STP, Ether-channel port into CISCO Switch

[chiragom2341]

Hi,

The two switche, namely A and B are connected with a single link and both switches have few vlan's.

here, Can I still make the switch A as the root bridge for specific vlan's that are associated with Switch B ?

I would like to know, is there any function/s available through that we can manipulate the root of specific vlan even though the switches are connected with single link.

If there are features available then can we applies the same feature/technique onto ether-channel port as well.....

Thank you for your time and support.

Chirag

[milan.kulik]

Hi,

you can configure the single link connecting your two switches as a trunk.

And then you can configure the first or the second switch as a root per VLAN as you want.

Best regards,

Milan

[Giuseppe Larosa]

Hello Chirag,

I make the assumption that the single link is a trunk link carrying multiple Vlans.

The Vlan set have to be defined on both devices.

Let's suppose these two devices are Cisco switches. You can choice to use VTP protocol to create Vlans on one switch and have them propagated to the other switch via VTP messages that travel over the trunk link. (VTP messages are not sent on access link only on trunk links).

Otherwise, without VTP both switches can be configured for VTP transparent mode and both define the same set of Vlans (10,20,30,40,50).

At this point both devices agree on the Vlan database either by VTP either by manual configuration.

The default STP type that runs on Cisco switches is PVST+ (Per Vlan Spanning Tree ) plus, that creates a different STP instance for each Vlan.

Here it is the answer, if you have Cisco switches running PVST+ or Rapid PVST  you can decide Vlan by Vlan which switch will be the root bridge just remembering that in case of a tie the lowest bridge-id wins.

So two commands like

spanning-tree vlan 10,30,50 priority 0

spanning-tree vlan 20,40 priority 4096

on Switch A

and

spanning-tree vlan 10,30,50 priority 4096

spanning-tree vlan 20,40 priority 0

on SwitchB

make Switch A root bridge for vlans 10,30,50 and Switch B root bridge for vlans 20,40.

Bridge-priority in multiple of 4096 is now the standard for cisco switch that uses the extended system-id concept: they use as bridge-id a base MAC address and the priority is actually given by

base priority + vlan-id + MAC address

This is made to save MAC address used as source MAC addresses for STP messages (BPDUs).

It is also called MAC address reduction.

Hope to help

Giuseppe