Spanning tree


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Spanning tree

  1. 1. SPANNING TREE PART 1 Spanning Tree Protocols Legacy STP PVST PVST+ RSTP RPVST+ MST Algorithm Legacy ST Legacy ST Legacy ST Rapid ST Rapid ST Rapid ST 802.1w, 802.1s, Defined By 802.1D-1998 Cisco Cisco Cisco 802.1D-2004 802.1Q-2003 Instances 1 Per VLAN Per VLAN 1 Per VLAN Configurable Trunking N/A ISL 802.1Q, ISL N/A 802.1Q, ISL 802.1Q, ISL Spanning Tree Instance Comparison STP PVST+ MST Root VLAN 1,10 Root VLAN 20,30 Root MSTI 0 Root MSTI 1 Root A B A B A B All VLANs VLAN 1 MSTI 0 (1, 10) x xx xx VLAN 10 x x MSTI 1 (20, 30) VLAN 20 C C VLAN 30 C BPDU Format Spanning Tree Specifications Link CostsField Bits Bandwidth Cost 802.1s 802.1Q-2003 802.1Q-2005Protocol ID 16 4 Mbps 250Version 8 10 Mbps 100BPDU Type 8 802.1D-1998 802.1D-2004 16 Mbps 62Flags 8 45 Mbps 39Root ID 64 802.1Q-1998 802.1w 100 Mbps 19Root Path Cost 32 155 Mbps 14 ISL PVST PVST+ RPVST+Bridge ID 64 622 Mbps 6Port ID 16 IEEE 802.1D-1998 Deprecated legacy STP standard 1 Gbps 4Message Age 16 IEEE 802.1w Introduced RSTP 10 Gbps 2Max Age 16 20+ Gbps 1 IEEE IEEE 802.1D-2004 Replaced legacy STP with RSTPHello Time 16 IEEE 802.1s Introduced MST Port StatesForward Delay 16 IEEE 802.1Q-2003 Added MST to 802.1Q Legacy ST Rapid ST Default Timers IEEE 802.1Q-2005 Most recent 802.1Q revision DisabledHello 2s PVST Per-VLAN implementation of legacy STP Blocking Discarding CiscoForward Delay 15s PVST+ Added 802.1Q trunking to PVST ListeningMax Age 20s RPVST+ Per-VLAN implementation of RSTP Learning Learning Forwarding Forwarding Spanning Tree Operation Determine root bridge Port Roles 1 The bridge advertising the lowest bridge ID becomes the root bridge Legacy ST Rapid ST Select root port Root Root 2 Each bridge selects its primary port facing the root Designated Designated Select designated ports 3 Alternate One designated port is selected per segment Blocking Block ports with loops Backup 4 All non-root and non-desginated ports are blockedby Jeremy Stretch v3.0
  2. 2. SPANNING TREE PART 2 PVST+ and RPVST+ Configuration Bridge ID Format 4 12 48spanning-tree mode {pvst | rapid-pvst} Pri Sys ID Ext MAC Address! Bridge priorityspanning-tree vlan 1-4094 priority 32768 Priority 4-bit bridge priority (configurable from 0 to 61440 in! Timers, in secondsspanning-tree vlan 1-4094 hello-time 2 increments of 4096)spanning-tree vlan 1-4094 forward-time 15 System ID Extensionspanning-tree vlan 1-4094 max-age 20 12-bit value taken from VLAN number (IEEE 802.1t) MAC Address! PVST+ Enhancementsspanning-tree backbonefast 48-bit unique identifierspanning-tree uplinkfast Path Selection! Interface attributes 1 Bridge with lowest root ID becomes the rootinterface FastEthernet0/1 spanning-tree [vlan 1-4094] port-priority 128 2 Prefer the neighbor with the lowest cost to root spanning-tree [vlan 1-4094] cost 19 3 Prefer the neighbor with the lowest bridge ID ! Manual link type specification 4 Prefer the lowest sender port ID spanning-tree link-type {point-to-point | shared} Optional PVST+ Ehancements ! Enables PortFast if running PVST+, or ! designates an edge port under RPVST+ PortFast spanning-tree portfast Enables immediate transition into the forwarding state (designates edge ports under MST) ! Spanning tree protection spanning-tree guard {loop | root | none} UplinkFast Enables switches to maintain backup paths to root ! Per-interface toggling BackboneFast spanning-tree bpduguard enable Enables immediate expiration of the Max Age timer in spanning-tree bpdufilter enable the event of an indirect link failure MST Configuration Spanning Tree Protection Root Guardspanning-tree mode mst Prevents a port from becoming the root port! MST Configuration BPDU Guardspanning-tree mst configuration Error-disables a port if a BPDU is received name MyTree revision 1 Loop Guard Prevents a blocked port from transitioning to listening ! Map VLANs to instances after the Max Age timer has expired instance 1 vlan 20, 30 BPDU Filter instance 2 vlan 40, 50 Blocks BPDUs on an interface (disables STP)! Bridge priority (per instance) RSTP Link Typesspanning-tree mst 1 priority 32768 Point-to-Point! Timers, in seconds Connects to exactly one other bridge (full duplex)spanning-tree mst hello-time 2 Sharedspanning-tree mst forward-time 15 Potentially connects to multiple bridges (half duplex)spanning-tree mst max-age 20 Edge! Maximum hops for BPDUs Connects to a single host; designated by PortFastspanning-tree mst max-hops 20 Troubleshooting! Interface attributesinterface FastEthernet0/1 show spanning-tree [summary | detail | root] spanning-tree mst 1 port-priority 128 show spanning-tree [interface | vlan] spanning-tree mst 1 cost 19 show spanning-tree mst […]by Jeremy Stretch v3.0