CCIE R&S Written Overview: STP

STP

  • 802.1d (STP/CST)
  • 1 STP calculation for all VLANs in Layer 2 Network
  • PVST/PVST+
  • Cisco Propertiary Per VLAN STP
  • 802.1w (RSTP)
  • 802.1s (MST)

802.1d

  • Elect one Root Bridge
  • Elect one Root Port per bridge
  • Elect Designated Ports

Root Bridge Election

  • Lowest Bridge ID
  • Bridge Priority (Inc of 4096) 0-61440
  • System ID Extension (VLAN NUMBER) Added to Bridge ID
  • MAC Address
  • 32768 = Default Priority
  • Once RB is elected, BPDUs flow down from the root of the tree to the ‘leaves’.
  • On newer versions of STP, BPDUs are sent from all switches, not just the RB.

Root Port Election

  • RP is upstream facing towards RB
  • Elected based on lowesr Root Path Cost
  • Cost of all links to get to the root
  • Higher bandwidth = lower cost
  • If tie in cost, then break tie with lowest Bridge ID or lowest Port ID.

Designated Port Election

  • DPs are downstream ports facing away from the RB.
  • All ports on RB have to be DPs.
  • Lowest Root Path Cost
  • Lowest BID
  • Lowest Port Identifer
  • All other ports will BLK. Will still receive BPDUs and will not/can not send traffic.
  • No MACs will be associated with a BLK port.

802.1d convergence

  • CST convergence based on timers set on RB
  • Hello – How often BPDUs are sent – 2 seconds
  • Forward Delay – Each stages of LISTEN/LEARN – 15 seconds
  • Max Age – 20 seconds – Time period to wait when no BPDU received? (CHECK CHRIS BRYANT DOC)
  • TCN BPDUS used to notify the root bridge of changes
  • Flag is set in TCN BPDU
  • Flows up to the root, root replies with ACK
  • CAM aging time set to Forward Delay to flush out MAC addresses in table to relearn new topology.

PVST/PVST+

PVST

• Per VLAN STP

– One instance of Legacy STP per VLAN

– Cisco ISL support

PVST+

– One instance of Legacy STP per VLAN

– Cisco ISL and 802.1Q support

– Provides interoperability between CST and PVST

– Default mode on most Catalyst platforms

– Allows root bridge/port placement per VLAN

Cisco’s STP Enhancements

• PortFast

– Edge ports shouldn’t be subject to Forwarding Delay or generate TCNs

• UplinkFast

– Direct Root Port failure should re-converge immediately if Alternate Port available

• BackboneFast

– Indirect failures should start recalculating immediately. (Discard the Max Age time)

Other Cisco STP Features

• BPDU Filter

– Filter BPDUs in and out

• BPDU Guard

– If BPDU is received shut port down

• Root Guard

– If superior BPDU is received shut port down

• Loop Guard & UDLD

– Prevent unidirectional links

802.1w STP Rapid Spanning-Tree Protocol

– Rapid convergence based on sync process (Proposal process) http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#agree

– Allows for faster initial convergence

• Simplifies port states

– Discarding, Learning, & Forwarding

All bridges generate BPDUs

– Send out all every Hello interval

– If three Hellos are missed neighbor is declared down and CAM is immediately flushed

– Allows for faster re-convergence

• Runs backwards compatibility with 802.1D

802.1s STP Multiple Spanning-Tree Protocol

– User defined instances are separate from VLANs

– PVST+ uses one instance per VLAN

• Uses 802.1w for rapid convergence

• Highly scalable

– Switches with same instances, configuration revision number, and name form a “region”

– Different regions see each other as virtual bridges

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