| By default, STP is enabled for every
port on the switch. If for some reason STP has been disabled, you
can reenable it on a set command-based switch with the set
spantree enable command.
You can also selectively enable or
disable specific ports on the switch.
Switch> (enable) set spantree
enable mod-num/port-num
Switch> (enable) set spantree disable mod-num/port-num
The following example shows how to
enable STP and verify that it is enabled on a selected virtual LAN (VLAN).
Switch> (enable) set
spantree enable all
Spantree enabled.
To enable STP on a Cisco IOS
command-based system, enter the following command in global
configuration mode.
Switch(config)#spantree vlan-list
Where vlan-list
is the number of the specific
VLAN on which you are enabling STP. You can include up to ten VLANs
in the list.
To disable STP on a VLAN, enter the no
spantree vlan-list command in
global configuration mode.
The most important switch command for
working with STP is the show
spantree command. Although
this command offers several useful parameters, this section explains
only the basic syntax. Figure
illustrates a sample of the show
spantree output from a set
command-based switch (Cat-B) in the sample network.
This show
spantree output in Figure
can be broken down into four sections as follows:
- Global statistics for the current
switch/bridge (lines 2-4)
- Root-bridge statistics (lines 5-9)
- Local-bridge statistics (lines
10-12)
- Port statistics (lines 13-16)
The global statistics appear at the
top of the screen. The first line of this section (VLAN 1)
indicates that the output contains information only for VLAN 1. The
second line indicates that STP is enabled on this Catalyst® Switch
for this VLAN. The final line of this section shows that the IEEE
version of STP is being utilized (this cannot be changed on most
Catalyst Switches).
The first two lines of the
root-bridge statistics display the BID of the current root bridge.
The BID subfields are displayed separately - Designated Root
shows the MAC address contained in the low-order six bytes, whereas Designated
Root Priority holds the high-order two bytes. The cumulative
root path cost to the root bridge is displayed in the Designated
Root Cost field. The fourth line of this section (Designated
Root Port) shows the current root port of the local device. The last
line of the root-bridge statistics section shows the timer values
currently set on the root bridge. These values are used throughout
the entire network (at least for VLAN 1) to provide consistency. The
term designated is used here to signify that these values pertain to
the bridge that this device currently believes is the root bridge.
However, because of topology changes and propagation delays during
network convergence, this information might not reflect the
characteristics of the true root bridge.
The local bridge statistics section
displays the BID of the current bridge in the first two lines. The
locally configured timer values are shown in the third line of this
section.
Note: The timer values shown in the
local-bridge statistics section are not utilized unless the current
bridge becomes the root bridge at some point.
The port statistics section is
displayed at the bottom of the screen. Depending on the number of
ports present in the Catalyst Switch, this display can continue for
many screens using the more
prompt. This information displays the path-cost value associated
with each port. This value is the cost that is added to the Root
Path Cost field contained in BPDUs received on this port. In other
words, Cat-B receives BPDUs on Port 1/1 with a cost of 0 because
they are sent by the root bridge. The Port 1/1 cost of 19 is added
to this zero-cost value to yield a designated root cost of 19. In
the outbound direction, Cat-B sends BPDUs downstream with a cost of
19-the port 1/2 path cost of 19 is not added to transmitted BPDUs
(see 5.2.3).
Figure
illustrates the show
spanning-tree command output
from a Cisco IOS command-based switch. As can be seen, this output
is formatted differently but contains the same information.
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Interactive
Lab Activity (Flash,
301 kB) |
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In
this activity, you will learn how to configure
a root bridge and port priorities on a
Catalyst 2900 series switch. |
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