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FRNT / RSTP

Headline. FRNT / RSTP. STP/RSTP. History Spanning Tree Protocol (STP) Invented by Radia Perlman in 1985 (Digital) standardized in IEEE Standard 802.1D (1990) STP is a link layer protocol designed to ensure loop-free network topology in a bridged mesh network. STP/RSTP.

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FRNT / RSTP

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  1. Headline FRNT / RSTP

  2. STP/RSTP • History • Spanning Tree Protocol (STP) • Invented by Radia Perlman in 1985 (Digital) standardized in IEEE Standard 802.1D (1990) • STP is a link layer protocol designed to ensure loop-free network topology in a bridged mesh network

  3. STP/RSTP • Rapid Spanning Tree Protocol (RSTP) • IEEE Standard 802.1w (1998) (RSTP extension) • IEEE Standard 802.1D (2004) obsoletes STP • RSTP can perform a topology convergence in a few seconds compared to STP 30-50s

  4. FRNT • Method and apparatus for Fast Re-configuration of a Network Topology (FRNT) • Invented by Øyvind Holmeide and Lennart Liljeström • Patent filed 2004 and granted 2008 • FRNT is a link layer protocol designed to ensure loop-free network topology in a bridged ring network • Designed to be very fast.

  5. FRNT • Based on link events instead of BPDU packets • Uses direct communication to focal point instead of hop based communication • Robust break before make principles • Converge a network topology within ~20ms

  6. Network redundancy • Redundant/alternative paths • Redundant networking is all about managing the alternative paths • Avoiding loops that causing storms • Converge fast

  7. Network redundancy Ring Topology Mesh Topology

  8. Network redundancy • Supported topologies differ between protocols from ring to mesh • Convergence time differ from tenth of seconds to milliseconds. • Mechanisms to detect non functional paths differ between protocols

  9. Network redundancy • Recovery time depends on: • How fast a non functional link can be detected • Time it takes to forward link state information to link manager / root / focal point • Time it takes to calculate proper action

  10. Network redundancy • Recovery time depends on: • Time it takes to forward command from link manager to bridge managing redundant link. • Direct or one hop at a time • Time it takes to put redundant path in forwarding • Clearing MAC tables

  11. RSTP protocol principles • RSTP is a plug and play protocol • When the network is established the bridge does not have any information on the topology • The bridges need to compute a spanning tree • The bridges needs to elect a root bridge • Determine the least cost path to the root bridge • Disable all other root paths (and ties) • BPDUs are used in the process above

  12. Bridge Protocol Data Units (BPDUs) • There are three types of BPDUs: • Configuration BPDU used for Spanning Tree computation • Topology Change Notification (TCN) BPDU, used to announce changes in the network topology • Topology Change Notification Acknowledgment (TCA) • Default BPDUs are exchanged each 2 seconds • Note: BPDUs can traverse blocked port on a bridge

  13. Root election • Root bridge of the spanning tree will be the bridge with the lowest bridge ID • Bridge ID is made up from bridge priority and bridge unique identifier • Bridge priority is a configurable parameter set by user • Unique identifier is for all practical cases the bridge MAC address. • Bridge priority is compared first then the MAC address

  14. Root election • Bridge priority • Range is 0-61440 in steps of 4096 • Recommended default 32768 • Port priority • Range is 0-240 in steps of 16 • Recommended default 128

  15. Determine least cost path to root • Path cost parameter • Sum of all path costs in the direction of root • Lowest path cost is selected

  16. Determine least cost path to root • Each bridge in the topology except root bridge calculate least cost path to root bridge. • Then the bridges disables the other paths to root and let the designated port go to forwarding state. • Note that root can go to forwarding on all ports immediately when it is elected as root

  17. RSTP improvements over STP • Detection of root bridge within one Hello time of 2 seconds (default). • Range 1-2 according to standard (1-10 possible) • Admin edge ports • Goes to forwarding state immediately but still listen to BPDUs • Handshake protocol instead of multiple port states • TCN/TCA instead of listening/learning port states

  18. FRNT V0 • Ring topology • Topology already known • Focal point • Managing a virtual break • Means holding one port in blocking until a real link failure is detected. • When focal point is informed of a link failure by a link down event message it can instantly go to forwarding and send a so called clearing message.

  19. FRNT V0 • Link up procedure • FLHP integrity check of link • Link up message to focal point • Focal point goes to blocking on one port • ACK message to members to go to forwarding • MAC table clearing message to all nodes

  20. FRNT V0 • FLHP (FRNT Link Health Protocol) • Check link integrity • Communication in both ways (ref. fiber) • Can also be used with Link aggregation and RSTP in some cases • Optimized for multicast data managed with IGMP (internet group management protocol) protocol • Video virtually not affected compared to RSTP • MC filters reapplied.

  21. Fast Robust IGMP optimized Ring topology Large topology Proprietary Slow but much faster than STP Fiber applications need FLHP to be robust Mesh topology Limited topology Industry standard FRNT V0 vs. RSTP • FRNT • RSTP

  22. RSTP and FRNT • RSTP and FRNT can coexist at the same time on WeOS based products • Ring towards backbone • Access ports supporting RSTP • FRNT and RSTP ports needs to be configured • Loops crossing between FRNT and RSTP administered domains can create storms

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