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Rapid Spanning Tree ProtocolPer VLAN Spanning Tree

Multiple Spanning Tree Protocol

RSTP and MSTP

Marvin Rambhadjan

UvA

February 25, 2010

Marvin Rambhadjan RSTP and MSTP



Overview

Rapid Spanning Tree Protocol

Per VLAN Spanning Tree





Rapid Spanning Tree Protocol

IEEE 802.1w 1998

Underlying protocol MSTP

I Similarities with STPI Calculate the same spanning tree topology with the same

configuration

I Differences with STPI Faster conversion timesI Different port states and new port rolesI Different definition topology changeI BPDUs used for Hello mechamism




RSTP Port States and Roles

I Port States

Forwarding MAC LearningForwarding traffic

Learning MAC LearningNo forwarding traffic

Disabled No MAC learningNo forwarding traffic

I Port Roles

Root Port Port to higher segments (to the Root Bridge)Designated Port Port to lower segmentsAlternate Port Backup Root PortBackup Port Backup Designated PortDisabled Port Not participating in RSTP




Port Roles




Definition Topology Change

I STPI A Forwarding or Listening state port that transits to Blocking (lost

connection)I A port that transits in Forwarding state, while the switch already has

a Designated port (new connection)

I RSTPI Non-Edge port that transits in Forwarding (new connection)




Rapid transit to Forwarding

I Edge PortsI End stationsI Not part of spanning tree process

I Point to PointI Connection between switchesI Proposal and Agreement mechanism




Proposal and Agreement Mechanism

If a designated port wants to transits into a non-discarding state, theupstream switch will do the following in order to agree with thedownstream switch.

1. Upstream Bridge sends proposal on Designated Port

2. Downstream Bridge receives proposalI If it agrees on port role, it starts synchronization.I If it not agree on the proposal, it will send is own proposal

3. Downstream Bridge blocks all non-edge Designated Ports and sendAgreement to upstream bridge

4. Synchronization is needed to make sure there are no loops in thetopology when when the upstream bridge unblocks its DesignatedPort




Proposal and Agreement

www.cisco.com


www.cisco.com



Proposal Agreement

www.cisco.com


www.cisco.com



Topology Changes

TC DetectionI Start TC While timer (2x Hello-Time) on all Non-Edge designated

and root portsI Flush all MAC addresses on these portsI Sends BPDUs with TC bit set, while TC While timer has not expired

TC PropagationI Flush all MAC addresses on all ports except receiving portI Start TC While timer and sends BPDUs with TC bit set on all

Designated and Root portsI TCN not longer used





I Many different variants

I Proprietary

I For both STP and RSTP

I Load Balancing

I Create independent spanning tree instances




Single Spanning Tree

I Not all connections are used





I 10 Root Bridges

I 10 BPDU’s

I 10 Spanning Tree calculations





IEEE 802.1s 2003

Built on RSTP

I MSTP vs (R)STPI Use all links

I MSTP vs PVSTI Minimize overhead

I Introduction Regions

I Two-Level Hierarchy

I Multiple independent Spanning Tree Instances




MSTP Terminology

I CIST Common Internal Spanning TreeI Topology of all switches in all regions

I CST Common Spanning TreeI Topology of all “pseudo bridges”

I IST (MSTI0) Internal Spanning TreeI Spanning Tree within a regionI Can communicate with other regionsI Root Bridge election first based on External Path Cost

I MSTIn Multiple Spanning Tree Instance nI Spanning Tree within a regionI Cannot communicate with other regions




MSTP parameters

I MSTP Region parametersI VLAN-to-instance mappingI Configuration NameI Configuration Revision

I Root Priority per MSTI

I Path cost per MSTI

I Port Priority per MSTI




Example IST




Example MSTIs




MSTP Multi Region

I Every region becomes a “pseudo-bridge”

I Topology of “pseudo-bridges” is called CST

I CST has no idea of internal MSTP regions

Hierarchy

I First LayerI Common Internal Spanning Tree (CIST)I Collection of Internal Spanning Tree (IST)

I Second LayerI Internal Spanning Tree (IST)I Multiple Spanning Tree Instance(s) (MSTI)




Example MSTP Multi Region Topology




Example MSTP Multi Region Root Ports




Example MSTP Multi Region Designated Ports




Example MSTP Multi Region CIST




Example MSTP “Pseudo-Bridges CST




Example MSTP Within a Region




Bridge Protocol Data Unit

Octet BPDU Content

1 - 2 Protocol Identifier Spanning Tree Protocol 0x0000

3 Protocol Version Identifier Multiple Spanning Tree 3

4 BPDU Type Rapid/Multiple Spanning Tree (0x02)

5 CIST Flags

6 - 13 CIST Root Identifier

14 - 17 CIST External Path Cost

18 - 25 CIST Regional Root Iden-tifier

26 - 27 CIST Port Identifier

28 - 29 Message Age

30 - 31 Max Age

32 - 33 Hello Time

34 - 35 Forward Delay

36 Version 1 Length 0

37 - 38 Version 3 Length Number of octets in MSTP extension64 octets MSTP Extension & 16 octetsper MSTI

39 - 102 MSTP ExtensionMarvin Rambhadjan RSTP and MSTP



Bridge Protocol Data Unit

Octet MSTP Extension Content

39 MST Config ID Format Selector 0

40 -71 MST Config Name

72 - 73 MST Revision

74 - 89 MST Config Digest

90 - 93 CIST Internal Path Cost

94 - 101 CIST Bridge Identifier

102 CIST Remaining Hops

MSTIDn MSTI Instances

Octet MSTID instance number Content

1 MSTI Flags

2 - 9 MSTI Regional Root Identifier

10 - 13 MSTI Internal Root Path Cost

14 MSTI Bridge Priority

15 MSTI Port Priority

16 MSTI Remaining Hops




Flags

Bits Description1 Topology Change Acknowledgement1 Agreement1 Forwarding1 Learning2 Port Role1 Proposal1 Topology Change




Questions?


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