11-13
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 11 Configuring STP Configuring Spanning-Tree Features
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the
extended system ID support:
•For Catalyst 2950 switches with the extended system ID (Release 12.1(9)EA1 and later), if all
network devices in VLA N 20 have the de fault p rior i ty of 3 2768, e nter ing the spanning- tree vlan
20 root primary command on the swit ch sets the switch priorit y to 24576, which causes this switc h
to become the root switch for VLAN 20.
•For Catalyst 2950 switches without the extended system ID (software earlier than
Release 12 .1(9) EA1) , if a ll ne twor k d evices in VLAN 1 00 have the defaul t pr iori ty of 32768,
entering the spanning-tree vlan 100 root primary command on the switch sets the switch prio ri ty
for VLA N 100 to 8192, which cause s this switch t o become the root switch for VLAN 100 .
Note If your network consists of switches that both do and do not support the extended system ID, it is unlikely
that the switch with the exten ded system ID support will become the root switch. The extended system
ID increases the switch priority value every time the VLAN number is greater than the priority of the
connected switches running older software.
Note Th e ro ot swi tch fo r eac h s pan ning -t ree inst anc e shoul d b e a ba ckbon e or dist ribution sw itch . D o no t
configure an access switch as the spanning-tree primary root.
Use the diameter keywo rd t o sp eci fy the L ayer 2 net work d i amet er (t hat is, t he m ax imum n umb er o f
switch hops b etw een a ny two end stat ion s in t he L ayer 2 ne twork ). Wh en yo u s pecify the n etwork
diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age
time for a network of that diameter, which can significantly reduce the convergence time. You can use
the hello keyword to over ride the automatically calculated hello time.
Note After conf iguring the switc h as the root switch, we recommend tha t you avoid manually conf ig uring the
hello time, forward-delay time, and maximum-age time by using the spanning-tree vlan vlan-i d
hello-time, spanning-tree vlan vlan-id forward-time, and t he spanning-tree vlan vlan-i d max-age
global configurati on comm ands.