Cisco Systems 3750E manual Understanding Rstp, Interoperability with Ieee 802.1D STP, 19-9

Chapter 19 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 receives a legacy 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 been 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-treedetected-protocolsprivileged 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 Version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch of which is either a single spanning-tree switch or a switch with a different MST configuration.

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, page 19-9

•Rapid Convergence, page 19-10

•Synchronization of Port Roles, page 19-11

•Bridge Protocol Data Unit Format and Processing, page 19-12

For configuration information, see the “Configuring MSTP Features” section on page 19-14.

Port Roles and the Active Topology

The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the 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 18-3. Then the RSTP assigns one of these port roles to individual ports:

•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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 19-9