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Figure 5-35:  802.1Q VLAN interface 
 
 
•  Group Configuration 
Edit the existing VLAN Group. 
•  Select the VLAN group in the table list. 
•  Click  EDIT . 
Figure 5-36:  Group Configuration interface 
  
						

							
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• You can modify the VLAN group name and VLAN ID. 
Figure 5-37:  Group Configuration interface 
 
• Click  APPLY . 
Rapid Spanning Tree Protocol 
The Rapid Spanning Tree Protocol (RSTP) is  an evolution of the Spanning Tree Protocol 
and provides for faster spanning tree co nvergence after a topology change. The 
system also supports STP and the system w ill auto-detect the connected device that 
is running STP or RSTP protocol. 
Theory 
The Spanning Tree protocol can be used to  detect and disable network loops, and to 
provide backup links between switches, bridges  or routers. This allows the switch to 
interact with other bridging devices in your  network to ensure that only one route 
exists between any two stations on the network, and provide backup links, which 
automatically take over when a primary link goes down. The spanning tree\
 
algorithms supported by this sw itch include these versions:  
•  STP - Spanning Tree Protocol (IEEE 802.1D)  
•  RSTP - Rapid Spanning Tree Protocol (IEEE 802.1w)  
The IEEE 802.1D Spanning Tree Protocol and IEEE 802.1W Rapid Spanning Tree 
Protocol allow for the blocking of links be tween switches that form loops within the 
network. When multiple links between switches are detected, a primary link is 
established. Duplicated links are blocke d from use and become standby links. The  
						

							
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protocol allows for the duplicate links to  be used in the event of a failure of the 
primary link. Once the Spanning Tree Prot ocol is configured and enabled, primary 
links are established and duplicated links are blocked automatically. The reactivation 
of the blocked links (at the time of a  primary link failure) is also accomplished 
automatically without operator intervention. 
This automatic network reconfiguration prov ides maximum uptime to network users. 
However, the concepts of the Spanni ng Tree Algorithm and protocol are a 
complicated and complex subject and must be  fully researched and understood. It is 
possible to cause serious degradation of  the performance of the network if the 
Spanning Tree is incorrectly configured. Please read the following before making any 
changes from the default values. 
The Switch STP performs  the following functions: 
•  Creates a single spanning tree from any  combination of switching or bridging 
elements. 
•  Creates multiple spanning trees - from an y combination of ports contained within 
a single switch, in user specified groups. 
•  Automatically reconfigures the spanning tree to compensate for the failure, 
addition, or removal of any element in the tree. 
•  Reconfigures the spanning tree without operator intervention. 
Bridge Protocol Data Units 
For STP to arrive at a stable network topo logy, the following information is used: 
•  The unique switch identifier 
•  The path cost to the root associ ated with each switch port 
•  The port identifier 
STP communicates between switches on the network using Bridge Protocol Data 
Units (BPDUs). Each BPDU contains the following information: 
•  The unique identifier of the switch that  the transmitting switch currently believes 
is the root switch. 
•  The path cost to the root fr om the transmitting port. 
•  The port identifier of the transmitting port. 
The switch sends BPDUs to communicate an d construct the spanning-tree topology. 
All switches connected to the  LAN on which the packet is transmitted will receive the 
BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses 
the information in the frame to calculate  a BPDU, and, if the topology changes, 
initiates a BPDU transmission. 
  
						

							
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The communication between switches via BPDUs results in the following: 
• One switch is elected as the root switch. 
•  The shortest distance to the root sw itch is calculated for each switch. 
•  A designated switch is selected. This is  the switch closest to the root switch 
through which packets will be  forwarded to the root. 
•  A port for each switch is selected. This is  the port providing the best path from the 
switch to the root switch. 
•  Ports included in the STP are selected. 
Creating a Stable STP Topology 
It is to make the root port a fastest link.  If all switches have STP enabled with default 
settings, the switch with the lowest MAC  address in the network will become the root 
switch. By increasing the priority (lowering  the priority number) of the best switch, STP 
can be forced to se lect the best switch  as the root switch. 
When STP is enabled using the default  parameters, the path between source and 
destination stations in a switched netw ork might not be ideal. For instance, 
connecting higher-speed links to a port th at has a higher number than the current 
root port can cause a root-port change. 
STP Port States 
The BPDUs take some time  to pass through a network. This propagation delay can 
result in topology changes where a port th at transitioned directly from a Blocking 
state to a Forwarding state co uld create temporary data loops. Ports must wait for 
new network topology info rmation to propagate throug hout the network before 
starting to forward packets. They must also  wait for the packet lifetime to expire for 
BPDU packets that were forwarded based  on the old topology. The forward delay 
timer is used to allow the network topology  to stabilize after a topology change. In 
addition, STP specifies a series  of states a port must transition through to further 
ensure that a stable network topology  is created after a topology change. 
Each port on a switch using STP exists  is in one of the following five states: 
•  Blocking - the port is blocked fr om forwarding or receiving packets. 
•  Listening - the port is waiting to receive  BPDU packets that may tell the port to go 
back to the blocking state. 
•  Learning - the port is adding addresse s to its forwarding database, but not yet 
forwarding packets. 
•  Forwarding - the port is forwarding packets. 
•  Disabled - the port only responds to  network management messages and must 
return to the blocking state first.  
						

							
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A port transitions from one  state to another as follows: 
•  From initialization (swi tch boot) to blocking. 
•  From blocking to list ening or to disabled. 
•  From listening to lear ning or to disabled. 
•  From learning to forwa rding or to disabled. 
•  From forwarding to disabled. 
•  From disabled to blocking. 
Figure 5-38: STP Port State Transitions 
 
You can modify each port state by usin g management software. When you enable 
STP, every port on every switch in the ne twork goes through the blocking state and 
then transitions through the states of listeni ng and learning at power up. If properly 
configured, each port stabilizes to the fo rwarding or blocking state. No packets 
(except BPDUs) are forwarded from, or re ceived by, STP enabled ports until the 
forwarding state is enabled for that port.  
						

							
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RSTP Parameters 
RSTP Operation Levels 
The Switch allows for two levels of operation: the switch level and the port level. The 
switch level forms a spanning tree consisting  of links between one or more switches. 
The port level constructs a s panning tree consisting of groups of one or more ports. 
The STP operates in much the same way for both levels. 
NOTE:  On the switch level, RSTP calculates th e Bridge Identifier for each switch and 
then sets the Root Bridge and the Designated Bridges. 
On the port level, RSTP sets the Ro ot Port and the Designated Ports. 
The following are the user-configurable  STP parameters for the switch level: 
 
Parameter Description Default Value 
Bridge Identifier (Not user 
configurable except by 
setting priority below) 
  A combination of the User-set priority and 
the switch’s MAC address.  
The Bridge Identifier consists of two parts: 
 
a 16-bit priority and a 48-bit Ethernet 
MAC address 32768 + MAC  32768 + MAC 
Priority  A relative priority for each switch – lower 
numbers give a higher priority and a 
greater chance of a given switch being 
elected as the root bridge  32768 
Hello Time  The length of time between broadcasts of 
the hello message by the switch  2 seconds 
Maximum Age Timer  Measures the age of a received BPDU for 
a port and ensures that the BPDU is 
discarded when its age exceeds the value 
of the maximum age timer.  20 seconds 
Forward Delay Timer  The amount time spent by a port in the 
learning and listening states waiting for a 
BPDU that may return the port to the 
blocking state.  15 seconds 
 
  
						

							
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The following are the user-configurable ST P parameters for the port or port group 
level: 
 
Variable Description Default Value 
Port Priority  A relative priority for each port –lower  numbers give a higher priority and a 
greater chance of a given port being 
elected as the root port  128 
Port Cost  A value used by STP to evaluate paths 
– STP calculates path costs and selects 
the path with the minimum cost as the 
active path  200,000-100Mbps Fast Ethernet 
ports 
20,000-1000Mbps Gigabit 
Ethernet ports 
0 - Auto 
 
Default Spanning-Tree Configuration 
 
Feature Default Value 
Enable state   STP disabled for all ports 
Port priority  128 
Port cost  0 
Bridge Priority  32,768 
 
User-Changeable STA Parameters 
The Switch’s factory default setting should
 cover the majority of installations. 
However, it is advisable to keep the default se ttings as set at the factory; unless, it is 
absolutely necessary. The user changeable par ameters in the Switch are as follows: 
Priority –  A Priority for the switch can be set fr om 0 to 65535. 0 is equal to the highest 
Priority. 
Hello Time  – The Hello Time can be from 1 to 10  seconds. This is the interval between 
two transmissions of BPDU packets sent by  the Root Bridge to tell all other Switches 
that it is indeed the Root Bridge. If you se t a Hello Time for your Switch, and it is not 
the Root Bridge, the set Hello Time will be  used if and when your Switch becomes the 
Root Bridge.  
						

							
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NOTE: The Hello Time cannot be longer than th e Max. Age. Otherwise, a configuration 
error will occur. 
Max. Age – The Max Age can be from 6 to 40 seco nds. At the end of the Max Age, if a 
BPDU has still not been received from the Ro ot Bridge, your Switch will start sending 
its own BPDU to all other Switches for perm ission to become the Root Bridge. If it 
turns out that your Switch has the lowest Bridge Identifier, it will become the Root 
Bridge. 
Forward Delay Timer  – The Forward Delay can be from  4 to 30 seconds. This is the 
time any port on the Switch spends in  the listening state while moving from the 
blocking state to the forwarding state. 
NOTE:  Observe the following formulas wh en setting the above parameters: 
Max. Age _ 2 x (Forward Delay - 1 second) 
Max. Age _ 2 x (Hello Time + 1 second) 
Port Priority – A Port Priority can be from 0 to  240. The lower the number, the greater 
the probability the port will be  chosen as the Root Port. 
Port Cost  – A Port Cost can be set from 0  to 200000000. The lower the number, the 
greater the probability the port will  be chosen to forward packets. 
Illustration of STP 
A simple illustration of three switches co nnected in a loop is depicted in the below 
diagram. In this example, you can anticipat e some major network problems if the STP 
assistance is not applied.  
If switch A broadcasts a packet to switch B,  switch B will broadcast it to switch C, and 
switch C will broadcast it to back to swit ch A and so on. The broadcast packet will be 
passed indefinitely in a loop, potentially ca using a network failure. In this example, 
STP breaks the loop by blocking the connection between switch B and C. The decision 
to block a particular connection is based on  the STP calculation of the most current 
Bridge and Port settings.  
Now, if switch A bro adcasts a packet to switch C, then  switch C will drop the packet 
at port 2 and the broadcast will end there. Setting-up STP using values other than the 
defaults, can be complex. Therefore, you are advised to keep the default factory 
settings and STP will automatically assign  root bridges/ports and block loop 
connections. Influencing STP to choose a parti cular switch as the root bridge using  
						

							
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the Priority setting, or influencing STP to  choose a particular port to block using the 
Port Priority and Port Cost settings is,  however, relatively straight forward. 
Figure 5-39: Before Applying the STA Rules 
 
In this example, only the default STP values are used. 
Figure 5-40: After Applying the STA Rules 
 
  
						

							
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The switch with the lowest Bridge ID (switch C) was elected the root bridge, and the 
ports were selected to give a high port  cost between switches B and C. The two 
(optional) Gigabit ports (defau lt port cost = 4) on switch A are connected to one 
(optional) Gigabit port on both switch B  and C. The redundant link between switch B 
and C is deliberately chosen as a 100 Mbps  Fast Ethernet link (default port cost = 19). 
Gigabit ports could be used, but the port cost  should be increased from the default to 
ensure that the link between switch  B and switch C is the blocked link. 
RSTP System Configuration 
This section provides RSTP-System Config uration from the Switch, the screen in 
Figure 5-41 appears. 
•  The user can view spanning tr ee information of Root Bridge. 
•  The user can modify RSTP state. After modification, click  APPLY. 
Figure 5-41:  RSTP System Configuration interface