Controlling STP by Removing It from Trunk Lines

In medium to large networ ks, it bgcom es crucia l to control how many instances of STP there are or and how many traverse the tsunks. Recal l that by default every VLAN w ill have a n icstan ce of STP, calls PVST+. Switch es wiil run an insta nce of STP on all truhks toe every VLAN they are aware of. If has five VLANs, th ere are five instance s of STP, a nd e ach i hatance h as a siegle root switch, and so ■ Catalyet 3550 s aaaorts 128 i nstance s of STP per switch. Ot aer switcues, such as C atalyst 3548XL a can support 64 STP insdances, end tNs var iem oa a switch- by-switch bas ¡s. To find out hove many in STP your switc It supports, tecer to ww w.c i If more VLAlSs aoe added, STP is disabled for tt VLANs on that switch. A more common problem is the amount of strain many STP instances put on or wiring-closet switches. Unfortunately, VTP pruning does not affect spanning tree. To remove an STP from a trunk, use the following interface command :

Switch(config-if)#switchport trunk allowed vlan [add | all | except | remove] vla

• add— Adds the following VLANs to the trunk

• except— Includes all VLANs except those specified

• remove— Removes the following VLANs from the trunk

To remove VLAN 3 through VLAN 6, you would use the following cryptic-looking command:

Switch(config-if)#switchport trunk allowed vlan remove 3-6

Figure 1-1 88 sh ows the sa me netw ork as Figure 1- 17, with updated interfaces names. In this examp switch, all VLANs are cleared, except 1 and 2, on the trunk to the crane switch.

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