Spanning Tree: Root Election and Path Tuning

The concept is rather simple - The Switches send these probes called the BPDUs (Bridge Protocol Data Units) to discover loops in the network. If the BPDU “returns” – there is a loop in the network!

BPDU = 4-bit-PRIORITY + MAC Address

Spanning tree is no game, so be extremely careful when tuning the Priorities, Costs and Port-Priorities in order to manually make the Switch set your desired path as preferred. Each problem and mis-configuration can easily cause a major critical situation, as most of the Layer2 Loop Problems cause your Switches to immediately increase the CPU usage drastically. For your own stress-free dreams be sure to test in the Pre-Production environment everything you need to change in your production network regarding the Spanning Tree.

I´m not going to get into the explaining the Spanning Tree basics here, as I guess most of the CCIE candidates should be familiar with it. The focus of this post will be the pure control of the Root Bridge in your network, and the preferred links.

Set the Root Bridge

There are two ways to set which Switch within your network will take the roll or the Root Bridge. They are both based on setting the priority parameter, and therefore the Bridge ID.

BRIDGE ID = PRIORITY* + VLAN No + MAC Address

*PRIORITY = N x 4096 (values between 0 and 61440, with 32768 being the mid-value chosen as a "default priority")

Remember about the STP: LOWER PRIORITY IS BETTER!!!

1^st Way of setting the PRIORITY – SET PRIMARY/SECONDARY

(config)# spanning-tree vlan 1 root [primary | secondary]

Honestly I don’t like this way because theres a general confusion among the network engineers that this command somehow dynamically sets the Switch to maintain its role as a Root Bridge. It doesn’t!!! It just sets the priority value to the lower value that the CURRENT Root Bridge has (case of primary), or sets the FIXED value to the 28672 (case of secondary).

2^nd way of setting the PRIORITY – Manual priority command

(config)# spanning-tree vlan 1 priority X*

*X = N x 4096 (values between 0 and 32768)

Personally I prefer this way, makes me that I´ve got much more control over the L2 network when I set the parameters myself, no automatic stuff and elections!

Be extremely careful when manually adjusting this parameter as it will totally alter your Layer 2 Switching Paths. What basically happens here is that each switch in the network does the following process:

1. Determine who the Root Bridge is

2. Find the shortest path to the Root Bridge looking at the COST parameter

1. Block all the other paths*

*In the basic version of the Per-VLAN Spanning Tree Protocol, 802.1D the ports have one of the following states:

- ROOT Port – Used to reach the Root Bridge (Port TOWARDS the Root Bridge)

- DESIGNATED Port – Forwarding Port (One-Per-Link, Port AWAY from the Root Bridge)

- BLOCKING (Non-DESIGNATED) Port – the link where “the tree fell”

On the NEWER versions of this protocol, such as Rapid STP (802.1w) two new port roles are introduced instead of the BLOCKING port:

-      - ALTERNATE Port – The improvement where the Switch actually “remembers” its alternative path to reach the Root Bridge in order to failover immediately in case the primary link goes down

-      - EDGE Port – towards the NON-SWITCH devices, has portfast feature configured

Link Cost

You´ve already chosen the Root Bridge, everything is fine, but for some reason – you don’t want your traffic to prefer a certain link… Why? Well, maybe you´ve got insider information that the cable is bad, that the cleaning lady sometimes unplugs it by accident, or… well, or you simply consider it better for your design. Cisco allows you to manually set the COST of one link to a higher value so that the other link is chosen as the preferred, and that way you gain the control of the Root Port election on your switch. 

This is a GNS3 diagram of 3 Switches, and it will serve for this example. Lets say SW3 is the Root Bridge, and all the links are 100Mbps, which would be the COST of 19. Naturally the SW1 chooses the direct link towards the SW3 cause it costs him 19 to get there.

Lets sat that for some reason we want to force the SW1 to use the other path to reach the SW3 (the link SW1-SW2-SW3). The cost of that link is 38, so what do we do? We set the cost of the direct link SW1-SW3 to the value greater than 38. Let’s go with the value of 39:

(config-if)# spanning-tree vlan 1 cost 39

*Notice that the configuration is done on the Interface level

Port Priority

This command is used only when there are more ways to reach the SAME SWITCH. Sometimes it’s a tie-breaker so be careful, it determines which ports are in the FWD and which in the BLK state. The default port priority is 128, so you need to define the lower value in order for the link to be preferred. The value range is between 0 and 255.

Check the output of the *show spanning tree interface x details* command on the Cat2 device:

Cat1(config-if)#spanning-tree vlan 1 port-priority 64
Cat2#show spanning-tree vlan 1 int fa0/24 detail Port 24 (FastEthernet0/24) of VLAN0001 is root forwarding   Port path cost 19, Port priority 128, Port Identifier 128.24.   Designated root has priority 24577, address 0018.1820.2700   Designated bridge has priority 24577, address 0018.1820.2700   Designated port id is 64.26, designated path cost 0   Timers: message age 2, forward delay 0, hold 0   Number of transitions to forwarding state: 2   Link type is point-to-point by default   BPDU: sent 1993, received 697

The important thing is that the UPSREAM value needs to be changed because it’s the RECEIVED port priority that really matters! For the port to be preferred we need to set a smaller value on the Cat1 device, so let’s go with 64:

And when we check the output of the *show spanning tree interface x details* command again:

Cat2#show spanning-tree vlan 1 interface fa0/24 details
Port 24 (FastEthernet0/24) of VLAN0001 is alternate blocking
Port path cost 19, Port priority 128, Port Identifier 128.24.
Designated root has priority 24577, address 0018.1820.2700
Designated bridge has priority 24577, address 0018.1820.2700
Designated port id is 128.26, designated path cost 0
Timers: message age 1, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 1992, received 306

For more details please check the Cisco official Spanning Tree Configuration reference: Cisco Configuration Guide