Advanced STP – Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Rapid Spanning Tree Protocol Overview

• RSTP significantly reduces the time taken for STP to converge when a link failure occurs
• With RSTP, failover to an alternate path or link can occur in the sub second timeframe
• RSTP is an extension of 802.1D
• RSTP performs better that the 802.1D with no additional configuration

The Modified RSTP BPDU

• The BPDU format used by RSTP is similar to that of the 802.1D BPDU
• In the RSTP BPDU, the Protocol Version ID and the BPDU Type fields are now set to 2
• Only 2 Flags are defined in the IEEE 802.1D standard
• RSTP uses all 6 bits of the Flag byte that remain to encode the role and state of the port

RSTP BPDU Handling

• RSTP BPDUs are sent every Hello Time (2 seconds) and are not simply relayed anymore
• Switches send BPDUs even if they do not receive any BPDUs from the Root Bridge
• Port information is now invalidated in 3 times the Hello Time interval (which is 6 seconds)
• In RSTP the Message Age information is simply used as a hop count

RSTP Port States

• RSTP defines only three distinct port states for a port under STP control
• These port states and their characteristics are listed in the following table:
  

  802.1D State	802.1w State	Default Port Operational Status	Port in Active Topology?	Port Learning MAC Addresses?
  Disabled	Discarding	Enabled	No	No
  Blocking	Discarding	Enabled	No	No
  Listening	Discarding	Enabled	Yes	No
  Learning	Learning	Enabled	Yes	Yes
  Forwarding	Forwarding	Enabled	Yes	Yes

• 802.1D Disabled, Blocking, and Listening states are merged into an 802.1w Discarding state
• RSTP also excludes the Listening state

RSTP Port Roles

• Spanning Tree Protocol operates internally with port roles
• Port roles are determined by received BPDUs and on the ports those BPDUs are received
• Ports change their Spanning Tree Protocol port states based on their assigned roles
• RSTP defines a set of port roles for Spanning Tree ports, which includes the following roles:
  1. Root Port (Forwarding state)
  2. Designated Port (Forwarding state)
  3. Alternate Port (Blocking state)
  4. Backup Port (Blocking state)
• The Root Port is defined as the port that receives the best BPDU from the Root Bridge
• A Designated Port is an active forwarding port that points away from the Root Bridge
• An Alternate Port is a non-forwarding (blocking) port that backs up a Root Port
• A Backup Port is a non-forwarding (blocking) port that backs up the Designated Port

RSTP Rapid Transition

• Rapid transition places ports into a  Forwarding state without having to rely on timers
• Rapid transition is based on edge ports and the link type
• An edge port is simply a port at the edge of the Spanning Tree network
• By default, a port that operates in full-duplex mode is considered a point-to-point link
• A port that operates in half-duplex mode is considered a shared port

RSTP Synchronization

• RSTP uses an explicit handshake mechanism between switches RSTP switches
• RSTP switches exchange BPDUs with the proposal and agreement flags
• If a switch receives no response to its proposal, it falls back to 802.1D
• The receiving switch must agree to the proposal before the port begins Forwarding

RSTP Integrated Enhancements

• RSTP includes several integrated 802.1D enhancements:
  1. Operation similar to Uplink Fast
  2. Operation similar to Backbone Fast

RSTP Topology Changes

• In 802.1D, when a port moves to Forwarding or Blocking, a TCN BPDU is originated
• The TCN BPDU is propagated to the Root, which sends it to all other switches
• In RSTP, a TC is initiated only when a non-edge port transitions to the Forwarding state
• RSTP does not use TCN or TCA BPDUs, unless a legacy bridge needs to be notified

802.1D and 802.1w Interoperability

• By default, 802.11D switches drop 802.1w BPDUs
• RTSP begins a Migration Delay timer after sending out a BPDU
• The Migration Delay timer runs for 3 seconds
• The switch accepts and processes all BPDUs received during the Migration Timer
• The switch uses the format of the last BPDU received after the timer expires

RSTP with PVST+

• Rapid Per VLAN Spanning Tree allows for the use of 802.1w with PVST+
• By default, when RSTP is enabled, R-PVST+ is enabled on the switch

Understanding Multiple Spanning Tree Protocol

• 802.1D supports two flavors of Spanning Tree:
  1. Common Spanning Tree (802.1Q)
  2. Per VLAN Spanning Tree
• CST uses a single Spanning Tree instance for all VLANs configured on the switch
• PVST allows for a single Spanning Tree instance for each individual VLAN
• MST provides the ability to group multiple VLANs into a single Spanning Tree instance

MST BPDU Format

• MST uses RSTP for rapid convergence
• Each switch must be capable of processing RSTP BPDUs in order to support MST
• The MST BPDU is similar to the RSTP BPDU, with two differences:
  1. The Protocol Version Identifier Field is 0x03
  2. The MST BPDUs contain an MST extension field

Understanding MST Regions

• An MST Region defines a boundary in which a single instance of Spanning Tree operates
• A Region is a collection of interconnected switches that have the same MST configuration
• Switches in an MST Region must have the following values set the same:
  1. The MST Region Name (up to 32 bytes)
  2. Config Revision Number (0 to 65,535)
  3. VLAN-To-Instance Mapping (up to 4096 entries)
• An MST Region consists of a two different components:
  1. Edge ports
  2. Boundary ports
• An edge port is simply a port that connects to a non bridging device
• An MST boundary port connects an MST region to:
  1. A single Spanning Tree region running Rapid Spanning Tree (802.1w)
  2. A single Spanning Tree region running traditional Spanning Tree (802.1D)
  3. Another MST Region with a different MST configuration
  4. The Designated Switch that belongs to a single Spanning Tree Instance
  5. The Designated Switch that belongs to another MST instance
• All other ports are simply referred to as internal MST ports

MST Spanning Tree Instances

• MST establishes and maintains two types of Spanning Trees instances:
  1. Internal Spanning Tree (IST) Instances
  2. Multiple Spanning Tree (MST) Instance
• The MST Region sees the outside world via its IST interaction only
• IST presents the entire MST Region as a single virtual bridge to the outside world
• BPDUs are exchanged at the Region boundary over the native VLAN of trunks
• The IST is the only Instance that sends and receives BPDUs
• All other Spanning Tree Instance information is contained in M-records
• When the IST converges, the Root of the IST becomes the IST master
• The switch with the lowest BID and path cost to the CST Root is elected IST master
• If there is only a single Region, then the IST master will also be the CST Root
• If the CST Root is outside of the Region, a boundary switch is selected as the IST master
• IST and MST Instances do not use the Message Age and Maximum Age information
• IST and MST Instances use the Path Cost to the Root and a hop-count mechanism
• The Root Bridge of the Instance sends a BPDU (or M-record) with a cost of 0
•  The Root Bridge of the Instance sends a BPDU (or M-record) with a hop count of 20
• MST Instances (MSTIs) are STP instances that exist only within the region
• MSTIs convey the Spanning Tree information for each instance within the Region
• Cisco Catalyst switches support up to 16 MSTIs on a single switch
• MST Instance 0 is mandatory and is always present; all other Instances are optional
• By default, all VLANs are assigned to the IST, which is MST Instance 0
• The IST Instance exists on all ports within the MST Region
• Only the IST Instance has timer-related parameters
• MST BPDUs are sent out of every port by all switches