Study Guide

Evolution of STP

• STP (Spanning Tree Protocol) – Eliminates loops at layer 2 topology

STP

• STP initially converges on a logically loop-free network topology.
• Elects one root bridge
• Selects the root port on all nonroot bridges
• Selects the designated port on each segment

STP Steps

• Elects One Root Bridge
• The protocol uses a process to elect a root bridge
• Only one bridge acts as the root bridge in a given network per VLAN
• On the root bridge, all ports act as designated ports
• Switch with lowest priority parameter is elected as root

STP Step 2

• Selects the Root Port on All Non-root Bridges
• The protocol establishes one root port on each nonroot bridge.
• The root port is the lowest-cost path from the nonroot bridge to the root bridge

Step 3

• Selects Designated Port on Each Segment
• On each segment, STP establishes one designated port on the bridge that has the lowest path cost to the root bridge
• The switch primarily chooses a designated port as the least-cost path to the root bridge
• In the event of a tie, the bridge ID acts as the tiebreaker

Port Roles

• The are four roles of ports under STP
• Root Port
• Designated Port
• Disabled Port
• Nondesignated Port

The Five Port States

• Blocking
• Listening
• Learning
• Forwarding
• Disabled

A variation of Spanning Tree Protocol (STP) designed to ensures a topology that doesn’t have loops.

IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w

• Detection of root switch failure is done in 3 hello times
• STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times *(default: 3 times 2 seconds) or within a few milliseconds of a physical link failure

• RSTP maintains backup details regarding the discarding status of ports.

RSTP bridge port roles:

• Root - A forwarding port that is the best port from Nonroot-bridge to Rootbridge
• Designated - A forwarding port for every LAN segment
• Alternate - An alternate path to the root bridge. This path is different than using the root port.
• Backup - A backup/redundant path to a segment where another bridge port already connects.
• Disabled - Not strictly part of STP, a network administrator can manually disable a port

RSTP Topology Change Mechanism

When an RSTP bridge detects a topology change it performs the following actions.

• The RSTP Bridge starts the TC while time with a value equal to twice the hello time for all its nonedge designated ports and its root, port if necessary. The TC While time is the interval during which the RSTP Bridge actively informs the rest of the bridges in the network of a topology. It then flushing the MAC addresses associated with all nonedge ports. And as long as the TC While timer is running the BPDUs will be sent out of the port will have the TC bit set.

Bridge Identifier for PVRST+

• Bridge Priority A 4-bit field still used to carry bridge priority
• Extended System ID
• Mac Address

• Only four high-order bits of the 16 bit bridge priority field carry actual priority
• Cisco STP Default Config
• PVST+
• PVRST+
• MST

• PortFast
• Spanning Tree Portfast causes an interface configured as a layer 2 access port to enter the forwarding state immediately.
• Conf t
• int fa 0/1
• spanning-tree portfast

• confirm with show spanning-tree interface Fastethernet 0/1.

• Config the Basic Parameters of PVRST+
• conf t
• spanning-tree mode rapid-pvst
• spanning-tree vlan 2 root primary
• spanning tree- vlan 3 root secondary

Multiple Spanning Tree

MST extends the IEEE 802.1w RST algorithm to multiple spanning trees. The main purpose of MST is to reduce the total number of spanning-tree instances.