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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP
Unders tan ding MSTP

Hop Count

The IST and M ST inst ances do not use the mes sage- age an d maximum -age informa tion in the
configurati on BPDU to comp ut e t he sp an ning- tre e t opolo gy. Instead , th ey use th e path cost to the r oot
and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops globa l c onfiguratio n c omma nd , you c an c onfigure t he
maxim um hops inside t he region and appl y it to the IST and a ll MST instanc es in that region. The hop
count achieves the same result as the message-age information (determines when to trigger a
reconfigur atio n). T he root sw itch of the ins tance always s ends a B PDU (o r M-reco rd) wi th a co st 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 th e por t.
The messa ge-age and maxim um-ag e inform ation in the RSTP portion of the BPDU re main the sam e
through out the region, and the sam e values a re propaga ted by the region’s designated ports at the
boundary.

Bounda ry Po rts

A boundary port is a a port that connects an MST region to a single spanning-tree region running RSTP,
or to a single sp an ning- tre e regi on runni ng 80 2.1D , or to a no ther M ST regi on w it h a d i fferent MST
configuration. A boundary port also connects to a LAN, the designated switch of which is either a single
spanning-tree switch or a switch with a different MST configuration.
At the boundary, the roles of the MST ports do not matter, and their state is forced to be the same as the
IST port state (MST ports at the boundary are in the forwarding state only when the IST port is
forwarding ). A n IST po rt at th e bou ndary c an have any po rt rol e exce pt a ba ckup po rt r ol e.
On a shared boundary link, the MST ports wait in the blocking state for the forward-delay time to expire
before transiti oning to the learning st ate. Th e MST por ts wait anot her forwar d-delay t ime befo re
transitioning to the forwarding state.
If the boundar y port is on a point- to-poin t link and it is the I ST root port , the MST por ts transition to th e
forwarding state as soon as the IST port transitions to the forwarding state.
If the IST port is a designated port on a point-to-point link and if the IST port transitions to the
forwarding state because of an agreement received from its peer port, the MST ports also immediately
transit ion to the for warding state .
If a bound ary por t transitions to th e for warding state in an IST instance, it is fo rwarding in all MST
instan ces, and a topo logy ch ang e i s tr ig gere d. If a b oun dary por t w ith th e IST ro ot or de signa ted po rt
role receives a topology change notice external to the MST cloud, the M STP switch tri ggers a topology
change in the IST instance and in all the MST instances active on that port.