Spanning Tree Protocol

Using the Spanning Tree Protocol makes your network more resilient to link failure and also provides a protection from loops - one of the major causes of broadcast storms.

This chapter explains more about the protocol and the protocol features supported by your Switch. It covers the following topics:

• What is STP?

• How STP Works

• Using STP on a Network with Multiple VLANs

• Connecting to STP Systems on Legacy Switch Units

• Enabling STP

  The protocol is a part of the 802.1D bridge specification defined by the IEEE Computer Society. To explain STP more effectively, your Switch will be defined as a bridge.

Using the Spanning Tree Protocol (STP) makes your network more resilient to link failure and also provides a protection from loops - one of the major causes of broadcast storms.

STP is a bridge-based system that allows you to implement parallel paths for network traffic and uses a loop-detection process to:

• Find and disable the less efficient paths (that is, the paths that have a lower bandwidth).

• Enable one of the less efficient paths if the most efficient path fails.

As an example, Figure 45 shows a network containing three LAN segments separated by three bridges. With this configuration, each segment can communicate with the others using two paths. Without STP, this configuration creates loops that cause the network to overload; however, STP allows you to have this configuration because it detects duplicate paths and prevents, or blocks, one of them from forwarding traffic.

Figure 46 shows the result of enabling STP on the bridges in the configuration. The STP system has decided that traffic from LAN segment 2 to LAN segment 1 can only flow through Bridges C and A.

If the link through Bridge C fails, as shown in Figure 47, the STP process reconfigures the network so that traffic from segment 2 flows through Bridge B.

Figure 45 A network configuration that creates loops

Figure 46 Traffic flowing through Bridges C and A

Figure 47 Traffic flowing through Bridge B

STP determines which is the most efficient path between each bridged segment and a specifically assigned reference point on the network. Once the most efficient path has been determined, all other paths are disabled. Thus, in the example above, STP initially decided that the path through Bridge C was the most efficient, and so blocked the path through Bridge B. After the failure of Bridge C, STP re-evaluated the situation and opened the path through Bridge B.

Before it can configure the network, the STP system requires the following:

• Communication between all the bridges. This communication is carried out using Bridge Protocol Data Units (BPDUs), which are transmitted in packets with a known multicast address.

• Each bridge to have a Bridge Identifier. This specifies which bridge acts as the central reference point, or Root Bridge, for the STP system - the lower the Bridge Identifier, the more likely the bridge is to become the Root Bridge. The Bridge Identifier is calculated using the MAC address of the bridge and a priority defined for the bridge. The default priority of your Switch is 32768.

• Each port to have a cost. This specifies the efficiency of each link, usually determined by the bandwidth of the link - the higher the cost, the less efficient the link. Table 9 shows the default port costs for your Switch.

Table 9 Default port costs

Port Type	Duplex	Cost
1000BASE-SX	Full	4
Port trunk containing 100BASE-TX/100BASE-FX	Full/Half	15
100BASE-TX/100BASE-FX	Full Half	18 19
Port trunk containing 10BASE-T only	Full/Half	90
10BASE-T	Full	95
10BASE-T	Half	100

CAUTION: If you are using STP on a network that contains various network devices, ensure that the cost for each port type is the same for each device. If the costs are different, STP cannot determine the efficiency of each link accurately. You can change the port costs of devices on your network using Transcend Network Management software.

STP Calculation

The first stage in the STP process is the calculation stage. During this stage, each bridge on the network transmits BPDUs that allow the system to work out:

• The identity of the bridge that is to be the Root Bridge - the central reference point from which the network is configured.

• The Root Path Costs for each bridge - that is, the cost of the paths from each bridge to the Root Bridge.

• The identity of the port on each bridge that is to be the Root Port - the one that is connected to the Root Bridge using the most efficient path, that is, the one that has the lowest Root Path Cost. Note that the Root Bridge does not have a Root Port.

• The identity of the bridge that is to be the Designated Bridge of each LAN segment - the one that has the lowest Root Path Cost from that segment. Note that if several bridges have the same Root Path Cost, the one with the lowest Bridge Identifier becomes the Designated Bridge.

  All traffic destined to pass in the direction of the Root Bridge flows through the Designated Bridge. The port on this bridge that connects to the segment is called the Designated Bridge Port.

After all the bridges on the network have agreed on the identity of the Root Bridge, and have established the other relevant parameters, each bridge is configured to forward traffic only between its Root Port and the Designated Bridge Ports for the respective network segments. All other ports are blocked, which means that they are prevented from receiving or forwarding traffic.

STP Reconfiguration

Once the network topology is stable, all the bridges listen for special Hello BPDUs transmitted from the Root Bridge at regular intervals. If a bridge does not receive a Hello BPDU after a certain interval (the Max Age time), the bridge assumes that the Root Bridge, or a link between itself and the Root Bridge, has gone down. The bridge then reconfigures the network to cater for the change. If the topology of your network changes, the first bridge to detect the change sends out an SNMP trap.

CAUTION: To ensure that the bridges can communicate after a reconfiguration, all potential Designated Bridge ports and Root Ports must belong to the same VLANs. For more information about VLANs, see "Virtual LANs (VLANs)".

An Example

Figure 48 shows a LAN that has STP enabled. The LAN has three segments, and each segment is connected using two possible links.

Figure 48 Port costs in a network

• Bridge A has the lowest Bridge Identifier in the network, and has therefore been selected as the Root Bridge.

• Because Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment A. Port 1 on Bridge A is therefore selected as the Designated Bridge Port for segment A.

• Port 1 of Bridges B, C, X and Y have been defined as a Root Ports because they are the nearest to the Root Bridge.

• Bridges B and X offer the same Root Path Cost for LAN segment B, however, Bridge B has been selected as the Designated Bridge for the segment because it has a lower Bridge Identifier. Port 2 on Bridge B is therefore selected as the Designated Bridge Port for LAN segment B.

• Bridge C has been selected as the Designated Bridge for LAN segment B, because it offers the lowest Root Path Cost for segment C - the route through Bridges C and B costs 200 (C-B=100, B-A=100), the route through Bridges Y and B costs 300 (C-B=200, B-A=100). Port 2 on Bridge C is therefore selected as the Designated Bridge Port for segment C.

Figure 49 (overleaf) shows three possible STP configurations using SuperStack Switch units.

• Configuration 1 - Redundancy for Backbone Link

  In this configuration, a Switch 1100 and a Switch 3300 both have STP enabled and are connected by two links. STP discovers a duplicate path and disables one of the links. If the enabled link breaks, the disabled link becomes re-enabled, therefore maintaining connectivity.

• Configuration 2 - Redundancy through Meshed Backbone

  In this configuration, four Switch 3300 units are connected such that there are multiple paths between each one. STP discovers the duplicate paths and disables two of the links. If an enabled link breaks, one of the disabled links becomes re-enabled, therefore maintaining connectivity.

• Configuration 3 - Redundancy for Cabling Error

  In this configuration, a Switch 1100 has STP enabled and is accidentally connected to a hub using two links. STP discovers a duplicate path and disables one of the links, therefore avoiding a loop.

Figure 49 STP configurations

Your Switch does not take into account VLANs when it calculates STP information - the calculations are only performed on the basis of duplicate connections. For this reason, some network configurations can result in VLANs being subdivided into a number of isolated sections by the STP system.

For example, Figure 50 shows a network containing VLANs 1 and 2. They are connected using the 802.1Q-tagged link between Switch B and Switch C. By default, this link has a path cost of 100 and is automatically blocked because the other Switch-to-Switch connections have a path cost of 36 (18+18). This means that both VLANs are now subdivided - VLAN 1 on Switch units A and B cannot communicate with VLAN 1 on Switch C, and VLAN 2 on Switch units A and C cannot communicate with VLAN 2 on Switch B.

Figure 50 A configuration that separates VLANs

To avoid any VLAN subdivision, 3Com recommend that all inter-Switch connections are made members of all available 802.1Q VLANs to ensure connectivity at all times. For example, the connections between Switches A and B, and Switches A and C should be 802.1Q tagged and carrying VLANs 1 and 2 to ensure connectivity.

For more information about VLAN Tagging, see Chapter 6, Virtual LANs (VLANs).

If you are connecting your Switch to legacy units that support STP, note the following:

• Your Switch supports one STP system; however legacy Switch units (for example, the SuperStack II Switch 1000) may support one STP system per VLAN. Consequently:

  • If the legacy Switch units use a single VLAN and you connect your Switch to them using an untagged link, the STP system of your Switch and the STP system of the legacy Switch units are combined.

  • If the legacy Switch units use multiple VLANs and you connect your Switch to them using a VLT tagged link, the STP system of your Switch and the STP system of the legacy Switch units are completely separate. This means that if you connect your Switch to a legacy Switch unit using multiple VLT links, a loop may created.

• Some legacy units cannot use VLAN 16 if they are using STP. If your Switch is connected to one of these units, VLAN 16 traffic is blocked on the port.

To enable STP on your Switch via the command line interface, see "Enabling and Disabling Spanning Tree on a Bridge" on page 116.

To enable STP on your Switch via the web management interface:

1 .   From the web interface, click the Configuration icon on the side-bar.

2 .   Click the Advanced Stack Setup hotlink. The Advanced Stack Setup page is displayed.

3 .   From the Spanning Tree listbox, select Enabled.

4 .   Click the Apply button.

You cannot enable STP if you have set up any resilient links on the Switch.