Implement Spanning Tree Protocols : Implement Spanning Tree Protocols LAN Switching and Wireless – Chapter 5
Objectives : Objectives Explain the role of redundancy in a converged network
Summarize how STP works to eliminate Layer 2 loops in a converged network
Explain how the STP algorithm uses three steps to converge on a loop-free topology
Implement rapid per VLAN spanning tree (rapid PVST+) in a LAN to prevent loops between redundant switches.
Need for Redundancy : Need for Redundancy Existing business practices make network access critical
Hierarchal design addresses need at Distribution/Core
Disadvantage of Redundancy : Disadvantage of Redundancy Multiple paths in a switched network cause switching loops
Frame contains no Time-To-Live field
IP at layer 3 has TTL
Loop causesframes to cycle endlessly
Result is high CPUusage and consumedbandwidth
Problems Caused by Redundant Links : Problems Caused by Redundant Links Broadcast Storms
Broadcast frames are always forwarded at Layer 2
Switch AND all connected devices must process frame
Valid network traffic is suppressed
Duplicate Unicast Fames
Multiple switches forward frame to destination
Out-of-sequence frame possibly disrupts application layer
MAC Database Instability
Switch learns correct MAC
Looped MAC is re-learnedon wrong interface
How Switching Loops are Introduced : How Switching Loops are Introduced In the well-managed network, physically included by designed
Logically limited by STP
Loops in the wiring closet
Occurs when multiple connections are created between two switches
Usually occurs because of poor documentation or cable labels
Loops in the cubicle
Created by end users trying to “expand” network
Unauthorized switches or hubs are added to access ports
May include switch modules on wireless access points
Solution: Spanning Tree Protocol (STP) : Solution: Spanning Tree Protocol (STP) Developed in the 1970’s, logically controls physical links
Published in 1990 as IEEE 802.1d
All switches allowed one path through network
Spanning Tree Algorithm (STA) calculates which links stay up
Remaining links are blocked (no traffic forwarded)
Re-calculates link status if switchingtopology changes
STP Terminology : STP Terminology Root Bridge – center of the network
i.e., all traffic flow through this switch
Elected based on Bridge ID (more later)
Path Cost/Port Cost – values associated with outgoing ports
Root Ports – ports closest to the root (i.e., lowest cost)
Designated Ports – non-roots that still forward traffic
Non-designated Ports – blocked ports (non-forwarding)
STP Example Topology : STP Example Topology
STP Terminology - more : STP Terminology - more Bridge ID (BID)
Identifier composed of Bridge Priority and MAC address
Low BID’s are always the winner in Root or Port elections
Bridge Priority – default value is 32769
Value ranges from 0 to 65535 (note: VLAN number added)
Set using config#spanning-tree vlan 1 priority 0
or config#spanning-tree vlan 1 root primary/secondary
View with command Show Spanning-tree
Bridge Protocol Data Units (BPDU) : Bridge Protocol Data Units (BPDU) Switches exchange BPDU’s by default every 2 seconds
Used to elect root bridge and establish port roles
Port States and Timers : Port States and Timers Five states (or conditions) for ports in STP
Disabled – administratively shutdown
Portfast – Cisco spanning-tree variable that move an access port directly to forwarding
Steps to STP Convergence : Steps to STP Convergence
Step 1: Election of the Root Switch (Bridge) : Step 1: Election of the Root Switch (Bridge) Key issues:
Bridge ID and Switch Priorty
Step 2: Election Root Port on non-Root Switch : Step 2: Election Root Port on non-Root Switch Key issue – path cost
Step 3: Selection of Designated Ports : Step 3: Selection of Designated Ports Key issues – port cost and port ID’s
Note: only one designated port per segment
Other port on segment is non-designated (blocking)
STP Practice Election : STP Practice Election
Variations on Spanning Tree : Variations on Spanning Tree Summarize the features of the PVST+, RSTP and rapid PVST+ variants of STP
Rapid per VLAN Spanning Tree : Rapid per VLAN Spanning Tree
Edge Ports and Link Types : Edge Ports and Link Types Edge ports are access ports connected to devices (same as PortFast concept)
Non-Edge Port Link Types are:
Point-to-Point
Shared
RSTP Port Roles : RSTP Port Roles Root and Designated port roles are the same as STP
New roles added are:
Alternate – same as non-designated
Backup – “redundant” link to the same network segment
RSTP Port Roles - more : RSTP Port Roles - more
RSTP Port States : RSTP Port States There are no Blocking or Disabled states with RSTP
Configuring Rapid PVST+ : Configuring Rapid PVST+
Design to Avoid STP Problems : Design to Avoid STP Problems
Chapter 5 Labs : Chapter 5 Labs Lab 5.5.1: Basic Spanning Tree Protocol
Lab 5.5.3: Troubleshooting Spanning Tree Protocol
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