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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP
Configuring RSTP and MSTP Featur es

Configur ing a Se co ndar y R oot S witch

When you configure a Catalyst 2950 switch that supports the extended system ID as the secondary root,
the spanning-tree switch priority is modified from the default value (32768) to 28672. The switch is then
likely to become the root switch for the specified instance if the primary root switch fails. This is
assuming that th e other netwo rk switche s use the defaul t switch pr iority of 32768 an d theref ore ar e
unlikely to become the root switch. For Catalyst 2950 switches without the extended system ID support
(software earlier than Release 12.1(9)EA1), the switch priority is changed to 16384.
You can e xecu te this comman d on more th an one switch to conf igure multip le backup r oot switches. U se
the same n etwor k di am eter a nd he llo -ti me values th at y ou use d w he n you co nfigured t he p ri mary r oot
switch with the spanning-tree mst instance-id root prim ary glo ba l configur atio n c om mand.
Beginning i n privileged EX EC mo de , follow these s teps to c on figure a swit ch as the seco nda ry root
switch:
To return the switch to its default setting, use the no spanning-tree mst instance-id root global
configurati on comm a nd.
Command Purpose
Step 1 configure terminal Enter g lo bal configur ation m ode .
Step 2 spanning-tree mst instance-id root
secondary [diameter net-diameter
[hello-time seconds]]
Configure a switch as t he secondar y root switch.
•For instance-id, th e range is 0 t o 15.
•(Optional) For diameter net-diameter, sp ecify the maximum
number of swit ches bet ween any t wo en d sta tions. T he r ange is
2 to 7 . Th is keyword is avail able o nly for M ST insta nce 0.
•(Optional) For hello-time seconds, specify the interval in
seconds be tween th e genera tion of co nfiguration messages by
the root sw itch. The rang e is 1 to 10 seco nds; the default is 2
seconds.
Use the sam e network dia meter and hello-t ime values that you used
when configuring the primary root switch. See the “Configuring the
Root Switch” section on pa ge 12-14.
Step 3 end Return to pr ivileged EX EC m ode.
Step 4 show spanning-tree mst instance-id Verify your entries.
Step 5 copy running-config startup-config (Optional) Save your entries in the configuration file.