23-28
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 23 Configuring STP and MST
MST Configuration Restrictions and Guidelines
To prevent a misconfiguration, the PortFast operation is turned off if the port receives a BPDU. You can
display the configured and operational status of PortFast by using the show spanning-tree mst interface
command.
Link Type
Rapid connectivity is established only on point-to-point links. You must configure ports explicitly to a
host or switch. However, cabling in most networks meets this requirement. By entering the
spanning-tree linktype command to treating all full-duplex links as point-to-point links, you can avoid
explicit configuration.
Message Age and Hop Count
IST and MST instances do not use the message age and maximum age timer settings in the BPDU. IST
and MST use a separate hop count mechanism that is very similar to the IP time-to live (TTL)
mechanism. You can configure each MST bridge with a maximum hop count. The root bridge of the
instance sends a BPDU (or M-record) with the remaining hop count that is equal to the maximum hop
count. When a bridge receives a BPDU (or M-record), it decrements the received remaining hop count
by one. The bridge discards the BPDU (M-record) and ages out the information held for the port if the
count reaches zero after decrementing. The nonroot bridges propagate the decremented count as the
remaining hop count in the BPDUs (M-records) they generate.
The message age and maximum age timer settings in the RST portion of the BPDU remain the same
throughout the region, and the same values are propagated by the region’s designated ports at the
boundary.
MST Configuration Restrictions and Guidelines
Follow these restrictions and guidelines to avoid configuration problems:
• Do not disable spanning tree on any VLAN in any of the PVST bridges.
• Do no use PVST bridges as the root of CST.
• Do not connect switches with access links because access links may partition a VLAN.
• Ensure that all PVST root bridges have lower, (numerically higher) priority than the CST root
bridge.
• Ensure that trunks carry all of the VLANs mapped to an instance or do not carry any VLANs at all
for this instance.
• Complete any MST configuration that incorporates a large number of either existing or new logical
VLAN ports during a maintenance window because the complete MST database gets reinitialized
for any incremental change (such as adding new VLANs to instances or moving VLANs across
instances).
Configuring MST
The following sections describe how to configure MST:
• Enabling MST, page 23-29
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