Toastmaster Recipe 1195

STP


Load Balancing using STP port priority

is a layer 2 protocol used to prevent layer 2 loops in collegamneto with multiple switches.
The election of the ports (designated, blocking, non-designated) takes into account several factors, including the priority of the ports. Playing with the priorities of the doors you get a load balancing of traffic between multiple VLANs. For example:





Obviously we have:

Or the traffic of the four VLANs properly balanced in the two trunk link.

There are two techniques you can use to get this: stp stp port priority and cost.
STP Port Priority

The scheme for the port fastethernet 0 / 1 S1:

PRIORITY
VLAN ---- ------- ----------- S1 4-2 16 3-5128 (default)
In this way, the door will be forwarding the traffic in vlan 4-2 and blocking traffic for VLANs 3-5.
S1 (config) # interface fastethernet 0 / 1 S1 (config-if) # vlan 2 spanning-tree port-priority 16
S1 (config-if) # spanning-tree vlan 4 port-priority 16


Check priorities:

S1 # show spanning-tree interface fastethernet 0 / 1 Vlan Role Sts

Cost Prio.Nbr
Type ---------------- ---- - - --------- -------- ------------------------------- -
VLAN0001 Desg FWD 19 128.1 P2p
VLAN0002 Desg FWD 19 1.16 P2p
VLAN0003 Desg FWD 19 128.1 P2p
VLAN0004 Desg FWD 19 1.16 P2p
VLAN0005 Desg FWD 19 128.1 P2p

The same is repeated for all trunk ports in order to respect the logical pattern above

S1 # show spanning-tree interface FastEthernet 0 / 2 Vlan Role Sts Cost
Prio.Nbr
Type ---------------- ---- --- --------- --- -------------------------------- -----
VLAN0001 Desg FWD 19 128.2 P2p
VLAN0002 Desg FWD 19 128.2 P2p
VLAN0003 Desg FWD 19 2.16 P2p VLAN0004 Desg FWD 19 128.2 P2p VLAN0005 Desg FWD 19 2.16 P2p



Version

Several levrsione are introduced to stp:
1) Common Spanning Tree (CST), referred to as 802.1D in which a single instance of STP runs for the entire network

2) PVST + In this case there is a instance of STP for each VLAN in the network

3) Rapid STP, 802.1w as shown in which there is only one instance of STP with a convergence time high.
4) MST, Multiple Spanning Tree in which more 'with the same VLAN traffic requirements can be arranged in individual instances of STP.

Definitions

STP Bridge ID: + bridge priority MAC address
Default priority: 32.768
Port ID: port priority + port number
Path Cost: cumulative cost to reach the root switch from a switch interface from which you want calcorae cost


Timer Hello-messages -> 2 sec-
're Listening -> 15 sec-Learning
state-> 15 sec-
Listening + Learning = 30 sec -> Forwarding
delay = 20 sec-Max_Age_Timer