Switch spanning tree algorithm monitor

Overview

The goal of this LoriotPro Plugin is to monitor bridges and switches running the Spanning Tree Algorithm. This algorithm is used to determine when multiple paths exist between two nodes which one should be active and which one should be in standby. This is necessary to avoid loop and broadcast packet storm.
Furthermore, the use of a dynamic protocol allows the network administrator to define backup path and thus improve network availability. Below two examples of Spanning Tree.

The Plugin through a graphical map performs the supervision. On this map, all devices behaving as bridge are represented. All ports and their state (blocking forwarding learning) are also represented.
Links (any kind of network) between bridge ports are also displayed.

Exemple of a network , Switches are green box, standby link are in red, active link are in green

The configuration is realized through a visual interface.

Management tasks are performed by direct acces to the Bridge MIB object

Plugin Installation

The Plug-In should be attach to one of your host of your Directory. It could anyway control all the bridge from the same Spanning Tree domain. Eihter attach it ot one of your switch/bridge or to the LoriotPro Icon

An alternative Way for Plug-in selection is to the Directory Plug-in task option of the menu which opens a PlugIn Loader box with explanations on each Plug-in.

Configuration

The configuration of the Bridge Plugin is done from the monitor Window. To access the monitor Window select the Directory object, one of your bridge to which the Plugin is attached and double click on the plugin.

The monitor window opened, you can now proceed to the next step : add bridges to the list.

You may do that in three ways:

1. Manually, if you know the IP address of your Bridges
2. Automatically by letting LoriotPro searching the Directory and discover Bridge devices
3. Semi automatically by browsing the directory and picking up devices

To add a bridge device manually, fill the the Bridge IP Address field. Press the Add Bridge Button

To let LoriotPro discover the Bridges, press the Discover Bridge Devices from the Directory Button

To select device from the directory press List All AVailable Host from the Directory

Select the host to add and press Add Selection

The added bridges appears in the right pane of the Window named STP Instances.

From that point each Bridge in the list will be used by LoriotPro to create the Map.

The left buttons of the Pane allows yout to :

The Properties button open the following window:

You could specify the Bridge Instance (stack) Name
The interval between each time that LoriotPro will check to see if the topology has changed.
The alarm number sent to the LoriotPro event manager if the topology has changed.

The topology change mainly occurs on the following events:

One of the bridge stop to work.
One the link (network) is broken.
You change parameter within the bridge (Path cost, priority, port status etc...)
A new bridge is added in the network.

VLAN Selection on CISCO Switch

On Cisco switch only, this Plug-in support Multi-Vlan. To select the VLAN specify it in the select box:

Review on Spanning Tree Concepts

To explain the concept of the Spanning Tree we will use a concrete example.

We defines a network with three Ethernet Switches (working as multiport Bridge) that are connected together by Ethernet uplinks. Switches are considered here as Transparent Bridges and support the RFC1493.

Transparent bridges are so named because their presence and operation are transparent to network hosts. When transparent bridges are powered on, they learn the network's topology by analyzing the source address of incoming frames from all attached networks. In our example, the switch sees a frame arrive on port 4 from Host A, the switch concludes that Host A can be reached through the network connected to line 1. Through this process, transparent bridges build a table such as the one below.

The bridge uses its table as the basis for traffic forwarding. When a frame is received on one of the bridge's interfaces, the bridge looks up the frame's destination address in its internal table. If the table contains an association between the destination address and any of the bridge's ports aside from the one on which the frame was received, the frame is forwarded out the indicated port. If no association is found, the frame is flooded to all ports except the inbound port. Broadcasts and multicasts are also flooded in this way.

How the Spanning tree is created ?

The first activity in spanning-tree computation is the selection of the root bridge (dot1dStpDesignatedRoot), which is the bridge with the lowest value bridge identifier. In our example, the root bridge is Switch 1. Next, the root port on all other bridges is determined. A bridge's root port (dot1dStpRootPort) is the port through which the root bridge can be reached with the least aggregate path cost. This value (the least aggregate path cost to the root) is called the root path cost.

Finally, designated bridges (dot1dStpPortDesignatedBridge) and their designated ports (dot1dStpPortDesignatedPort) are determined. A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge.

In some cases, two or more bridges can have the same root path cost. In this case, the bridge identifiers are used again, this time to determine the designated bridges.

Cost of a port

To determine the path cost, use this formula:
Interface Path Cost (dot1dStpPortPathCost) = 1000/Attached LAN speed in Mb/s

We applied the Spanning Tree Algorithm to our network and we get the following results. All values are visible from MIB object that we will detail after.

Supervision

The supervision of the spanning tree is performed directly from the Bridge Plugin.

The right pane of the window display all the bridge participating in the Spanning Tree and from there you could check the current status of each bridge.

In our example we get the following information for each bridge

Switch 1

The bridge Switch 1 is root, clearly identified by the earth icon.
We found next the following information :

Global parameter for this bridge

The MAC address of the Bridge

The bridge type defined by the MIB Object dot1dBaseType

The Spanning tree version protocol used defined by the MIB object dot1dStpProtocolSpecification

The Bridge priority defined by the MIB Object dot1dStpPriority. The value is decimal but it is ofently defined in hexa (here 32768 = 0x8000).

The last time the topologie changed defined by the MIB Object dot1dStpTopChanges

Specific parameter for each port

Port name are identified from the MIB object :
(...mib-2(1).interfaces(2).iftable(2).ifentry(1).ifdescr(2))

The operating status of the port defined by the MIB Object (dot1dStpPortEnable)

The Spanning Tree status defined by the MIB Object (dot1dStpPortState).
The values are 1 disabled, 2 blocking, 3 listenning, 4 learning, 5 forwarding, 6 broken)

The designated bridge for this Lan defined by the MIB Object (dot1dStpPortDesignatedBridge)

The designated port defined by the MIB Object (dot1dStpPortDesignatedPort)Port name on the designated bridge identified from the MIB object :
(...mib-2(1).interfaces(2).iftable(2).ifentry(1).ifdescr(2))

We get the same information for the two other bridges

Switch 2

We could see that for the bridge Switch 2 the root Bridge is Swicth 1. The value here is the concatenation of the Priority value 0x8000 (32768) of Switch 1 and its MAC address.
The Switch 2 Port 2 is connected to Switch 3 Port 1.
The Switch 2 Port 2 is connected to Switch 3 Port 1.

Switch 3

We could see here that the status of the second port of the Switch 3 is Blocking. This is the normal behavior of the Spanning tree, all the network are available and the best route from Switch 3 to switch 1 (The root) is by Switch 2. The total cost by this way is 20 against 100 by the direct link.
This is explained here by the link speed between 1-2 and 2-3 which are Ethernet 100 Mbps against cost 100 the 10 Mbps between 1-3.

On the MAP you could see the same status with color.

The color of the link and port have the following significance :

Management

The management of the Spanning tree parameters could be done from the MAP menu. The commands sent from here are SNMP SET and thus knowing the write community of the bridge agent is necessary to perform them.

The menu provide the following option:

The Menu is divided in three sections:

Tools allow you to do a Ping, Telnet or Browse the MAP selected Bridge.

Next three options provides you with informations on the Bridge Device, list of ports, statistic on the Spanning tree protocol, STP timer values. Value in purple could be change.

The dot1dStpPriority could be change here to force a bridge to become root. The Bridge with the lowest value in a Spanning tree become root. In our exemple the Swicth 1 has value 0x8000 and is lower than swicth 2 (0x9000) and switch 3 (0xa000).

The timing value should not be changed or only if your are experimented. The modification of these values could for example decrease the convergence (tree reconfiguration).

Anyway, be careful when you change the values here.

The last five otpions give you Table contains of Spanning Tree information.

Select a device on the Map and select monitor -> Set/Display dot1stpPort Entry

To see the significance of each table parameter, refers to the MIB Object dot1dStpPortEntry

The dot1dTpFdbTable stand for Transparent Protocol Forwarding DataBase entry and contains for each port of the Bridge (Switch here) the learned MAC addresses of your hosts.

Restrictions

The Bridge Plugin Support the IEEE 802.1d standard only. If you use switch and set one STP instance per VLAN you will not be able to see them except for the Cisco SWITCH.

Spanning Tree Types and switch