Stp Configuration Examples - Cisco Catalyst 2000 Configuration Handbook

120 Cisco LAN Switching Configuration Handbook
10. (Optional) Improve STP stability with Loop Guard:
(interface) spanning-tree loopguard
Loop Guard should be enabled only on the ports that you know are root or alternate
root ports. For example, the uplink ports on an access layer switch would always be
root or alternate root ports because they are closest to the root bridge. (This
assumes that you have placed the root bridge toward the center of your network.)
Displaying Information About STP
Table 7-2 lists the switch commands that you can use to display helpful information
about STP.
Table 7-2 Switch Commands to Display STP Information
Display Function
STP for a specific VLAN
STP state for all VLANs on a trunk

STP Configuration Examples

As a good practice, you should always configure one switch in your network as a primary
root bridge for a VLAN and another switch as a secondary root. Suppose you build a
network and forget to do this. What might happen if the switches are left to sort out a
spanning-tree topology on their own, based on the default STP parameters?
Poor STP Root Placement
The top half of Figure 7-2 shows an example network of three Catalyst switches connect-
ed in a triangle fashion. Catalysts C1 and C2 form the core layer of the network, whereas
Catalyst A connects to the end users in the access layer. (C1 and C2 might also be consid-
ered distribution layer switches if the overall campus network doesn't have a distinct core
layer. In any event, think of them as the highest layer or the backbone of the network.)
problem has been corrected. On the Supervisor IOS, use the EXEC command
udld reset to reenable all ports that are disabled by UDLD.
Command
(exec) show spanning-tree vlan vlan
(exec) show spanning-tree interface
mod/num