SPANNING TREE PROTOCOL (STP) VARIANTSRapid Spanning Tree Protocol (RSTP) -The reason behind the word «rapid»

Multiple Spanning Tree Protocol (MSTP)

Introduction• Spanning Tree Protocol (STP) developed in the late 80s

• Later standardized by IEEE (IEEE-802.1D, 1990)

• Switches and Bridges do not age-out packets• Loops in the network -> frames may live forever -> congestion

• STP prevents loops allowing redundant connections• But STP is too slow

• After a failure -> recovery time 30-50 seconds

• Rapid Spanning Tree Protocol is an improved and faster version

• Preserves the basic concepts of STP• Also standardized (IEEE-802.1W)• In IEEE-802.1D from 2004 STP has been suppressed

Tree Topology• Spanning Tree can be thought of a tree:

• Root -> Root Bridge• Branches -> LANs and Designated Switches• Leaves -> End nodes

• No disconnected parts

• No loops

• Only one path from leaf to leaf

Root and Designated Bridges• Both STP and RSTP use Root and Designated Bridges

• Root bridge -> from which all branches spring• There is only one• Any switch could be the Root (Bridge ID)

• Designated bridge -> traffic from the Root to any link• Only one Designated bridge per link• No loops

• The Root bridge is the Designated bridge for all links connected to it

Port Roles – STP (I)• Three types of ports in STP

• Root port: closest to the Root bridge (path cost)

• Designated port: connectivity in the direction away from the Root

• Sends the best Bridge Protocol Data Unit (BPDU) on the segment it is connected

• Blocking port: disables redundant links• Do not forward data• Prevents loops

Port Roles – STP (II)

Port Roles – RSTP (I)• Maintains Root and Designated ports• Splits Blocking port into two (do not forward data):• Alternate port

• Provides redundant connection to the Root bridge• May become a new Root port

• Backup port• Connected to the same LAN segment as a Designated port• Or two ports are connected together in a loopback

• Edge ports• Connected directly to end stations -> cannot create loops• Do not follow regular states

Port Roles – RSTP (I)

Port States – STP (I)• 5 states• Disabled: not receiving or transmitting any data• Blocking: enabled and listen for BPDU messages• Listening: not forwarding data, but listening and sending

BPDU messages• Learning: preparing to forward data -> building up

forwarding table• Forwarding: forwards data

• Duration of listening and learning states is 15 seconds by default (forwarding delay timer)

Port States – STP (II)

Port States – RSTP• RSTP has only 3 port states• Forwarding: forwards data and learns MAC addresses• Learning: does not forward data, but learns MACs• Discarding: does not forward data and does not learn

MACs

BPDUs• Bridge Protocol Data Units (BPDUs) to learn and

exchange information• STP uses two BPDUs

• Configuration BPDUs: from Root every hello time (typically 2 seconds)• Other bridges forward on Designated ports

• Topology Change (TCN) BPDUs: from the bridge that detected a change to the Root• Root answers setting a Topology Change (TC) flag• A bridge receiving a BPDU with a TC flag -> switches aging time to short

• RSTP uses one BPDU• All the bridges• Includes TC flag, role and state of the port and flags for handshake

Filtering Database Aging• Database of MAC-to-port entries• STP

• Bridge detecting a topology change do not flush its filtering database• Send a TCN BPDU to Root• The Root responds with the TC flag activated• Bridges wait the aging timer before removing entries from database

• RSTP• Switches detecting a topology change send a BPDU with TC flag• Purges old entries• Every switch receiving the BPDU purges old entries

«Keep-alive» BPDUs• STP bridges do not generate BPDUs (unless failures)

• Receive them on Root port and forward them on Designated ports• If no BPDU is received in a “max age time” (default 20 seconds) the

Root is declared dead• The bridge assumes to be the Root and starts from the beginning

• RSTP bridges send BPDUs every “hello time”• If no BPDU is received in three “hello times” -> connection is lost• Immediately assumes it is the new Root or• Alternate ports can move to Forwarding state without delay

RSTP Behavior• RSTP does not relies on timers:

• Monitors MAC operational states and retires ports• Processes inferior BPDUs (STP discards them)• If a Root port fails, an Alternate port can be put into

operation without delay• If bridges are connected via point-to-point links,

handshake is used to transition a Designated port to Forwarding state

Example

Example (II) – STP Case• 222 and 444 wait max age timer (default 20 seconds)

before deciding connection to the Root is broken• 444 ages out information -> path to Root through port 02

-> advertises to 222 through port 01• 444’s port 02 is new Root port -> port 01 is Designated

port• Both ports must move through listening and learning

states -> other switches agree -> 30 seconds (15 each)• 222 makes port 03 a new root port -> transition through

listening and learning• Total time: 20 + 15 + 15 = 50 seconds

Example (III) –RSTP Case• 222 loses connection to Root -> decides it is the new Root• 444 recognizes BPDUs from 222 as inferior -> connection

to Root through 222 is broken• 444’s Alternate port 02 is immediately placed in

Forwarding state• 444’s port 01 is set as Designated port -> advertises new

path to the Root to 222• 222 accepts and makes port 03 Root port• 444 performs a handshake (“sync operation) with 222 to

transition port 01 to Forwarding state• No timers

Multiple Spanning Tree Protocol (I)• MSTP is based on RSTP and aims at

• A more balanced load across the network• Failures only affect a region of the network

• The network is divided in regions (MST regions):• Internal Spanning Tree (IST)

• Spanning Tree within a region• Can communicate with other regions

• Multiple Spanning Tree Instance (MSTIn)• Spanning Trees within a region• Cannot communicate with other regions

• Multiple VLANs could be mapped to a Spanning Tree Instance

Multiple Spanning Tree Protocol (II)• MST regions are interconnected using a Common

Spanning Tree (CST)• Using one Regional Root Bridge

• The Common Internal Spanning Tree is comprised of:• The CST connecting all regions• The IST providing connectivity inside each region

• MST regions are seen as “big bridges” (pseudobridge or superbridge) by CST

• Allows separated management of the regions• No change in internal topologies is influenced or produced by outside

region changes

Multiple Spanning Tree Protocol (III)

References• W. Wojdak, “Rapid Spanning Tree Protocol: A New Solution

from an old Technology”, CompactPCI Systems Magazine, Telecom Special Feature, March 2003

• G. Prytz, “Redundancy in Industrial Ethernet Networks”, IEEE International Workshop on Factory Communication Systems, 2006

• Cisco White Paper, “Understanding Spanning-Tree Protocol, Cisco Systems Inc., 1997

• Cisco White Paper, “Understanding Rapid Spanning Tree Protocol”, Cisco Systems Inc., 2006

• G. Ibanez, A. Garcia, A. Azcorra, “Alternative Multiple Spanning Tree Protocol (AMSTP) for Optical Ethernet Backones”, Proc. of LCN’04, November 2004