Example 128 Viewing Spanning Tree for VLAN

3550_switch#show spanning-tree summary

Root Bridge for: none.

Extended system ID is enabled. PortFast BPDU Guard is disabled

EtherChannel misconfiguration guard is enabled

UplinkFast is disabled

BackboneFast is disabled

Default pathcost method used is short

Name Blocking Listening Learning Forwarding STP Active

VLAN0001 0 0 0 5 5

yin#

To properly set th e STP root, it hel ps to recali t he four-step decision process of STP and h ow spann determines root. The root is selected by the lowest-co st BID . The B ID is composed oP p riority follov address.

1. Lowest root BID (priority followed by MAC address, adjacent to root bridge)

2. bowest- path cost to root bridge; t he cumulative cost of aN paths to root

3. Lowest sender BID

4. Lowe st port ID

From this process, you can influence the decision of the root at multiple levels. At times, you may w port to have the same STP priority; at other times, however, you may want a specific port to have priority, such as in a load-sharing environment. Table 1-12 lists the four primary STP election state Catalyst 3550 global configuration command.

Table 1-12. Ethernet STP Configuration Outcomes

STPElection State

Catalyst 3550 Configuration Command

1. Lowest BID

I±Jspanning-tree [ vlanvlan_id ] [priorityO-65535]

1+Ispanning-tree vlanvlan_idroot [primary |secondary] [diameter2-

timeseconds]]

2. Lowest patld cost to root

I*spanning-tree [ vlanvlan_id ] [cost1-2OOOOOOOO]

3. Lowest sender BID

!±!spanning-tree [ vlanvlan_id ] [priorityO-65535]

4. Lowest port ID

I*Jspanning-tree [vlanvlan_id] [port-priorityO-255]

[+] Global configuration commands [*] Interface configuration commands

[+] Global configuration commands [*] Interface configuration commands

You can influence the root bridge selection process in many ways. The way you choose depends on trying to accomplish by setting root. The higher up in the election process you use to influence root will help safeguards against possible ties or other STP configurations present on other switches tha have control over.

• The global spanning-tree [ vlanvlan_id ] [priorityO-65535] command can influence the Prior the BID; th e lower the priority, the more likely the switch wi II become root. It can be set on a level or g l obally for the entire switch. The valid values for the VLAN ID are 1 through 4094. V. values are 4096, 8192, 12,288, 16,384, 20,480, 24,576, 28,672, 32,768, 36,864, 40,960, 45 53,248, 57^44, an d 61,440. All other val ues tre rejxcted.

• The global command spanning-tree vlanvcan^'droot [ari ieiary|secondaey] [diameter2-7

seconds]] is a macro much like the set root macro on Cat OS. When the command is entere< primary keyword , it examin es the VLAN(s) on the swi^h for th e highes t p riority, the ro ot, an pno eity less t Ipan eha^ The commfnd may als o adjust thg max age, hello, and forwarding deli comman d also sses the extended oysterm ID. The optiosal diameter keywoad specifi es the m; number ofovvitch es between any two eyd stati0n s° The val id range is 2 to 7. The optional hel sineci fies t he inrejvai , in secon dsn between the generation of configuration m e^anes Iny the ro hang e is 1 to n.0 seconds; th e default value is 2 secon dn. Example 1-29) demon strates the use macro command.

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