This configuration support for :Catalyst 2900/3500XL, Catalyst 2950, Catalyst 3550, Catalyst 4000 supervisor III/IV, and Catalyst 6000.
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.Show Current STP Status on Catalyst DOn Catalyst D, as expected, port 5/2 is blocked for both VLANs 1 and 2.Catalyst_R#show spanning-tree interface FastEthernet 3/1 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ------------------ VLAN0001 Desg FWD 19 128.129 P2p VLAN0002 Desg FWD 19 128.129 P2p Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ------------------ VLAN0001 Desg FWD 19 128.130 P2p VLAN0002 Desg FWD 19 128.130 P2p
Tune the Port Priority on Catalyst RYou 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 128).Catalyst_D#show spanning-tree interface FastEthernet 5/1 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Root FWD 19 128.129 P2p VLAN0002 Root FWD 19 128.129 P2p Catalyst_D#show spanning-tree interface FastEthernet 5/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Altn BLK 19 128.130 P2p VLAN0002 Altn BLK 19 128.130 P2p
Catalyst_R#config terminal Catalyst_R(config)#interface FastEthernet 3/2 Catalyst_R(config-if)#spanning-tree vlan 1 port-priority 64 Catalyst_R(config-if)#end Catalyst_R#
You can check that the port priority value has changed for VLAN 1:
Catalyst_R#show spanning-tree interface FastEthernet 3/1 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ---------------- VLAN0001 Desg FWD 19 128.129 P2p VLAN0002 Desg FWD 19 128.129 P2p Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Desg FWD 19 64.130 P2p VLAN0002 Desg FWD 19 128.130 P2p
Check Result on Catalyst D
You can see on Catalyst D that, for VLAN 1, port 5/1 is now blocking and port 5/2 is forwarding, as was expected.Catalyst_D#show spanning-tree interface FastEthernet 5/1 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Altn BLK 19 128.129 P2p VLAN0002 Root FWD 19 128.129 P2p Catalyst_D#show spanning-tree interface FastEthernet 5/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ---------------- VLAN0001 Root FWD 19 128.130 P2p VLAN0002 Altn BLK 19 128.130 P2p
Details on the port-priority and vlan port-priority Commands
There are two ways to define VLAN port priority:- The "global" port priority value (128 by default) which can be modified per interface by the port-priority command
- The "per VLAN" port priority value which can be modified per interface and per VLAN by the VLAN port-priority command
Change the global priority for the port, using the spanning-tree port-priority interface configuration command:Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Desg FWD 19 64.130 P2p VLAN0002 Desg FWD 19 128.130 P2p VLAN0003 Desg FWD 19 128.130 P2p
Notice that all the VLANs belonging to the "global" group changed their priority to 160. Now assign VLAN 3 its own priority 48 with the spanning-tree vlan port-priority interface command:Catalyst_R(config)#interface FastEthernet 3/2 Catalyst_R(config-if)#spanning-tree port-priority 160 Catalyst_R# Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- --------------- VLAN0001 Desg FWD 19 64.130 P2p VLAN0002 Desg FWD 19 160.130 P2p VLAN0003 Desg FWD 19 160.130 P2p
Notice that only VLAN 3 has changed its priority to 48. To put VLAN 3 back in the default group, use the no spanning-tree vlan port-priority interface command:Catalyst_R(config)#interface FastEthernet 3/2 Catalyst_R(config-if)#spanning-tree vlan 3 port-priority 48 Catalyst_R# Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ----------------- VLAN0001 Desg FWD 19 64.130 P2p VLAN0002 Desg FWD 19 160.130 P2p VLAN0003 Desg FWD 19 48.130 P2p
Catalyst_R(config)#interface FastEthernet 3/2 Catalyst_R(config-if)#no spanning-tree vlan 3 port-priority Catalyst_R# Catalyst_R#show spanning-tree interface FastEthernet 3/2 Vlan Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- ------------------ VLAN0001 Desg FWD 19 64.130 P2p VLAN0002 Desg FWD 19 160.130 P2p VLAN0003 Desg FWD 19 160.130 P2p
Conclusion
The VLAN load balancing configuration just completed optimizes the use of redundant trunks between two Catalysts.Keeping the default STP values leads all redundant links between the two Catalysts to end up in blocking mode. Tuning the STP priority allows several links to be used at the same time, for different VLANs. This increases the overall bandwidth available between the two devices. In case of failure of a link, the STP re-dispatches the VLANs to the remaining trunks as it reconverges.
The only drawback left with this design is that it can only load-balance traffic on a VLAN basis. If in the previous example, you had a 130 Mb/s traffic flowing through VLAN 1 and only 10 Mb/s traffic on VLAN 2, you still drop packets on VLAN 1, even though you have, in theory, 200 Mb/s between Catalyst R and Catalyst D. The EtherChanneling feature addresses this, by providing load balancing between several links on a packet basis. If your hardware supports it, use FastEtherchannel (or GigabitEtherChannel) rather than the configuration described in this document
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