in MST. MST preserves the VLAN 1 disabled
functionality except that BPDUs are still transmitted in VLAN 1.
– MST switches operate as if MAC reduction is enabled.
– For private VLANs (PVLANs), you must map a secondary VLAN to the same instance as the
primary.
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RSTP selectively sends 802.1D-configured BPDUs and Topology Change Notification (TCN)
BPDUs on a per-port basis.
When a port initializes, the migration delay timer starts and RSTP BPDUs are transmitted. While
the migration delay timer is active, the bridge processes all BPDUs received on that port.
If the bridge receives an 802.1D BPDU after a port's migration delay timer expires, the bridge
assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs.
When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU after the migration delay
expires, RSTP restarts the migration delay timer and begins using RSTP BPDUs on that port.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
About MST
"Configuring MST"
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