VLAN Load Balancing Between Trunks Using the Spanning-Tree Protocol Port Priority.pdf

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Cisco − VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority
Table of Contents
VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority.......................1
Document ID: 10555
................................................................................................................................1
Introduction..........................................................................................................................................................1
Before You Begin................................................................................................................................................1
Conventions............................................................................................................................................1
Prerequisites............................................................................................................................................1
Components Used...................................................................................................................................1
Introduction to VLAN Load Balancing Between Trunks....................................................................................1
How STP Decides Which Port to Block.................................................................................................2
Details on the portvlanpri Command......................................................................................................3
Details on the port−priority and vlan port−priority Commands.............................................................5
Configuring VLAN Load Balancing on Catalyst Switches Running CatOS......................................................7
Configuring VLAN Load Balancing on Catalyst Switches Running Integrated IOS.........................................8
Conclusion...........................................................................................................................................................9
Related Information...........................................................................................................................................10
i
VLAN Load Balancing Between Trunks Using the
Spanning−Tree Protocol Port Priority
Document ID: 10555
Introduction
Before You Begin
Conventions
Prerequisites
Components Used
Introduction to VLAN Load Balancing Between Trunks
How STP Decides Which Port to Block
Configuring VLAN Load Balancing on Catalyst Switches Running CatOS
Details on the portvlanpri Command
Configuring VLAN Load Balancing on Catalyst Switches Running Integrated IOS
Details on the port−priority and vlan port−priority Commands
Conclusion
Related Information
Introduction
This document provides the theory behind VLAN load balancing between trunks, and also provides
configuration examples for switches running CatOS and Integrated IOS.
Before You Begin
Conventions
For more information on document conventions, see the Cisco Technical Tips Conventions.
Prerequisites
There are no specific prerequisites for this document.
Components Used
This document is not restricted to specific software and hardware versions.
Introduction to VLAN Load Balancing Between Trunks
The commands used in this document are available on the Catalyst 4000, 5000, and 6000 families running
CatOS and Integreated IOS. The theoretical sections of this document are related to spanning−tree protocol
(STP) and are platform independent.
The configuration shown in figure 1 below, in which two switches are directly connected through more than
one trunk, is very common for redundancy purposes. If one of the two links fail, the second soon becomes
available to transmit traffic. When both links are up, the spanning−tree algorithm (STA) disables one of them
in order to avoid a bridging loop between the two switches.
Cisco − VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority
In the figure 1 configuration above, with two FastEthernet trunks linking Catalyst R and Catalyst D, the STP
elects the same blocking port for all the VLANs configured. In this case, Catalyst R is the Root bridge and
Catalyst D decides to block port D2 for both VLAN 1 and VLAN 2. The major issue with this design is that
link R2−D2 is simply sacrificed and there is only 100 Mb/s available between the two switches. To take
advantage of both links, you can change the configuration and allow VLAN 1 only on link R1−D1, and
VLAN 2 only on link R2−D2.
The resulting network, shown in figure 2, lost its redundancy. Nowyou have both links forwarding at the same
time, and you are practically able to use 200 Mb/s between the two switches. However, if one link fails, you
completely loose connectivity for one VLAN. The ideal solution is the one described in figure 3:
In figure 3, you keep the trunks between the two switches, but port D1 is blocking VLAN 1 and forwarding
VLAN 2; port D2 is blocking VLAN 2 and forwarding VLAN 1. This design keeps the best features of figure
1 and figure 2:
•
Both links are forwarding, providing 200 Mb/s aggregate connectivity between the two switches.
•
If one link fails, the remaining one unblocks the corresponding VLAN and maintains connectivity for
both VLANs between the switches.
This document explains how to achieve such a configuration, after a short explanation of the STP operations.
How STP Decides Which Port to Block
The detailed description of how the STA works is beyond the scope of this document. However, it briefly
summarizes how the algorithm decides, in this case, whether a port blocks or forwards. It focuses on the most
simple configuration possible with only one VLAN; Catalyst R is the Root bridge in this VLAN and Catalyst
D has multiple redundant connections to Catalyst R. Catalyst D blocks all of its ports to Catalyst R but its
Root port. How does Catalyst D select its Root port? Bridges running the STA exchange Bridge Protocol Data
Units (BPDUs) across the links, and these BPDUs can be strictly classified depending on their content. A
BPDU is superior than another if it has:
Cisco − VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority
1. A lower Root Bridge ID.
2. A lower path cost to the Root.
3. A lower Sending Bridge ID.
4. A lower Sending Port ID.
These four parameters are examined in order, that is, you only care about parameter 2 if parameter 1 is the
same in the two BPDUs you are comparing.
The port that is elected Root Port on Catalyst D is the port
that is receiving the best BPDU.
In this specific case, figure 4, all the BPDUs sent by Catalyst R have the same Root Bridge ID, the same path
cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending
Port ID. The Sending Port ID is a 16 bit parameter, split in two fields: the port priority and a port index. The
default value for the port priority is 32 and the port index is unique to each port on the switch.
Port Priority
Size in bits
Default value
6
32
10
Fixed unique value
Port Index
Figure 4 represents the port ID parameter in the BPDUs. In this case, Catalyst D chooses port D1 as its Root
port because the port index R2 is inferior to R1. If you want D2 to be eventually forwarding, you need to force
it as the Root port. The only way to do this is to reduce R2 port priority value (or increase R1 port priority
value). This is what was done on figure 5.
In order to achieve load−balancing between two trunks, you tune on a VLAN basis the port priority on
Catalyst R.
Configuring VLAN Load Balancing on Catalyst Switches
Running CatOS
Lab Diagram
Cisco − VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority
Show Current STP Status on Catalyst R
Here is the current STP status on Catalyst R. It is the Root for both VLAN 1 and 2, so all its ports are
forwarding.
Catalyst_R> (enable)
show spantree 3/1
Port
Vlan Port−State
−−−−−−−−−−−−−−−−−−−−−−−− −−−− −−−−−−−−−−−−−
3/1
1
forwarding
3/1
2
forwarding
Catalyst_R> (enable)
show spantree 3/2
Port
Vlan Port−State
−−−−−−−−−−−−−−−−−−−−−−−− −−−− −−−−−−−−−−−−−
3/2
1
forwarding
3/2
2
forwarding
Catalyst_R> (enable)
Cost Priority Portfast
Channel_id
−−−−− −−−−−−−− −−−−−−−−−− −−−−−−−−−−
19
32 disabled
0
19
32 disabled
0
Cost Priority Portfast
Channel_id
−−−−− −−−−−−−− −−−−−−−−−− −−−−−−−−−−
19
32 disabled
0
19
32 disabled
0
Show Current STP Status on Catalyst D
On Catalyst D, as expected, port 5/2 is blocked for both VLANs 1 and 2.
Catalyst_D> (enable)
show spantree 5/1
Port
Vlan Port−State
Cost
Priority
−−−−−−−−− −−−− −−−−−−−−−−−−− −−−−− −−−−−−−−
5/1
1
forwarding
19
32
5/1
2
forwarding
19
32
Catalyst_D> (enable)
show spantree 5/2
Port
Vlan Port−State
Cost
Priority
−−−−−−−−− −−−− −−−−−−−−−−−−− −−−−− −−−−−−−−
5/2
1
blocking
19
32
5/2
2
blocking
19
32
Catalyst_D> (enable)
Fast−Start
−−−−−−−−−−
disabled
disabled
Fast−Start
−−−−−−−−−−
disabled
disabled
Group−Method
−−−−−−−−−−−−
Group−Method
−−−−−−−−−−−−
Tune the Port Priority on Catalyst R
You are going to decrease the port priority value for VLAN 1 on port 3/2. This way, the corresponding port
5/2 on Catalyst D receives better BPDUs than the ones that are sent on port 5/1 (that still has a port priority
value of 32).
Catalyst_R> (enable)
set spantree portvlanpri 3/2 16 1
Port 3/2 vlans 1 using portpri 16.
Port 3/2 vlans 2−1004 using portpri 32.
Port 3/2 vlans 1005 using portpri 4.
Catalyst_R> (enable)
Check Result on Catalyst R
You can check that the port priority value has changed for VLAN 1:
Catalyst_R> (enable)
show spantree 3/1
Cisco − VLAN Load Balancing Between Trunks Using the Spanning−Tree Protocol Port Priority

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