How to make a Root Switch in STP

[Anand Narayana]

I have got 6509 as main & 3750 as standby, running hsrp, enabled STP also, now my question is, how do i make my 6509 as root switch & what benefit do i get once if i make it as root?

following are the summary

6509

Cat6509> (enable) sh spantree summary

MAC address reduction: disabled

Root switch for vlans: none.

Global loopguard is disabled on the switch.

Global portfast is disabled on the switch.

BPDU skewing detection disabled for the bridge.

BPDU skewed for vlans: none.

Portfast bpdu-guard disabled for bridge.

Portfast bpdu-filter disabled for bridge.

Uplinkfast disabled for bridge.

Backbonefast disabled for bridge.

3750

Cisco3750-Standby#sh spanning-tree summary

Switch is in pvst mode

Root bridge for: none

Extended system ID is enabled

Portfast Default is disabled

PortFast BPDU Guard Default is disabled

Portfast BPDU Filter Default is disabled

Loopguard Default is disabled

EtherChannel misconfig guard is enabled

UplinkFast is disabled

BackboneFast is disabled

Configured Pathcost method used is short

[mahmoodmkl]

HI

On the switch which u want to act as root just reduce its priority.for example

u can give this command

spanning-tree vlan port-priority root

Thanks

Mahmood

[Anand Narayana]

does it really must to make a switch as ROOT? if not what will happen?

[mahmoodmkl]

HI

Yeh it is better to make a ROOT switch in the network becoz the path taken by switches to forward the frames might not be that much preferable when it is done automatically with STP.so by defining the root u can make u r switches follow more optimal path than using the default path.

Thanks

Mahmood

[Francois Tallet]

The final topology computed by the spanning tree will look like a star centered on the root bridge. You'd rather make sure your root (and even a secondary root, in case of failure of the primary) are located in something that looks like the center of your network. If most of the traffic goes to a default gateway or to some servers from edge bridges, the root must be close to this point. Letting STP decide where you root bridge is not a good idea because you don't want the traffic between two of your core switches to go through an access switch for instance;-) Also, if a switch with a default configuration is inserted in your network, you don't want it to be a possible root because it could cause a complete reconvergence of your bridged domain. It is thus better (and so simple) to configure a root bridge.

As already mentioned, just reducing the priority to low value is enough to select a root bridge.

Regards,

Francois

[glen.grant]

Also if you are running hsrp make sure the active side of your hsrp is the side that the root for the vlan is on.