Cisco Catalyst 2960 Series, Руководство по настройке

Страница: 339 / 2288

• Notification — Unlike IEEE 802.1D, which uses TCN BPDUs, the RSTP does not use them. However,
for IEEE 802.1D interoperability, an RSTP switch processes and generates TCN BPDUs.
•
Acknowledgement
—
When an RSTP switch receives a TCN message on a designated port from an IEEE
802.1D switch, it replies with an IEEE 802.1D configuration BPDU with the TCA bit set. However, if
the TC-while timer (the same as the topology-change timer in IEEE 802.1D) is active on a root port
connected to an IEEE 802.1D switch and a configuration BPDU with the TCA bit set is received, the
TC-while timer is reset.
This behavior is only required to support IEEE 802.1D switches. The RSTP BPDUs never have the TCA
bit set.
• Propagation — When an RSTP switch receives a TC message from another switch through a designated
or root port, it propagates the change to all of its nonedge, designated ports and to the 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 IEEE 802.1D switches, RSTP selectively sends
IEEE 802.1D configuration BPDUs and TCN BPDUs on a per-port basis.
When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which
RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all
BPDUs received on that port and ignores the protocol type.
If the switch receives an IEEE 802.1D BPDU after the port migration-delay timer has expired, it assumes
that it is connected to an IEEE 802.1D switch and starts using only IEEE 802.1D BPDUs. However, if
the RSTP switch is using IEEE 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.
Protocol Migration Process
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 MST BPDU (Version 3)
associated with a different region, or an RST 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 also might continue to assign a boundary role to a port when the
switch to which it is connected has joined the region.
Related Topics
Restarting the Protocol Migration Process , on page 278
Default MSTP Configuration
Table 33: Default MSTP Configuration
Default Setting Feature
MSTP Spanning-tree mode
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
257
Information About MSTP