Cisco Cisco ONS 15454 SONET Multiservice Provisioning Platform (MSPP) Guía De Diseño
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Processing Inferior BPDU Information
If a designated ML-Series port receives an inferior BPDU (higher bridge ID, higher path cost, etc.,
than currently stored for the port) with a designated port role, it immediately replies with its own
information.
Topology Changes
This section describes the differences between the RSTP and the 802.1D in handling spanning
tree topology changes.
If a designated ML-Series port receives an inferior BPDU (higher bridge ID, higher path cost, etc.,
than currently stored for the port) with a designated port role, it immediately replies with its own
information.
Topology Changes
This section describes the differences between the RSTP and the 802.1D in handling spanning
tree topology changes.
Detection: Unlike 802.1D in which any transition between the blocking and the forwarding
state causes a topology change, only transitions from the blocking to the forwarding state
cause a topology change with RSTP. (Only an increase in connectivity is considered a
topology change.) State changes on an edge port do not cause a topology change.
When an RSTP switch detects a topology change, it flushes the learned information on all
of its non-edge ports.
cause a topology change with RSTP. (Only an increase in connectivity is considered a
topology change.) State changes on an edge port do not cause a topology change.
When an RSTP switch detects a topology change, it flushes the learned information on all
of its non-edge ports.
Notification: Unlike 802.1D, which uses TCN BPDUs, the RSTP does not use them.
However, for 802.1D interoperability, an RSTP switch processes and generates TCN
BPDUs.
BPDUs.
Acknowledgement: When an RSTP switch receives a TCN message on a designated port
from an 802.1D switch, it replies with an 802.1D configuration BPDU with the topology
change acknowledgement bit set. However, if the TC-while timer (the same as the
topology-change timer in 802.1D) is active on a root port connected to an 802.1D switch
and a configuration BPDU with the topology change acknowledgement bit set is received,
the TC-while timer is reset. This behavior is only required to support 802.1D switches.
The RSTP BPDUs never have the topology change acknowledgement bit set.
change acknowledgement bit set. However, if the TC-while timer (the same as the
topology-change timer in 802.1D) is active on a root port connected to an 802.1D switch
and a configuration BPDU with the topology change acknowledgement bit set is received,
the TC-while timer is reset. This behavior is only required to support 802.1D switches.
The RSTP BPDUs never have the topology change acknowledgement bit set.
Propagation: When an RSTP switch receives a TC message from another switch through
a designated or root port, it propagates the topology change to all of its non-edge, edge,
designated ports, and root port (excluding the port on which it is received). The switch
starts the TC-while timer for all such ports and flushes the information learned on them.
designated ports, and root port (excluding the port on which it is received). The switch
starts the TC-while timer for all such ports and flushes the information learned on them.
Protocol migration: For backward compatibility with 802.1D switches, RSTP selectively
sends 802.1D configuration BPDUs and TCN BPDUs on a per-port basis.
When a port is initialized, the timer is started (which specifies the minimum time during which
RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the ML-Series
card processes all BPDUs received on that port and ignores the protocol type.
If the ML-Series card receives an 802.1D BPDU after the port’s migration-delay timer has
expired, it assumes that it is connected to an 802.1D switch and starts using only 802.1D BPDUs.
However, if the RSTP ML-Series card is using 802.1D BPDUs on a port and receives an RSTP
BPDU after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port.
Interoperability with 802.1D STP
An ML-Series card running RSTP supports a built-in protocol migration mechanism that enables it
to interoperate with legacy 802.1D switches. If this card receives a legacy 802.1D configuration
BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port.
However, the ML-Series card does not automatically revert to the RSTP mode if it no longer
receives 802.1D BPDUs, because it cannot determine whether the legacy switch has been
removed from the link unless the legacy switch is the designated switch. Also, an ML-Series card