What is 802.1D Spanning Tree Protocol?
Spanning Tree Protocol (STP) was created to allow for loop-free topologies in switch architectures. Prior to STP, any switch architecture that had a loop would end up with duplicate frames flooding between switches causing an outage. Radia Perlman’s algorithm resolved the physical loop by creating a logical blocking solution stopping normal switch forwarding. STP became a standard with IEEE 802.1D. The STP algorithm requires all the switches in the layer 2 domain resolve a root switch which all traffic will flow through. The root is defined by a priority in updates sent between switches using Bridge Protocol Data Units (BPDUs).
|
LENGTH (bytes) |
FIELD |
DEFINITION |
|
2 |
Protocol ID |
0x0000 |
|
1 |
Version ID |
0x00 Config BPDU & TCN BPDU 0x02 RST BPDU |
|
1 |
Message Type |
0x00 Config BPDU 0x80 TCN BPDU 0x02 RST BPDU |
|
1 |
Flag |
1 : Topology Change Flag 2 : unused 0 3 : unused 0 4 : unused 0 5 : unused 0 6 : unused 0 7 : unused 0 8 : Topology Change Ack |
|
8 |
Root ID |
2 bytes Priority default 32768 6 bytes Bridge MAC of Root Bridge |
|
4 |
Root Path Cost |
Defined by Hops and Speeds of links to reach Root Bridge |
|
8 |
Bridge ID |
2 bytes Priority default 32768 6 bytes Bridge MAC of RSender Bridge |
|
2 |
Port ID |
Switch Port that send the BPDU |
|
2 |
Message Age |
Elapsed time from when message was sent |
|
2 |
Maximum Age |
20 seconds |
|
2 |
Hello Time |
2 Seconds |
|
2 |
Forward Delay |
15 Seconds |
A BPDU consists of the Priority and MAC address of the root switch, the Priority and MAC address of the switch sending the BPDU, it’s cost to root and additional data dealing with timers on how long switches are to wait before moving between forwarding states.
|
SPEED |
COST 1994 |
COST 1998 |
|
4M |
NA |
250 |
|
10M |
100 |
100 |
|
16M |
NA |
62 |
|
100M |
10 |
19 |
|
1G |
1 |
4 |
|
2G |
1 |
3 |
|
10G |
1 |
2 |
|
PORT STATE |
DEFINITION |
|
Blocking |
Port stops forwarding and receiving Frames, except for Management Frames – LLDP, CDP |
|
Listening |
Port receives and process BPDU Frames |
|
Learning |
Updates MAC Address Table but does not forward frames |
|
Forwarding |
Port operates normally, sends and receives frames |
|
Disabled |
Port is not operating, functionally shutdown by admin |
As switching technologies have improved STP was enhanced to keep up with improvements in speed and functionality. Spanning Tree Protocol was updated several times and variety of iterations. Cisco created PVST and PVST+. IEEE updated STP with RSTP and MSTP.
Iterations of Spanning Tree Protocol updates:
- STP the original
- PVST Cisco proprietary using ISL encapsulation
- 802.1q tagging of Ethernet Frames
- PVST+ Cisco proprietary using 802.1q encapsultion
- RSTP incorporation of speed enhancements and new modes
- MSTP Open standard for “Per Vlan” STP
Algorhyme by Radia Perlman
I think that I shall never see
A graph more lovely than a tree.
A tree whose crucial property
Is loop-free connectivity.
A tree that must be sure to span
So packets can reach every LAN.
First, the root must be selected.
By ID, it is elected.
Least-cost paths from root are traced.
In the tree, these paths are placed.
A mesh is made by folks like me,
Then bridges find a spanning tree.
###
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