Prioritizing traffic by VLAN

Prioritizing traffic by VLAN
VLANS 1–2
VLANS 3–4
set spantree portvlanpri 1/1 16 1-2
set spantree portvlanpri 1/2 16 3-4
1/1
1/2
Scaling the Spanning Tree Protocol 515
Terry_4000> (enable) set spantree portvlanpri 1/1 16 1-2
Port 1/1 vlans 1-2 using portpri 16.
Port 1/1 vlans 3-1004 using portpri 20.
Port 1/1 vlans 1005 using portpri 4.
Terry_4000> (enable) set spantree portvlanpri 1/2 16 3-4
Port 1/2 vlans 1-2,5-1004 using portpri 32.
Port 1/2 vlans 3-4 using portpri 16.
Port 1/2 vlans 1005 using portpri 4.
Terry_4000> (enable)
The preceding switch output displays the VLAN priority information. We set both VLAN port
priorities to 16. Notice that for VLANs 1–4, the priority is 16. However, on port 1/1, all the other
VLANs are listed as having a port priority of 20 because that is what we set the port priority to
earlier in this chapter. On port 1/2, the switch thinks all the other ports have a port priority of 32,
except for VLAN 1005, which becomes a default priority of 4.
You can view the changes by using the show spantree slot/port command, as shown here:
Terry_4000> (enable) show spantree 1/1
Port Vlan Port-State Cost Priority Fast-Start
--------- ---- ------------- ----- -------- ----------
1/1 1 forwarding 10 16 disabled
1/1 2 forwarding 10 16 disabled
1/1 3 forwarding 10 20 disabled
1/1 4 forwarding 10 20 disabled
1/1 1003 not-connected 10 20 disabled
1/1 1005 not-connected 10 4 disabled

Terry_4000> (enable) show spantree 1/2
Port Vlan Port-State Cost Priority Fast-Start
--------- ---- ------------- ----- -------- ----------
1/2 1 forwarding 19 32 disabled
1/2 2 forwarding 19 32 disabled
1/2 3 forwarding 19 16 disabled
1/2 4 forwarding 19 16 disabled
1/2 1003 not-connected 19 32 disabled
1/2 1005 not-connected 19 4 disabled
Terry_4000> (enable)
Setting the VLAN priority on the IOS-based switches is carried out using the interface command
spanning-tree vlan vlan_number port-priority priority. Looking at the default
configuration, we can see that the port priority is set to 128.


Terry_2950#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
[Output cut]
Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------------- -------------------- --------
Fa0/1 128.1 100 BLK 0 1 00b0.6414.1180 128.1
Fa0/24 128.24 20 FWD 0 1 00b0.6414.1180 128.12
If we want to change the VLAN port priority on the 2950 switch to make the port more
desirable, then we can reduce the priority as follows:
Terry_2950#conf t
Terry_2950(config)#interface fa0/1
Terry_2950(config-if)#spanning-tree vlan 1 port-priority 20
Terry_2950(config-if)#^Z
Terry_2950#sho span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 00b0.6414.1180
Cost 20
Port 24 (FastEthernet0/24)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000b.be53.2c00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- ------------- --------- --------------------
Fa0/1 20.1 20 FWD 0 1 00b0.6414.1180 128.1
Fa0/24 128.24 20 BLK 0 1 00b0.6414.1180 128.12


By changing either the port priority or the port cost, you can persuade the switch to use your
chosen paths. However, there are some miscellaneous other STP variables that you can change.
We’ll discuss those next.