HP 5500 Ei 5500 Si Switch Series Configuration Guide

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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...

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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...

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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...

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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...

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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...

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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...

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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...

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•  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...

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•  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...

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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...