Root Bridge Configuration

[yuanguitao]

Hei,guys. As i got from the Cisco Official documents that the cmd to configure a root bridge has two styles. one is to set the switch to be root directly, and the other is to deploy the priority of the switch. However, I am confused by the cmd.

Switch(config)#spanning-tree Vlan 1 root primary

why should mention the Vlan 1 in the cmd. if it is the native vlan or other reasons?

[Peter Paluch]

Hi Wei,

why should mention the Vlan 1 in the cmd. if it is the native vlan or other reasons?

Cisco's STP implementation runs a separate copy of STP process for each VLAN. There is no "global" STP setting for a Cisco switch because there is no "global" STP process. All STP settings that can be done on a Catalyst switch always refer to a particular VLAN in which the corresponding STP instance runs.

So regardless of using the spanning-tree vlan vlan-range priority or spanning-tree vlan vlan-range root primary command, the VLAN number for which the switch priority is being modified must be explicitly specified.

Best regards,

Peter

[Karsten Iwen · Accepted solution]

On Cisco Switches you typically run one spanning-tree instance per VLAN (there are better ways if you have many VLANs like MST). So you can have different topologies configured in every VLAN. For that you can specify the vlan when setting the priority. And remember that the referenced command doesn't do anything else then setting the priority to a lower level.

So how do you use these multiple topologies?

If you have a "traditional" 3 layer network (core-distribution-access) you have two ways from access to your distribution layer. To use both uplinks you can set the root-bridge for half the VLANs to distribution-switch1 and the other half to distribution-switch2. By that you split your traffic over both links.

-- 
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
http://www.kiva.org/invitedby/karsteni

[yuanguitao]

Hello, i am not well in implementing STP. In fact, i don't know when and how to use every STP standards, like CST,PVST,MPVST and so on. can you show me details about these protocols. or show me some simple documents, configuration instance. thank you !

[Peter Paluch · Accepted solution]

Wei,

A gross simplification but perhaps helpful to you:

• Use Rapid PVST+ if you have at most tens of VLANs and the entire network is based on Cisco switches
• Use MSTP if you have hundreds of VLANs or the network is composed of Cisco and non-Cisco switches
• Do not either use or activate STP. Both Rapid PVST+ and MSTP will automatically downgrade their operation to STP if they detect an old neighbor on a per-port basis while retaining their advantages on remaining ports.

A good reading can be found here:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

http://blog.ine.com/2008/07/17/pvst-explained/

http://blog.internetworkexpert.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/

http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/

Best regards,

Peter

[yuanguitao]

Thank you very much.