Spanning tree Protocol (STP) Variants

1. Spanning tree Protocol (STP) Variants Rapid Spanning Tree Protocol (RSTP) -The reason behind the word «rapid» Multiple Spanning Tree Protocol (MSTP)

2. 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

3. 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

4. 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

5. 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

6. Port Roles – STP (II)

7. 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

8. Port Roles – RSTP (I)

9. 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)

10. Port States – STP (II)

11. 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

12. 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

13. 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

14. «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

15. 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

16. Example

17. 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

18. 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

19. 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

20. 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

21. Multiple Spanning Tree Protocol (III)

22. 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