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HP 5500 Ei 5500 Si Switch Series Configuration Guide

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    							 60 
Configuring spanning tree protocols 
As a Layer 2 management protocol, the Spanning Tree Protocol (STP) eliminates Layer 2 loops by 
selectively blocking redundant links in a network, putting them in a standby state, which still also allows 
for link redundancy. 
The recent versions of STP include the Rapid Spanning Tree Protocol (RSTP), Per VLAN Spanning Tree 
(PVST), and the Multiple Spanning Tree Protocol (MSTP). 
STP 
STP was developed based on the 802.1d standard of IEEE to eliminate loops at the data link layer in a 
local area network (LAN). Networks often have redund ant links as backups in case of failures, but loops 
are a very serious problem. Devices that run STP detect loops in the network by exchanging information 
with one another, and eliminate loop s by selectively blocking certain ports to prune the loop structure 
into a loop-free tree structure. This avoids proliferation and infinite cycling of packets that would occur in 
a loop network, and prevents received duplicate pac kets from decreasing the performance of network 
devices.  
In the narrow sense, STP refers to IEEE 802.1d STP. In the broad sense, STP refers to the IEEE 802.1d STP 
and various enhanced spanning tree pr otocols derived from that protocol. 
STP protocol packets 
STP uses bridge protocol data units (BPDUs), also  known as configuration messages, as its protocol 
packets. 
STP-enabled network devices exchange BPDUs to est ablish a spanning tree. BPDUs contain sufficient 
information for the network devices to  complete spanning tree calculation. 
STP uses the following types of BPDUs: 
•   Configuration BPDUs —Used by network devices to calculat e a spanning tree and maintain the 
spanning tree topology 
•   Topology change notification (TCN) BPDUs—Notify network devices of the network topology 
changes 
Configuration BPDUs contain sufficient information fo r the network devices to complete spanning tree 
calculation. Important fields in a configuration BPDU include the following: 
•   Root bridge ID —Consisting of the priority and MAC address of the root bridge. 
•   Root path cost —Cost of the path to the root bridge denoted by the root identifier from the 
transmitting bridge. 
•   Designated bridge ID —Consisting of the priority and MA C address of the designated bridge. 
•   Designated port ID —Consisting of the priority and global  port number of the designated port. 
•   Message age —Age of the configuration BPDU while it propagates in the network. 
•   Max age —Maximum age of the configuration BPDU stored on the switch. 
•   Hello time —Configuration BPDU transmission interval. 
•   Forward delay —Delay that STP bridges use to transition port state.
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    							 61 
Basic concepts in STP 
Root bridge 
A tree network must have a root bridge. The entire network contains only one root bridge. The root bridge 
is not permanent, but can change with changes of the network topology. 
Upon initialization of a network, each device generates and periodically sends configuration BPDUs with 
itself as the root bridge. After network convergence, only the root bridge generates and periodically 
sends configuration BPDUs, and the other devices forward the BPDUs.  
Root port 
On a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates with 
the root bridge. Each non-root bridge has only one root port. The root bridge has no root port. 
Designated bridge and designated port 
Table 8  Description of designated br idges and designated ports 
Classification  Desi
gnated bridge  Designated port 
For a device
 Device directly connected with the local 
device and responsible for forwarding 
BPDUs to the local device
 Port through which the designated bridge 
forwards BPDUs to this device 
 
For a LAN Device responsible for forwarding BPDUs 
to this LAN segment
  Port through which the designated bridge 
forwards BPDUs to this LAN segment
 
 
As shown in Figure 17, Device B and Device C are directly connected to a LAN. If Device A forwards 
BPDUs to Device B through port A1, the designated br idge for Device B is Device A, and the designated 
port of Device B is port A1 on Device A. If Device  B forwards BPDUs to the LAN, the designated bridge 
for the LAN is Device B, and the designated port for the LAN is port B2 on Device B. 
Figure 17  Designated bridges and designated ports 
 
 
Path cost 
Path cost is a reference value used for link selection in STP. STP calculates path costs to select the most 
robust links and block redundant links that are less robust, to prune the network into a loop-free tree.
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    							 62 
Calculation process of the STP algorithm 
The spanning tree calculation process described in the following sections is a simplified process for 
example only. 
The STP algorithm uses the following calculation process: 
1. Initial state 
Upon initialization of a device, each port genera tes a BPDU with the port as the designated port, 
the device as the root bridge, 0 as the root path cost, and the device ID as the designated bridge 
ID. 
2.  Root bridge selection 
Initially, each STP-enabled device on the network as sumes itself to be the root bridge, with its own 
device ID as the root bridge ID. By exchanging  configuration BPDUs, the devices compare their 
root bridge IDs to elect the device with the  smallest root bridge ID as the root bridge. 
3. Non-root bridge: Selection of r oot port and designated ports 
Table 9  Selection of the root po rt and designated ports 
Step   Description 
1  A non-root-bridge device regards the port on wh
ich it received the optimum configuration BPDU 
as the root port.  Table 10 describes how the o ptimum configuration BPDU is selected.
 
2 Based on the configuration BPDU and the path cost
 of the root port, the device calculates a 
designated port configuration BPDU for each of the other ports. 
•  The root bridge ID is replaced with that of the configuration BPDU of the root port. 
• The root path cost is replaced with that of the configuration BPDU of the root port plus the path 
cost of the root port. 
• The designated bridge ID is replaced with the ID of this device. 
• The designated port ID is replaced with the ID of this port.  
3 The device compares the calculated configuration 
BPDU with the configuration BPDU on the port 
whose port role will be defined, and acts  depending on the result of the comparison. 
• If the calculated configuration BPDU is superior, the device considers this port as the 
designated port, replaces the configuration BPDU on the port with the calculated configuration 
BPDU, and periodically sends the calculated configuration BPDU. 
• If the configuration BPDU on the port is superior, the device blocks this port without updating its 
configuration BPDU. The blocked port can receive BPDUs, but cannot send BPDUs or forward 
data traffic.  
 
 NOTE: 
When the network topology is stable, only the root port and designated ports forward user traffic, while 
other ports are all in the blocked state to receiv e BPDUs but not forward BPDUs or user traffic.
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    							 63 
Table 10 Selection of the optimum configuration BPDU 
Step   Actions 
1  Upon receiving a configuration BPDU on a port, the device compares the priority of the received 
configuration BPDU with that of the config
uration BPDU generated by the port, and: • If the former priority is lower, the device discards the received configuration BPDU and keeps 
the configuration BPDU the port generated. 
• If the former priority is higher, the device replaces the content of the configuration BPDU 
generated by the port with the content of the received configuration BPDU.  
2 The device compares the configuration BPDU
s of all the ports and chooses the optimum 
configuration BPDU.  
 
The following are the principles of configuration BPDU comparison: 
•  The configuration BPDU with the lowest root bridge ID has the highest priority. 
•   If configuration BPDUs have the same root bridge ID, their root path costs are compared. For 
example, the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The 
configuration BPDU with the smallest S value has the highest priority. 
•   If all configuration BPDUs have the same ports valu e, their designated bridge IDs, designated port 
IDs, and the IDs of the receiving ports are compared in sequence. The configuration BPDU that 
contains the smallest ID wins. 
A tree-shape topology forms when the root bridge , root ports, and designated ports are selected. 
Figure 18  de
 scribes with an example how the STP algorithm works. This example shows a simplified 
spanning tree calculation process. 
Figure 18  The STP algorithm  
 
 
As shown in Figure 18, the p riority values of Device A, Device B, and Device C are 0, 1, and 2, and the 
path costs of links among the three devices are 5, 10, and 4, respectively. 
4.  Initial state of each device 
Device A
Priority = 0
Device B
Priority = 1 Device C
Priority = 2
Port A1 Port A2
Port B1 Port B2 Port C1
Port C2
P at
h
 c
o st 
=  5
P
at
h
 c
o
st 
=
 1 0
Path cost = 4
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    							 64 
Table 11 Initial state of each device 
Device   Port name  Confi
guration BPDU on the port
Device A  Port A1 
{0, 0, 0, Port A1} Port A2 {0, 0, 0, Port A2} 
Device B  Port B1
  {1, 0, 1, Port B1} Port B2  {1, 0, 1, Port B2} 
Device C  Port C1
  {2, 0, 2, Port C1} Port C2  {2, 0, 2, Port C2} 
 
 NOTE: 
In Table 11 , e
ach configuration BPDU contains the following fields: root bridge ID, root path cost, 
designated bridge ID, and designated port ID. 
 
5.  Comparison process and result on each device 
Table 12  Comparison process and result on each device 
Device  Comparison process   Configuration BPDU on 
ports after comparison
 
Device A 
• Por t A1 receives the configuration BPDU of Por t B1 {1, 0, 1, Por t 
B1}, finds that its existing configuration BPDU {0, 0, 0, Port A1} 
is superior to the received configuration BPDU, and discards the 
received one. 
• Po r t  A 2  re c e ive s  t h e  c o n f i g u ra t i o n  B PD U  o f  Po r t  C 1  { 2,  0 ,  2,  Po r t  
C1}, finds that its existing configuration BPDU {0, 0, 0, Port A2} 
is superior to the received configuration BPDU, and discards the 
received one. 
• Device A finds that it is both the root bridge and designated 
bridge in the configuration BPDUs of all its ports, and considers 
itself as the root bridge. It does not change the configuration 
BPDU of any port and starts to periodically send configuration 
BPDUs. 
• Port A1: {0, 0, 0, Port 
A1} 
• Port A2: {0, 0, 0, Port 
A2} 
Device B 
• Po r t  B 1  re c e ive s  t h e  c o n f i g u ra t i o n  B PD U  o f  Po r t  A 1  { 0 ,  0 ,  0 ,  Po r t  
A 1 } ,  fi n d s  t h a t  t h e  re c eive d  c on fig u ra t ion  B PD U  i s  s up erior  t o  i t s  
existing configuration BPDU {1, 0, 1, Port B1}, and updates its 
configuration BPDU. 
• Por t  B 2  re c eives  t h e  c on fig u ra t ion  B PD U  of  Por t  C 2  { 2,  0 ,  2,  Po r t  
C2}, finds that its existing configuration BPDU {1, 0, 1, Port B2} 
is superior to the received configuration BPDU, and discards the 
received one.  
• Port B1: {0, 0, 0, Port 
A1} 
• Port B2: {1, 0, 1, Port 
B2}
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Device Comparison process   Configuration BPDU on ports after comparison
 
• Device B compares the configuration BPDUs of all its ports, 
decides that the configuration BPDU of Port B1 is the optimum, 
and selects Port B1 as the root port with the configuration BPDU 
unchanged. 
• Based on the configuration BPDU and path cost of the root port, 
Device B calculates a designated port configuration BPDU for 
Port B2 {0, 5, 1, Port B2}, and compares it with the existing 
configuration BPDU of Port B2 {1, 0, 1, Port B2}. Device B finds 
that the calculated one is superi or, decides that Port B2 is the 
designated port, replaces the configuration BPDU on Port B2 
with the calculated one, and periodically sends the calculated 
configuration BPDU.   • Root port (Port B1): {0, 
0, 0, Port A1} 
• Designated port (Port 
B2): {0, 5, 1, Port B2} 
Device C 
• Po r t  C 1  r e c e i v e s  t h e  c o n f i g u r a t i o n  B P D U  o f  Po r t  A 2  { 0 ,  0 ,  0 ,  Po r t  
A 2 } ,  fi n d s  t h a t  t h e  re c eive d  c on fig u ra t ion  B PD U  i s  s up erior  t o  i t s  
existing configuration BPDU {2, 0, 2, Port C1}, and updates its 
configuration BPDU. 
• Port C2 receives the original configuration BPDU of Port B2 {1, 
0, 1, Port B2}, finds that the received configuration BPDU is 
superior to the existing configuration BPDU {2, 0, 2, Port C2}, 
and updates its configuration BPDU.  
• Port C1: {0, 0, 0, Port 
A2} 
• Port C2: {1, 0, 1, Port 
B2} 
• Device C compares the configuration BPDUs of all its ports, 
decides that the configuration BPDU of Port C1 is the optimum, 
and selects Port C1 as the root port with the configuration BPDU 
unchanged. 
• Based on the configuration BPDU and path cost of the root port, 
Device C calculates the configuration BPDU of Port C2 {0, 10, 2, 
Port C2}, and compares it with the existing configuration BPDU 
of Port C2 {1, 0, 1, Port B2}. Device C finds that the calculated 
configuration BPDU is superior to the existing one, selects Port 
C2 as the designated port, and replaces the configuration 
BPDU of Port C2 with the calculated one.  • Root port (Port C1): {0, 
0, 0, Port A2} 
• Designated port (Port 
C2): {0, 10, 2, Port C2}
 
• Port C2 receives the updated configuration BPDU of Port B2 {0, 
5, 1, Port B2}, finds that the received configuration BPDU is 
superior to its existing configuration BPDU {0, 10, 2, Port C2}, 
and updates its configuration BPDU. 
• Port C1 receives a periodic configuration BPDU {0, 0, 0, Port 
A2} from Port A2, finds that it is the same as the existing 
configuration BPDU, and discards the received one.  
• Port C1: {0, 0, 0, Port 
A2} 
• Port C2: {0, 5, 1, Port 
B2}
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Device Comparison process   Configuration BPDU on ports after comparison
 
• Device C finds that the root path cost of Port C1 (10) (root path 
c o s t  o f  t h e  r e c e i v e d  c o n f i g u r a t i o n  B P D U  ( 0 )  p l u s  p a t h  c o s t  o f  Po r t  
C1 (10)) is larger than that of Port C2 (9) (root path cost of the 
received configuration BPDU (5) plus path cost of Port C2 (4)), 
decides that the configuration BPDU of Port C2 is the optimum, 
and selects Port C2 as the root port with the configuration BPDU 
unchanged. 
• Based on the configuration BPDU and path cost of the root port, 
Device C calculates a designated port configuration BPDU for 
Port C1 {0, 9, 2, Port C1} and compares it with the existing 
c o n f i g u ra t i o n  B P D U  o f  Po r t  C 1  { 0 ,  0 ,  0 ,  Po r t  A 2 } .  D e vi c e  C  f i n d s  
that the existing configuration BPDU is superior to the calculated 
one and blocks Port C1 with the configuration BPDU 
unchanged. Then Port C1 does not forward data until a new 
event triggers a spanning tree calculation process, for example, 
the link between Device B and Device C is down.  
• Blocked port (Port C1): 
{0, 0, 0, Port A2} 
• Root port (Port C2): {0, 
5, 1, Port B2} 
 
  NOTE: 
In Table 12 , e
ach configuration BPDU contains the following fields: root bridge ID, root path cost, 
designated bridge ID, and designated port ID. 
 
After the comparison processes described in  Tabl e  12, a spanning tr ee with Device A as the root bridge 
is established, and the topology is shown in  Figure 19. 
Figure 19  The final calculated spanning tree 

 
 
 
The configuration BPDU forwarding mechanism of STP 
The configuration BPDUs of STP are forwarded following these guidelines: 
•  Upon network initiation, every device regards itself as the root bridge, generates configuration 
BPDUs with itself as the root, and sends the co nfiguration BPDUs at a regular hello interval. 
•   If the root port received a configuration BPDU and the received configuration BPDU is superior to 
the configuration BPDU of the port, the device increases the message age carried in the 
configuration BPDU following a certain rule and star ts a timer to time the configuration BPDU while 
sending this configuration BPDU through the designated port. 
•   If the configuration BPDU received on a designated port has a lower priority than the configuration 
BPDU of the local port, the port immediately  sends its own configuration BPDU in response. 
A
BC
Root port
Designated port
Root bridge
Normal link
Blocked link
Blocked port
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    							 67 
•  If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs and 
the old configuration BPDUs will be discarded due to timeout. The device generates a configuration 
BPDU with itself as the root and sends the BPDU s and TCN BPDUs. This triggers a new spanning 
tree calculation process to establish a new path to restore the network connectivity. 
However, the newly calculated configuration BPDU cannot be propagated throughout the network 
immediately, so the old root port s and designated ports that have  not detected the topology change 
continue forwarding data along the old path. If th e new root ports and designated ports begin to 
forward data as soon as they are elected, a temporary loop might occur. 
STP timers 
The most important timing parameters in STP calcul ation are forward delay, hello time, and max age. 
•   Forward delay 
Forward delay is the delay time for port state transition.  
A path failure can cause spanning tree re-calculati on to adapt the spanning tree structure to the 
change. However, the resulting new configuration BPDU cannot propagate throughout the 
network immediately. If the newly elected root  ports and designated ports start to forward data 
immediately, a temporary loop will likely occur. 
For this reason, as a mechanism for state transi tion in STP, the newly elected root ports or 
designated ports require twice the forward delay time before they tr ansit to the forwarding state to 
make sure that the new configuration BPDU  has propagated throughout the network. 
•   Hello time 
The device sends hello packets at the hello time in terval to the neighboring devices to make sure 
that the paths are fault-free. 
•   Max age 
The device uses the max age to determine whethe r a stored configuration BPDU has expired and 
discards it if the max age is exceeded. 
RSTP 
RSTP achieves rapid network convergence by allowing a newly elected root port or designated port to 
enter the forwarding state  much faster under certain conditions than STP. 
A newly elected RSTP root port rapidly enters the forw arding state if the old root port on the device has 
stopped forwarding data and the upstream desi gnated port has started forwarding data.  
A newly elected RSTP designated port rapidly enters the fo rwarding state if it is an edge port (a port that 
directly connects to a user terminal  rather than to another network device or a shared LAN segment) or 
it connects to a point-to-point link  (to another device). Edge ports di rectly enter the forwarding state. 
Connecting to a point-to-point link, a designated port  enters the forwarding state immediately after the 
device receives a handshake response from the directly connected device.  
PVST 
PVST was introduced to improve link bandwidth usage in network environments where multiple virtual 
LANs (VLANs) exist. Unlike STP and RSTP whose bridges in a LAN must forward their VLAN packets in 
the same spanning tree, PVST allows each  VLAN to build a separate spanning tree. 
PVST uses the following BPDUs:
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    							 68 
•  STP BPDUs —Sent by access ports according to the VLAN  status, or by trunk ports and hybrid ports 
according to the status of VLAN 1. 
•   PVST BPDUs—Sent by trunk port and hybrid ports acco rding to the status of permitted VLANs 
except VLAN 1. 
MSTP 
STP, RSTP, and PVST limitations 
STP does not support rapid state tran sition of ports. A newly elected port must wait twice the forward 
delay time before it transits to the forwarding state, even  i f  i t  c o n n e c t s  t o  a  p o i n t - t o - p o i n t  l i n k  o r  i s  a n  e d g e  
port. 
Although RSTP supports rapid network convergence, it has the same drawback as STP—All bridges 
within a LAN share the same spanning tree, so redu ndant links cannot be blocked based on VLAN, and 
the packets of all VLANs are forwarded along the same spanning tree. 
The number of PVST BPDUs generated grows with that of permitted VLANs on trunk ports. When the 
status of a trunk port transitions, network devices mi ght be overloaded to re-calculate a large number of 
spanning trees. 
MSTP features 
Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP, RSTP, and PVST. In addition to 
supporting rapid network convergence, it provides  a better load sharing mechanism for redundant links 
by allowing data flows of different VLAN s to be forwarded along separate paths. 
MSTP provides the following features: 
•   MSTP supports mapping VLANs to spanning tree  instances by means of a VLAN-to-instance 
mapping table. MSTP can reduce communication overheads and resource usage by mapping 
multiple VLANs to one instance. 
•   MSTP divides a switched network into multiple regions, each of which contains multiple spanning 
trees that are independent of one another. 
•   MSTP prunes a loop network into a loop-free tree, which avoids proliferation and endless cycling of 
packets in a loop network. In addition, it supports load balancing of VLAN data by providing 
multiple redundant paths for data forwarding.  
MSTP basic concepts 
Figure 20  shows a switched network that comprises four MST regions, each MST region comprising four 
MSTP devices.  Figure 21 sho
 ws the networking topology of MST region 3.
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    							 69 
Figure 20 Basic concepts in MSTP  
 
 
Figure 21 Network diagram and topology of MST region 3 
 
 
MST region 
A multiple spanning tree region (MST region) consists of multiple devices in a switched network and the 
network segments among them. All these devices have the following characteristics: 
•   A spanning tree protocol enabled 
•   Same region name 
MST region 1
MST region 2 MST region 3 MST region 4
VLAN 1  MSTI 1
VLAN 2  MSTI 2
 Other VLANs  MSTI 0
VLAN 1  MSTI 1
VLAN 2  MSTI 2
 Other VLANs  MSTI 0
VLAN 1  MSTI 1
VLAN 2  MSTI 2
 Other VLANs  MSTI 0
VLAN 1  MSTI 1
VLAN 2&3  MSTI 2
 Other VLANs  MSTI 0CST
MST region 3
Device A
Device C Device B
Device D
VLAN 1  MSTI 1
VLAN 2&3  MSTI 2
 Other VLANs  MSTI 0
To MST region 4
BA
CD
MSTI 1
AB
CD
MSTI 0
B
D
MSTI 2C
A
Regional rootMSTI
Topology of MSTIs in MST region 3
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