spanning tree load balance question

[mahesh18]

hi all,

i have question here for spanning tree load balance  which was in Switching exam

switch A config

interface fas0/10

spanning tree vlan 1-5 port priority 10

switchport mode trunk

interface fas0/12

spanning tree vlan 6-10 port priority 10

switchport mode trunk

question is how vlan traffic will flow  between switch A  and B

Switch A  has two trunk ports to switch B  which  are fa0/10  and fa0/12.

i know default port priority is 128 and  lower is better.

many thanks

mahesh

[Giuseppe Larosa]

Hello Mahesh,

the starting point is always the root bridge

With PVST+ or Rapid PVST a different STP instance is running for each Vlan.

Let's suppose that Switch A is the root bridge for all Vlans of interest of that is nearest to the root bridge

When Switch B has to decide for each Vlan what will be the root port it examines the received BPDUs on both fas0/10 and fas0/12.

the cost of the root bridge is the same so this is a tie

the bridge id of the BPDU sender is the same so thi is a tie

the lowest priority is preferred so port fas0/12 is preferred for those Vlans where priority is set to 10 as you noted.

Notice also that if SwitchB were the root bridge or were better connected to the root bridge these settings on Switch A ports are not effective.

another way to achieve this is to change spanning tree cost on the ports of Switch B:

the cost of the port is added to the path cost on the received BPDU

so you can make STP load sharing per Vlan either by:

changing STP priority per vlan on the upstream switch ports

OR

by changing STP cost per vlan on the downstream switch ports

to be noted with no settings all Vlans will be in forwarding mode only on link fa0/10 - fas0/10, for the lowest port index of fas0/10 in comparison to fas0/12.

Hope to help

Giuseppe

[mahesh18]

Hi  Giuseppe,

thanks for reply

as per correct answer  it says

. VLANs 6-10 will use fa0/10 as a backup only.

. VLANs 1-10 are configured to load share between fa0/10 and fa0/12.

we know that it is tied on por priority.so lower port number wins and it is fa0/10

so as per you note we can say that port fa0/10 will pass all the vlan traffic right ??

but answer says it will use vlan 6 - 10   as backup only?

thanks

mahesh

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

often test answers are not direct and require some effort

direct answers:

LANs 1-10 are configured to load share between fa0/10 and fa0/12.

traffic of Vlan 6-10 goes on fas0/12 link

traffc of  Vlan 1-5 goes on  fas0/10 link

if I tell that fas0/10 is a backup for vlans 6-10 is equivalent to say:

traffic of Vlan 6-10 goes on fas0/12 link

the vlans for which the priority is lowered to 10 will use the link if SwitchA is the root bridge or nearest to the root bridge for all Vlans 1-10

>> so as per you note we can say that port fa0/10 will pass all the vlan traffic right ??

no, fas0/10 would be used for all Vlans only with default configuration that is what I meant in my previous post and I'm sorry if it wasn't clear

Hope to help

Giuseppe

[mahesh18]

Hi Giuseppe,

Many thanks for wonderfull explanation

regards

mahesh