1. BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS Budapest University of Technology and Economics Verification of RSTP Convergence and Scalability by Measurements and Simulations István Moldován, Saad Abuguba, Csaba Lukovszki 11-14 December 2006

2. WPC1 — 2 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Background  Metro Ethernet, Ethernet Access, Ethernet aggregation  All using Spanning Tree Protocol  In the LAN  No special requirements, just simplicity  In the provider’s network  Carrier grade requirements!  Carrier Grade Questions:  How fast is the restoration?  Does it scale?

3. WPC1 — 3 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics STP – Why needed?  Ethernet does not have TTL field  Loop protection is required  Spanning Tree Protocol reduces the active topology to a tree  Alternatives  Disable STP – not needed on tree topologies  VLAN based trees  VLAN topologies configured as p2p or p2mp connections  management is responsible for loop control  a configuration error becomes FATAL

4. WPC1 — 4 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Spanning Tree standards, problems  Basic STP – IEEE 802.1D – 1998  timer based operation  slow (~1minute restoration)  does not scale  Rapid STP– IEEE 802.1w - 2001 – IEEE 802.1D - 2004  much faster  but still no upper bound on convergence  several problems revealed (like count to infinity)  Scalability?  Multiple STP – IEEE 802.1s - 2002  multiple regions  multiple trees within regions  scalable

5. WPC1 — 5 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Loop-free Connectivity Spanning Tree Basics  One root bridge per network  One root port per non-root bridge  One designated port per segment  Non-designated ports are blocked Root Port (Fwd): Port receiving the best BPDU for the bridge – shortest path to the Root in terms of path cost Designated Port (Fwd): Port sending the best BPDU on a segment Alternate Port (Disc): Port blocked by BPDUs from a different bridge – redundant path to the Root Backup Port (Disc): Port blocked by BPDUs sent from the same bridge – redundant path to a segment A B D R Root AB RR DD D AB

6. WPC1 — 6 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Bridge Protocol Data Unit (BPDU)

7. WPC1 — 7 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Proposal Block Proposal Agreement Forward Edge port Proposal Agreement Forward IEEE 802.1w sequence of events  Receive a proposal  Block all other non-edge ports  Send an agreement back  Put the new root port to forwarding  Send out proposals on other ports  Receive agreement from others  Put ports into forwarding

8. WPC1 — 8 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Simulations on convergence  We used OPNET Modeler 10.5 simulation tool  RSTP supported by default  RSTP standard compliance verified (bug removed)  Simulations on different resilient topologies  Dual Homing topologies  deep: multiple levels  wide: high aggregation  Ring topologies  Mixed ring and dual homing  Simulation objective: restoration time after failure

9. WPC1 — 9 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Dual-homing scenarios  Dual homing connections for resilience  Simulations with increasing number of layers  Traffic between node_5 and node_8  primary path fails  we measure restoration time Recovery happens almost instantaneously by accepting a proposal sent earlier on the other link We have found similar results for all investigated dual-homing topologies -no matter of the width, depth and failure location, since there is always an alternate port to the root

10. WPC1 — 10 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics RSTP restoration on ring topology  Different ring sizes  from 6-14  Different bridge BPDU processing times  10-5000 BPDU/s  Recovery time observed  From port state changes Basically limited by the bridge BPDU processing time Measured BPDU processing time on real bridge: - 0.025s, std. deviation of 0.001s

11. WPC1 — 11 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Hold Timer  TxHoldCount  At least one BPDU per HelloTime interval, and not more than (TxHoldCount + 1) BPDUs in one second  By Standard selectable  Between 1-10  The value of 1 introduces delays  Hello BPDU + other BPDU can not go in the same second

12. WPC1 — 12 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Count to infinity problem  Documented behavior  On failure of root long failover times  Several seconds!  Reason:  Old information persists and circulates  Until  message ages out or  Root path costs reaches maximum value Root

13. WPC1 — 13 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics RSTP scalability issues  Big ring  BPDU information ages  ports where BPDU ages out becomes forwarding (standard says so)  Two spanning trees formed!  2 roots LOOP CONDITION!

14. WPC1 — 14 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Maximum Bridge Diameter  IEEE 802.1D – 2004 does not contain this statement  Diameter limit is given by the value of Maximum Age value  20 by default, up to 40  Thus theoretically topologies with diameter up to 40 hops can be created  Diameter  distance from root!  Diameter = the maximum length path in the network  Important since in case of failure longer paths may be formed  2 trees, possible loops

15. WPC1 — 15 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Conclusions  RSTP convergence is limited by BPDU processing speed  Fast convergence in typical topologies (below 1 second)  No upper limit on convergence  Depends on topology and bridge BPDU processing speed  Vulnerable to count to infinity problem  Limited scalability  By default up to diameters of 20  Can be tuned up to 40 (theoretical limit)

16. WPC1 — 16 Broadband Europe, Geneva 2006 Budapest University of Technology and Economics Thank you for your attention! Questions?