STP and load balancing using port priority

[fabio.marino]

Hi to all, i'm Fabio and i have a simple question reguarding load-balancing in a trunk link using port-priority.

Topology is:

and logically i want to get:

1)This link is forwarding traffic for vlan 2,4

2)This link is forwarding traffic for vlan 3,5:

Switch0's configuration is:

interface FastEthernet0/1
switchport mode trunk
switchport nonegotiate
spanning-tree vlan 2,4 port-priority 16
!
interface FastEthernet0/2
switchport mode trunk
switchport nonegotiate
spanning-tree vlan 3,5 port-priority 16

Switch1's configuration is:

interface FastEthernet0/1
switchport mode trunk
switchport nonegotiate
spanning-tree vlan 2,4 port-priority 16
!
interface FastEthernet0/2
switchport mode trunk
switchport nonegotiate
spanning-tree vlan 3,5 port-priority 16

Based on the logical schema on point 2, i expected interface Fa0/2 of switch 1 to be in forwarding for VLAN 3 e 5 and in blocking for VLAN 2,4.

Instead i have:  

S1#show spanning-tree vlan 3
VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    32771
             Address     0040.0BAA.4B63
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     0060.5C85.A2B1
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Root FWD 19        128.1    P2p
Fa0/2            Altn BLK 19        16.2     P2p
Fa0/3            Desg FWD 19        128.3    P2p

Fa0/2 is in blocking for vlan 03.

Why?

Sure, i did not get how STP load balancing works.....

Thanks, Fabio.

[cadet alain]

Hi,

Surely packet tracer behaving weird but anyhow you can remove these statements on sw1 :spanning-tree vlan 2,4 port-priority 16  spanning-tree vlan 3,5 port-priority 16

because the tie-breaker must be on sw1( as sw0 is the root bridge) and it is the lowest priority received.

Regards.

[lgijssel]

In your example, you have configured equal port costs on all interfaces between S0 and S1.

In that way, all paths will be equal and hence the topology for all vlans will be identical as well.

STP then resorts to putting the lowest interface in forwarding mode.

What also strikes me as incorrect is the output of the show spanning tree command on S1.

It lists all costs as 19 which is the default value. I also notice that S0 is probably the root for all vlans but at least for vlan 3.

The port with the lowest cost will become the root port (ad hence forwarding).

This can be established by lowering the cost for vlan 3,5 on S1-fa0/2 while keeping them at their default value for S1-fa0/1.

Please check your settings, you are not too far from the solution!

regards,

Leo

[cadet alain]

Hi Leo,

STP then resorts to putting the lowest interface in forwarding mode.

Is it still true when port-priority is configured?

Regards.

[lgijssel]

It is when all ports have the same priority.

[cadet alain]

Leo,

interface FastEthernet0/2
 switchport mode trunk
 switchport nonegotiate
 spanning-tree vlan 3,5 port-priority 16

so on this switch (sw0) f0/2 will have port-priority of 16.2 for vlans 3 and 5 but 128.2 for vlans 2 and 4, no?

so when sw1 must choose root port it will look first for cost and as they are the same on each interface then he looks for lowest sending bid which again are the same and then he looks for port-priority and by default sw0 will have f0/1 as 128.1 for all vlans except 2 and 4 where it will be 16.1 so f0/2 for vlan 3 is sending port-priority of 16.2 which is better than 128.1 Where am I wrong in my reasoning?

Regards.

[fabio.marino]

Hi Cadetalain, you got my question.

My reasoning was the same and i have some doubt how load balancing work usning stp/port-priority

[lgijssel]

First of all, S0 is the root so it will be sending bpdu's with a priority of 0 out on all ports.

On S1, both f0/1 and f0/2 receive these bpdu's and will determine the least-cost path to the root.

Because you are running rapid-PVST, this occurs four times, one instance per vlan.

The cost is calculated by adding the port cost -in outbound direction- to the received cost.

This results in a cost of 19 without changing the cost on any port/vlan.

Changing the cost on S0 is useless; these are not taken into consideration because they are designated ports.

Simply set the interface cost for vlan 3,5 on S1-fa0/2 to 16 and it works.

OK, well. The above is about how I would do it but you intend to do this by using port priority. That's somewhat different and you must take care not to mess up the two like I did by misreading your original post. Perhaps it is the best when you check the attached link:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml

regards,'

Leo

[fabio.marino]

Hi Leo,

i read the suggested document and there you can see that for swicth Catalyst_D there is the same situation, but the output is how i exepected:

Catalyst_D> (enable) show spantree 5/1
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-Method
--------- ----  -------------  -----  --------  ----------  ------------
5/1      1     blocking          19        32   disabled              
5/1      2     forwarding        19        32   disabled              
Catalyst_D> (enable) show spantree 5/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-Method
--------- ----  -------------  -----  --------  ----------  ------------
5/2      1     forwarding        19        32   disabled              
5/2      2     blocking          19        32   disabled              
Catalyst_D> (enable)

--Port 5/1 in blocking for vlan 1 and forwarding for vlan 2

--Port 5/2 in blocking for vlan 2 and forwarding for vlan 1

So, may be my problem is due to packet tracer?????

[lgijssel]

So, may be my problem is due to packet tracer?????

I don't think so. You must configure it like in the document: set a different priority on the root bridge for fa0/1 and fa0/2.

regards,

Leo

[fabio.marino]

What also strikes me as incorrect is the output of the show spanning tree command on S1.
It
 lists all costs as 19 which is the default value. I also notice that S0
 is probably the root for all vlans but at least for vlan 3.
The port with the lowest cost will become the root port (ad hence forwarding).
This can be established by lowering the cost for vlan 3,5 on S1-fa0/2 while keeping them at their default value for S1-fa0/1.
Please check your settings, you are not too far from the solution!
regards,
Leo

Hi Leo,

thanks for your response.

Your observation is correct, but my doubt is different.

My target is to have load balancing using port priority.

I don't care about port cost, and it is set to the default value.

The question is that based on the priority used, i want to have (for example on switch 1):

-VLAN 3-5 traffic carried by port 0/2

-VLAN 2-4 traffic carried by port 0/1

-port 0/1 has to be a backup port for VLAN 3-5 traffic

-port 0/2 has to be a backup port for VLAN 2-4 traffic

Sure i can get this using port cost or using one STP instance for each vlan, but i need to use priority.

Thanks,

Fabio.

[lgijssel · Accepted solution]

Hi Fabio,

This became a bit frustrating for me so I built a quick GNS3 lab with a (good old) 3640 and an NM-16ESW.

In this setup, I simulated your scenario and it works as designed with the following config:

R1 (root switch)

interface FastEthernet0/1
switchport mode trunk
!
interface FastEthernet0/2
switchport mode trunk
spanning-tree vlan 3 port-priority 16
spanning-tree vlan 4 port-priority 16
!

That was basically all I needed to modify from a default config. On R2, the only setting on fa0/1 and fa0/2 is trunk mode:

!
interface FastEthernet0/1
switchport mode trunk
!
interface FastEthernet0/2
switchport mode trunk
!
This gives the following results:

VLAN1
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     cc00.120c.0000
             Cost        19
             Port        2 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     cc01.120c.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet0/1      128.2    128    19 FWD     0 32768 cc00.120c.0000 128.2
FastEthernet0/2      128.3    128    19 BLK     0 32768 cc00.120c.0000 128.3

VLAN3
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     cc00.120c.0002
             Cost        19
             Port        3 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     cc01.120c.0002
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet0/1      128.2    128    19 BLK     0 32768 cc00.120c.0002 128.2
FastEthernet0/2      128.3    128    19 FWD     0 32768 cc00.120c.0002  16.3

And the results are the same for vlan's 2 and 4, i.e. vlan 2 is same as vlan 1, vlan 4 is as vlan 3.

If you need more details, feel free to ask!

reagards,

Leo