To make a switch root use the following command and understand what it does to switch A or switch B spanning tree database:
Spanning-tree vlan 11 root primary
By using above command on the switch, it will try to become the root for vlan 11 by reducing its bridge priority by 8192 default. Try it for as many vlans as you need on the switch you want.
First check the vtp config on both the switches and if either is server than create vlan on server and it will be propagated to client provided domain name and md5 matches. If both are in transparent mode please add vlan to their respective databases by getting into global command
config# vlan 21 name engineering etc....
create/Verify if vlan 21,22,23 exist on switch A/B, if yes then good or else create it.
Assign ports properly to their respective vlans.
All the ports that should transition to forwarding must be port fast enabled. This saves the switch-port transition which normally takes 30 sec of blocking to listening to learning and then forwarding. Use the interface range command for quick configuration or use individual but you need to be quick.
Make the vtp mode of B same as that of switch A.
For Switch B to be in same spanning-tree mode as switch A, run a show spanning-tree command or show run on Switch A will also show the configured spanning-tree .
Configure svi vlan 1 on switch B if it doesn't exist and assign it the required IP and use no shut.
Interswitch connectivity:
1. For trunking use dot1q as it requires tagging unlike isl which encapsulates and is cisco prop.
2. Allow vlan on the trunk link only by using switchport trunk allowed vlan vlan_number. This needs to be run on port which has to be configured for trunking purpose be it one port or bundles of port.
3. For maximum use of available bandwidth one can combine many port under one port-channel so that the spanning tree is not required as loop won't happen and collective capacity of the link also is increased many times.
For the above to work do following:
1. Configure individual port which then is to be trunked but on port-channel only.
2. use show run to check if it has any configuration done on it that is not in line with config of other port it has to be bundled with. Make sure it is not in any access-mode otherwise it will make a port channel for that vlan only and not multi-vlan. Normally we don't make configuration on member but we do it on the etherchannel which is then applicable to members as whole. This is always the good way.
3. Use non proprietary protocol LACP. It can bundle up-to 16 ports as such.
Use following cmd on member interface;
channel-protocol lacp
channel-group xyz(number) mode (active/passive)
Then create a port-channel interface with xyz(number) on config mode and do trunking configuration on it. Make sure you assign right native vlan to the port channel interface.
Do the same config on the other side of the switch. It is up to you to decide which switch will initiate lacp bundling by choosing right mode.
Good luck!
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