STP

Redundant Layer 2 Topologies

Redundancy

Simple network VLAN mangement

Starting point

Path Failure - Access to Distribution

Path Failure - Distribution to Core

Switch Failure - Distribution layer

Switch Failure - Core layer

Issues with Redundancy

Layer 2 Loops

Broadcast loops

Real-world Redundancy Issues

Non-Redundant network topology

Loops from 2 connections in the same switch

Loops from connections to a second switch

Loops int the cubicles

Introduction to STP

The Spanning Tree Algorithm

STP Topology

STP Redundancy

STP Port names

Root ports - Switch ports closest to the root bridge.
Designated ports - All non-root ports that are still permitted to forward traffic on the network.
Non-designated ports - All ports configured to be in a blocking state to prevent loops.

Root Bridge

BID fields

Port cost

Configuring port costs

Path Cost

Verify port and path costs

STP BPDU

BPDU Fields

Version - The version field indicates the version of the protocol, This field contains the value zero.
Message type - The message type field indicated the type of message, this field contains the value zero.
Flags - The flags field includes one of the following: Topology change(TC) bit, which signals a topology change in the event a path to the root bridge has been disrupted. Topology change acknowledgment(TCA) bit, which is set to acknowledge receipt of a configuration messege with the TC bit set.
Root ID - The Root ID field indicates the root bridge by listing its 2-byte prority followed by its 6-byte MAC address ID.
Bridge ID - The Bridge ID field indicates the priority and MAC address ID of the bridge sending the message
Port ID - The Port ID field indicates the port number from which the configuration message was sent.
Forward Delay - The Forward delay field indicates the length of time that bridges should wait before transitioning to a new state after topology change.

BPDU Fields

BPDU Process Step 1

BPDU Process Step 2

BPDU Process Step 3

BPDU Process Step 4

BPDU Process Step 5

BPDU Process Step 6

BPDU Process Step 7

BPDU Process Step 8

BPDU Process Step 9

Bridge ID

BID Fields

Priority-based desicions

MAC Address-based desicions

Configure STP

Verify STP

Port Roles

Root Port: The root port exists on non-root bridges and is the switch port with the best path to the root bridge.
Designated Port: The designated port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. Only one designated port is allowed per segment. Designated ports are capable of populating the MAC table.
Non-designated Port: The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses.
Disabled Port: The disabled port is a switch port that is administratively shut down.

STP Port Roles

Configure port priority

Verify Port Roles and Priority

STP Port States and BPDU Timers

Port States:

Blocking: The port is a non-designated port and does not participate in frame forwarding. The port receives BPDU frames to determine the location and root ID of the root bridge switch and what port roles each switch port should assume in the final active STP topology.
Listening: STP has determined that the port can participate in frame forwarding according to the BPDU frames that the switch has received thus far. At this point, the switch port is not only receiving BPDU frames, it is also transmitting its own BPDU frames and informing adjacent switches that the switch port is preparing to participate in the active topology.
Learning: The port prepares to participate in frame forwarding and begins to populate the MAC address table.
Forwarding: The port is considered part of the active topology and forwards frames and also sends and receives BPDU frames.
Disabled: The Layer 2 port does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.

STP Port States

BPDU Timers

BPDU Roles and Timers

Configure network diameter

PortFast

PortFast is a Cisco technology. When a switch port configured with PortFast is configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states.

Cisco Portfast technology

Configure portfast

Verify portfast

STP Convergence

Step 1. Electing A Root Bridge

Electing a root bridge

Step 2. Elect Root Ports

Elect a root port step 1

Elect a root port step 2

Elect a root port step 3

Elect a root port step 4

Step 3. Electing Desgnated Ports and Non-Designated Ports

Elect port step 1

Elect port step 2

Elect port step 3

Elect port step 4

Elect port step 5

Elect port step 6

Elect port step 7

STP Topology Change

Topology change notification(TCN) messages are flooded out the root port, until it reaches the root bridge.

STP Broadcast notification

The Root Bridge broadcasts Topology change messages

PVST+, RSTP and Rapid-PVST+

Cisco and STP Variants

STP Variants

PVST+

PVST+ Bridge ID

PVST+ Bridge ID example

Default PVST+ configuration

Configure PVST+

Configure PVST+ Priority

Verify PVST+

RSTP

Characteristics of RSTP

RSTP BPDU

Edge Ports

Link Types

Point-to-point Link Type - Links attaches to switch ports that are operating in full-duplex mode
Shared Link Type - This link is attached to a port that is operating in half-duplex mode