Day 21 - Spanning Tree Protocol (STP) (2)

Jeremy’s IT Lab lecture video:

Day 21 - Spanning Tree Protocol (STP) (2)

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Commands

Commands List

Spanning Tree Show Commands

6. Spanning Tree

• show spanning-tree --> Shows general STP status and information
• show spanning-tree vlan vlan-id --> Shows spanning-tree information for a specific VLAN
• show spanning-tree detail --> Shows detailed STP information
• show spanning-tree summary --> Shows a summary of the current STP configuration

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Spanning Tree Interface Commands

Spanning Tree

1. General

• spanning-tree mode mode {mst / pvst / rapid-pvst} (Versions)

2. Spanning Tree Portfast / BPDU Guard

• spanning-tree portfast --> Enables portfast on the current interface.
• spanning-tree portfast default --> Enables Portfast on all access ports by default.
• spanning-tree bpduguard enable --> Enables BPDU Guard on the current interface.
• spanning-tree portfast bpduguard default --> Enables BPDU Guard on all Portfast-enabled interfaces by default.

3. Spanning Tree VLAN Related

• spanning-tree vlan vlan-id root primary --> Sets the STP bridge priority to 24576 (Or 4096 lower than the current lowest STP priority.)
• spanning-tree vlan vlan-id root secondary --> Sets the STP bridge priority to 28672
• spanning-tree vlan vlan-id cost number --> Changes the interface’s STP root cost
• spanning-tree vlan vlan-id port-priority number --> Changes the interface’s port priority

4. Spanning Tree Link Type

• spanning-tree link-type type { point-to-point / shared }

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Classic Spanning Tree Information

The protocol version identifier for classic STP is 0

Spanning Tree Port States

Classic Spanning Tree has 5 different port states, being:

1. Blocking / Stable
2. Listening / Transitional
3. Learning / Transitional
4. Forwarding / Stable
5. Disabled

A forwarding interface can move directly into a blocking,

However, a blocking cannot move directly into a forwarding state. It must go through the listening and learning states first.

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Spanning Tree Timers

There are 3 different spanning tree timers, being:

1. Hello / 2 seconds
2. Forward delay / 15 seconds
3. Max age / 20 seconds (10 multiplied by hello timer)

BPDU forwarding

Switches do not forward the BPDUs out of their root ports and non-designated ports, only their designated ports.

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Spanning Tree Optional Features (STP Toolkit)

1. Portfast
   • Portfast allows a port to move immediately into the forwarding state, bypassing listening and learning. This is used for switchports which are connected to end hosts, as they do not pose a risk to causing loops in the network.
   • Portfast will only work on access ports.
2. BPDU Guard
   • BPDU Guard makes it so when an interface with BPDU Guard enabled receives a BPDU from another switch, the interface will be shut down to prevent a loop from forming.
3. Root Guard
   • If you enable root guard on an interface, even if it receives a superior BPDU (lower bridge ID) on that interface, the switch will not accept the new switch as the root bridge. The interface will be disabled.
4. Loop Guard
   • If you enable loop guard on an interface, even if the interface stops receiving BPDUs, it will not start forwarding. The interface will be disabled.

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Spanning Tree Primary and Secondary Root Bridges

There are two commands to easily change the bridge priority of a switch, they are:

1. spanning-tree vlan vlan-id root primary
   • The command sets the STP priority to 24576.
   • If another switch already has a priority lower than 24576, it sets this switch’s priority to 4096 less than the other switch’s priority.
2. spanning-tree vlan vlan-id root secondary
   • The command sets the STP priority to 28672.

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Spanning Tree Port Settings

There are two commands you need to know for spanning tree port settings, they are:

1. spanning-tree vlan vlan-id cost number
   • Changes an interface’s root cost, therefore overriding the default STP port costs.
2. spanning-tree vlan vlan-id port-priority number
   • Changes an interface’s port priority, therefore overriding the default STP port priority of 128.