SPANNING TREE PROTOCOL (STP) VARIANTS Rapid 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 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 ’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: = 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 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