CISCO CCNA - SPANNING TREE PROTOCOL - 802.1d - Algorithm by Jagdeep Yadav
Spanning Tree Protocol
The Spanning Tree Protocol was created by DEC (Digital Equipment Corporation) now Compaq. This is not compatible with the
IEEE 802.1d version which Cisco use.
The Spanning Tree Protocol:
- Prevents loops, loops cause broadcast storms
- Allows redundant links
- Resilient to topology changes
- STA (Spanning Tree Algorithm) - Used to calculate loop-free path
- BPDUs (Bridge Protocol Data Units) are sent and received by switches in the network every 2 seconds (default) to determine
spanning tree topology.
Bridge Priority - Numerical value held by switches. All Catalyst switches are 32768
Bridge
ID = MAC Address
| Spanning Tree States |
| Blocking |
No frames forwarded, BPDUs heard |
| Listening |
No frames forwarded, listening for frames |
| Learning |
No frames forwarded, learning addresses |
| Forwarding |
Frames forwarded, learning addresses |
| Disabled |
No frames forwarded, no BPDUs heard |
STA - Spanning Tree Algorithm
Spanning Tree Algorithm is used to calculate a loop-free path.
All switch ports are in blocking mode to begin with. It takes approx 30 seconds until packets can be forwarded.
Step 1 : Elect Root Bridge - Lowest bridge priority, if there is a tie then switch with lowest bridge
ID
Step 2 : Elect Root Ports - Locate redundant paths to root bridge; block all but on root. Root Path
Cost is cumulative cost of path to root bridge. Ports directly connected to Root Bridge will be root ports, otherwise lowest
root path cost used.
Step 3 : Elect Designated Ports - Single port that sends and receives traffic from
a switch to and from Root Bridge - Lowest cost path to Root Bridge.
Spanning Tree Overview
- There can only be one Root Bridge.
- Root-Bridge ports are called 'Designated' and are set to send and receive traffic (forwarding state). All other redundant
links to the root bridge are shutdown.
- Blocked ports still receive BPDUs.
- Convergence occurs when switches have transitioned to either forwarding or blocking states. No other data is forwarded
during this time.
- Forward delay - Time taken for a switch to go from Listening to Learning (50 seconds default).
- IEEE default priority = 32,768, this is true for all devices running STP IEEE version.
- Port Fast Mode - Immediately brings a port from blocking to forwarding state by eliminating forward delays.
- Bridges can only have one spanning tree instance compared to switches which can have many.
- Bridge Protocol Data Units send confirmation messages using multicast frames.
