Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MSTP (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 6 of 70
Only the CST instance sends and receives BPDUs, and MST instances add their spanning-tree
information into the BPDUs to interact with neighboring switches and calculate the final spanning-
tree topology. Because of this, the spanning-tree parameters related to BPDU transmission (for
example,
hello-time
,
forward-delay
,
max-age
, and
max-hops
) are configured only on the CST
instance but affect all MST instances. Parameters related to the spanning-tree topology (for
example,
switch priority
,
port cost
,
port priority
) can be configured on both the CST instance
and the MST instance.
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.
By using the
mstp max-hops
command, in Protocol Configuration mode, 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.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the
same throughout the region, and the region’s designated ports at the boundary propagate the same
values.
MST-to-SST Interoperability
A virtual bridged LAN may contain interconnected regions of SST and MST bridges.
To enable running STP in the SST region, an MST region appears as a single SST or pseudobridge,
which operates as follows:
•
Although the values for root identifiers and root path costs match for all BPDUs in all
pseudobridges, a pseudobridge differs from a single SST bridge as follows:
o
The pseudobridge BPDUs have different bridge identifiers. This difference does not affect
STP operation in the neighboring SST regions because the root identifier and root cost are
the same.
o
BPDUs sent from the pseudobridge ports may have significantly different message ages.
Because the message age increases by one second for each hop, the difference in the
message age is measured in seconds.
•
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port
follows a path entirely contained within the pseudobridge or MST region. Data traffic
belonging to different VLANs might follow different paths within the MST regions established
by MST.
•
The system prevents looping by doing either of the following:
o
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the
boundary and blocking all other ports
o
Propagating correct information about the CST part of the BPDU so that other switches
can determine redundant paths and block the corresponding ports.