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Revision as of 14:09, 28 April 2009
The Spanning Tree protocol are used on Ethernet Switches to avoid broadcast storms.
Contents
Problems with Switches without Spanning Tree
Broadcast Storms
A broadcast starts when a Ethernet switch receives a broadcast from a Host and there exist a loop. See example below:
Duplicate Ethernet Frames
Another problem with Ethernet loops is duplicate Ethernet frames. In the picture below, switch X can see PC-B mac-address on two ports and send the frame out of each port.
Instability in Switch MAC-Address table
Another problem with with Ethernet loops is instability in the Switches MAC-Address table See the picture below and consider:
- The MAC-Address off PC-B is timed out on both Switches.
- PC-A send a unicast packet to PC-B's MAC-Address.
- PC-B's MAC-address is unknown to Switch X which send the Frame out of all ports. (Except the originating port 3)
- Switch Y receives the Frame to PC-B on port 1 and on port 2.
- PC-B's MAC-address is unknown to Switch X which send the Frame out of all ports. (Except originating port)
- Switch X know receives Frames on port 1 and port 2 with source MAC-address o fPC-A
- Switch X now thinks that PC-A is on Port 1, Port 2 and Port 3.
The Spanning Tree protocol principle
If the Switches has enabled the Spanning Tree Protocol - STP - the Switches discover the loop and close one of the links for traffic. The closed link will be enabled if one of the other links breaks down.
BPDU: Bridge Protocol Data unit
Connected Switches sends BPDU packets to each other, to make a hierarchy among them. The purpose is to build a loop free network.
- BPDU packets are send as 802.1d multicast packets.
- Switches not participating in the STP sends the BPDU packets out all ports. (Broadcast)
- Switches participating in STP receives the packets on the multicast addresses 01-80-C2-00-00-00 and 01-80-C2-00-00-10
- BPDU packets are send every 2 seconbds.
BPDU Packet
| Bytes | Field name | Notes |
|---|---|---|
| 2 | Protocol ID | Always 0 |
| 1 | Version | Always 0 |
| 1 | Message Type | Configuration or TCN BPPU |
| 1 | Flags | |
| 8 | Root Bridge ID | 2 Bytes priority and 6 Bytes MAC address |
| 4 | Path Cost | Cost of all links from the transmitting switch to the root bridge
See table below |
| 8 | Bridge ID | 2 Bytes priority and 6 Bytes MAC address |
| 2 | Port ID | Transmitting switch port ID |
| 2 | Message age | in 256's of a second |
| 2 | Max age | in 256's of a second |
| 2 | Hello Time | in 256's of a second |
| 2 | Forward delay | in 256's of a second |
BPDU Path Cost
| Link Bandwidth | Old STP Cost | New STP Cost |
|---|---|---|
| 4 Mbps | 250 | 250 |
| 10 Mbps | 100 | 100 |
| 16 Mbps | 63 | 62 |
| 45 Mbps | 22 | 39 |
| 100 Mbps | 10 | 19 |
| 622 Mbps | 2 | 6 |
| 1 Gbps | 1 | 4 |
| 10 Gbps | 0 | 2 |
Enhancements to the Spanning Tree Protocol
- Portfast
- Per VLAN Spanning Tree Protocol (PVST+)
- Rapid Spanning Tree Protocol (RSTP)
- Multiple Spanning Tree Protocol (MSTP)
- Per VLAN Rapid Spanning Tree (PVRST)
MST: Multiple Spanning Tree protocol
hostname Switch-1 spanning-tree mode mst spanning-tree extend system-id ! spanning-tree mst configuration name TEKKOM revision 1 instance 1 vlan 10-50 instance 2 vlan 51-99 ! spanning-tree mst 1 priority 24576 spanning-tree mst 2 priority 32768 ! vlan internal allocation policy ascending
hostname Switch-2 spanning-tree mode mst spanning-tree extend system-id ! spanning-tree mst configuration name TEKKOM revision 1 instance 1 vlan 10-50 instance 2 vlan 51-99 ! spanning-tree mst 1 priority 32768 spanning-tree mst 2 priority 24576 ! vlan internal allocation policy ascending