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STP

STP - Foundation per-VLAN Spanning Tree Concepts

  • Switches forward broadcast packets out all ports by design
  • Redundant connections are necessary in business networks
  • The place of spanning tree: drop trees on redundant links (until they are needed)

The Facts About Spanning Tree

  • Original STP (802.1D) was created to prevent loops
  • Switches send "probes" into the network called Bridge Protocol Data Units (BPDUs) to discover loops
  • The BPDU probes also help elect the core switch of the network, called the Root Bridge
  • The simplistic view of STP: all switches find the best way to reach the root bridge than block all redundant links

Understanding BPDUs and Elections

  • BPDUs are sent once every two seconds
    • Priority and Mac address in the packet
  • Priority is some value between 0 and 61440 (default is 32768); increments of 4096 - lower is better
  • Three port types
    • Root Port: used to reach the root bridge
    • Designated Port: forwarding port, one per link
    • Blocking / Non-designated port: where the tree fell

How STP Finds the Best Path

  1. Elect the root
  2. Switches find lowest cost path to port
  3. Use lower bridge ID on equal paths
  4. Use lower port to break a tie
Link Bandwidth STP Cost
4Mbps 250
10Mbps 100
16Mbps 62
45Mbps 39
100Mbps 19
155Mbps 14
622Mbps 6
1Gbps 4
10Gbps 2

Initial Enhancements: Per VLAN Spanning Tree

  • One root bridge elected for each VLAN
  • Helps load-balance more effectively.

The Spanning-Tree Command

Switch(config)#spanning-tree vlan x root primary
Switch(config)#spanning-tree vlan x root secondary
Switch(config)#spanning-tree vlan x priority <number>

Problems with Spanning-Tree

  • Listening
    • 15 seconds of listening for BPDUs
    • Switch sends / receives BPDUs
  • Learning
    • 15 seconds of learning MAC addresses
    • Populates CAM table
  • Forwarding
    • Port is forwarding traffic (happy)
  • Blocking
    • BONUS - switch will wait up to 20 seconds (max-age) before moving a blocked port into a listening phase.

Rapid Spanning Tree Concepts and Configuration

Problems and Solutions

  • Problems with PCs: Modern PCs can boot faster than 30sec
  • Solution: Portfast (spanning-tree portfast)
  • Problems with Uplink Ports: 50 sec of down time causes big problems
  • Solution: Rapid Spanning-Tree
    • 802.1w
    • Proactive System
    • Redefined Port Roles
    • Many STP Similarities

Rapid STP Port States

  • Discarding
  • Learning
  • Forwarding

Rapid STP Port Roles

  • Root Port
  • Designated Port
  • Alternate Port
  • Edge Port

Why RSTP is Better

  1. Because it doesnt forget ports
  2. Because of the proactive nature, many "safety timers" of STP are eliminated
  3. Any change to trunk ports flood through the network to other switches (TC packets)
  4. Because the name says "rapid"

Configuring RSTP

spanning-tree mode rapid-pvst
int g0/0
spanning-tree portfast
show spanning-tree