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 December 2006

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?

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

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 – IEEE 802.1D  much faster  but still no upper bound on convergence  several problems revealed (like count to infinity)  Scalability?  Multiple STP – IEEE 802.1s  multiple regions  multiple trees within regions  scalable

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

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

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

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

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

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  BPDU/s  Recovery time observed  From port state changes Basically limited by the bridge BPDU processing time Measured BPDU processing time on real bridge: s, std. deviation of 0.001s

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

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

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!

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

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)

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