Lab: Spanning Tree customization

Most real networks only choose where the root is. They leave everything else at the default settings. But exams, and some special cases, ask you to fine-tune the tree. This lab practises the three other settings you can change: port cost, port priority and timers.

⚠️ Based on Cisco IOS / IOS XE documentation, not run in a lab here. Change the interface names to match your switches. All links are 1 Gbps trunks (cost 4) carrying VLANs 10 and 20. The switches run PVST+.

Topology

Requirements

1. VLAN 10: SW4's traffic to SW1 must go via SW2.
2. VLAN 20: SW1 must reach SW2 over the second cable (its Gi1/0/2), without changing any cost on SW1.
3. VLAN 20: the root must send BPDUs every second. If the root goes silent, switches must stop waiting for it after 10 seconds.

Try it yourself first. Then compare with the solutions.

Remember the decision order

A switch chooses its root port by going down this list until it finds a difference (the “Spanning Tree port roles” lesson):

1. Lowest root bridge ID
2. Lowest total root path cost
3. Lowest sender bridge ID
4. Lowest sender port ID

You already used rule 1 when you chose the roots. Requirement 1 uses rule 2, and requirement 2 uses rule 4.

Requirement 1: VLAN 10 via SW2 (port cost)

Plan first. The cost adds up from switch to switch. So a change on one port can also change things for switches further down. Write down the paths before you change anything:

You have two choices: lower the cost of the path you want, or raise the cost of the path you don't want. Here, the simplest is to raise the cost of the direct port. It only affects SW4, because no switch is connected below SW4.

SW4#show spanning-tree vlan 10 root
                                        Root    Hello Max Fwd
Vlan                   Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
VLAN0010         24586 0200.0000.0001         8    2   20  15  Gi1/0/2

Requirement 2: VLAN 20 over the second cable (port priority)

SW1 sees two equal paths to SW2: the same root, the same cost (4) and the same sender switch (SW2). So rule 4 decides: the sender port ID. This is the port ID of SW2's port at the other end of each cable.

The two parts are shown together as Prio.Nbr, for example 128.1. You are not allowed to change SW1's costs, and the rule looks at the sender's port ID. So the change goes on SW2.

SW1#show spanning-tree vlan 20
...
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/1             Altn BLK 4         128.1    P2p
Gi1/0/2             Root FWD 4         128.2    P2p
Gi1/0/3             Desg FWD 4         128.3    P2p

Requirement 3: VLAN 20 timers

The timers are sent in the root's BPDUs. So you change them only on the root for that VLAN, and every other switch follows. The allowed values are:

⚠️ The timers must fit together. Max age must be at least 2 × (hello + 1), and at most 2 × (forward delay − 1). Hello 1, max age 10 and forward delay 15 fit (4 ≤ 10 ≤ 28). If timers are too short, ports can start forwarding before the tree is ready, and cause loops.

SW2#show spanning-tree vlan 20
VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    24596
             Address     0200.0000.0002
             This bridge is the root
             Hello Time   1 sec  Max Age 10 sec  Forward Delay 15 sec

💡 Shortcut: spanning-tree vlan 20 root primary diameter 3 hello-time 1 makes the switch the root. It also works out matching timers for a network up to 3 switches across, so you don't have to choose each value.

Wrap-up questions