Spanning tree

The Spanning Tree protocol are used on Ethernet Switches to avoid broadcast storms.

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:

Broadcast storm example

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.

Duplicate Ethernet frame example

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.

Broadcast storm example

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.

Broadcast storm example

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.

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-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 32768
spanning-tree mst 2 priority 24576

!
vlan internal allocation policy ascending