IBM 12.1(22)EA6 Manuale Utente
10-4
Cisco Systems Intelligent Gigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter 10 Configuring MSTP
Understanding MSTP
Hop Count
The IST and MST instances do not use the message-age and maximum-age information in the
configuration BPDU to compute the spanning-tree topology. Instead, they use the path cost to the root
and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
configuration BPDU to compute the spanning-tree topology. Instead, they use the path cost to the root
and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the
maximum hops inside the region and apply it to the IST and all MST instances in that region. The hop
count achieves the same result as the message-age information (determines when to trigger a
reconfiguration). The root switch of the instance always sends a BPDU (or M-record) with a cost of 0
and the hop count set to the maximum value. When a switch receives this BPDU, it decrements the
received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs
it generates. When the count reaches zero, the switch discards the BPDU and ages the information held
for the port.
maximum hops inside the region and apply it to the IST and all MST instances in that region. The hop
count achieves the same result as the message-age information (determines when to trigger a
reconfiguration). The root switch of the instance always sends a BPDU (or M-record) with a cost of 0
and the hop count set to the maximum value. When a switch receives this BPDU, it decrements the
received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs
it generates. When the count reaches zero, the switch discards the BPDU and ages the information held
for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same
throughout the region, and the same values are propagated by the region designated ports at the
boundary.
throughout the region, and the same values are propagated by the region designated ports at the
boundary.
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 can also 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).
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 can also 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).
IST master
and CST root
and CST root
IST master
IST master
A
MST Region 1
D
Legacy 802.1D
B
MST Region 2
MST Region 3
C
74009