BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI
Publicat de
Universitatea Tehnică „Gheorghe Asachi” din Iaşi
Tomul LVII (LXI), Fasc. 3, 2011
SecŃia
AUTOMATICĂ şi CALCULATOARE
ACHIEVE FASTER SPANNING TREE CONVERGENCE
BY
ROXANA STĂNICĂ1 and EMIL PETRE2∗∗
1
S.C. Polystart Security S.R.L., Craiova
2
University of Craiova,
Department of Automatic Control
Received: August 6, 2011
Accepted for publication: September 19, 2011
Abstract. Spanning Tree Protocol (STP) is a Layer-2 protocol that provides
path redundancy and ensures a loop-free topology for bridged Local Area
Networks (LANs). Rapid Spanning Tree Protocol (RSTP) is an evolution of
STP. RSTP provides faster spanning tree convergence after a topology change.
While STP can take 30 to 50 sec to respond to a topology change, RSTP is able
to respond to changes within 3*hello-timer or within a few milliseconds of a
physical link failure. This paper contains detailed explanations of STP and RSTP
features and new developed commands to ensure a faster convergence. The STP
and RSTP structures permit the configuration of different parameters such as
hello, forward-delay and MaxAge timers and priority for Bridge Protocol Data
Units (BPDUs).
Key words: network protocol, LAN bridge, fault tolerance, topology
change.
2000 Mathematics Subject Classification: 94C12.
∗
Corresponding author: e-mail: [email protected]
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Roxana Stănică and Emil Petre
1. Introduction
Spanning Tree Protocol (STP), also known as IEEE 802.1d, is a
network protocol that ensures a loop-free topology for any bridged Ethernet
local area network. Using this protocol, a network can include redundant links
that provide automatic backup paths in case of failure of an active link.
STP is based on an algorithm invented by Radia Perlman (Perlman,
1985; Perlman, 2000) while working for Digital Equipment Corporation.
STP creates a spanning tree as a mesh network of connected Layer-2
bridges (typically Ethernet switches), and disables the links that are not part of
the spanning tree, leaving a single active path between any two network nodes.
An Ethernet network functions properly if only one active path exists between
any two stations. STP operation is transparent to end stations, which cannot
perceive whether they are connected to a single Local Area Network (LAN)
segment or to a switched LAN with multiple segments.
To prevent the delay when hosts are connected to a switch and during
some topology changes, Rapid STP (RSTP) was developed and standardized by
IEEE 802.1w, which allows a switch port to rapidly transition into the
Forwarding state (Table 3).
RSTP shares most of STP basic operation characteristics. However,
there are some major differences (http://www.cisco.com/en/US/tech/tk389/
tk621/technologies_tech_note09186a0080094797.shtml):
− The detection of Root switch failure is done in 3*hello-timer (Table 2).
− Ports can be configured as edge ports (Table 5) if they are attached to
a LAN that has no other bridges attached. RSTP can also be configured to
automatically detect edge ports. As soon as the bridge detects a Bridge Protocol
Data Unit (BPDU) coming to an edge port, the port becomes a non-edge port.
− Unlike STP, RSTP will respond to BPDUs sent from the direction of
the Root bridge. An RSTP bridge will “propose” its spanning tree information
to its Designated ports (Table 4). If another RSTP bridge receives this
information and determines this is the superior Root information, it sets all its
other ports to Discarding. The bridge sends an “agreement” to the first bridge
confirming its superior spanning tree information. This allows RSTP to achieve
faster convergence times than STP (Wojdak, 2003).
− RSTP maintains backup details regarding the Discarding status of
ports. This avoids timeouts if the current Forwarding ports failed or BPDUs
were not received on the Root port in a defined interval.
2. Spanning Tree Protocol
An example of a network is presented in Fig. 1. The boxes are the
bridges and the number associated with each box is the bridge identifier (ID).
Each bridge has a unique ID and a configurable priority number (the bridge ID
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Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
contains both numbers). The letteres, in Fig. 1, are the network segments. The
bridge with ID 5 has the smallest ID, therefore it is the Root bridge. If the cost
of traversing any network segment is 1, the lowest path cost from bridge 7 to
the Root bridge (bridge 5) goes through network segment c. Therefore, the Root
port for bridge 7 is the one on network segment c. The lowest path cost to the
Root from network segment f goes through bridge 15. Therefore, the designated
port for network segment f is the port that connects bridge 15 to network
segment f.
To compare two bridge IDs, the priority is compared first. If two
bridges have equal priority, then their MAC addresses are compared.
Root bridge
5
b
a
27
7
15
d
c
7
f
e
25
g
10
Fig. 1 – Network example.
2.1. STP Algorithm
The STP algorithm (Perlman, 1985) calculates each path cost for all
switches in the network’s spanning tree. The paths with the lower cost are active
paths and the other paths are blocked. STP activates the blocked paths in case
the active link fails or if the path cost changes.
− Root Bridge
First step of the algorithm is to choose the active paths in a network,
STP first determines a Root bridge. (The Root is the switch towards which all
other switches calculate the path cost.) STP then choose the path with the
lowest cost from each switch to the Root as the active path, while blocking all
other redundant paths.
− Designated bridge
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The second step is to choose the Designated bridge, per network
segment, after choosing the Root bridge. This is the closest bridge to the Root,
and forwards packets from that segment towards the Root bridge.
Each segment has only one Designated bridge. The Designated
bridge has one Designated port that forwards packets from the Root bridge
to this segment.
− Root and Alternate ports
The last step is to select a Root port (per bridge) that sends data towards
the Root bridge.
To avoid loops, all other ports that provide redundant paths to the Root
bridge are set as Alternate ports. These ports do not forward traffic unless the
Root port goes down.
Each bridge has only one Root port, as a single path toward the Root
bridge.
2.2. Line Error Detection
STP allows interchanging the roles of the Root and Alternate ports
when the Cyclic Redundancy Check (CRC) errors reach a critical level
(Angelescu, 2010). In this case, the Root port’s path cost automatically changes
into a higher value, triggering the interchange of the Root and Alternate port
statuses. CRC is an error-detecting code designed to detect accidental data
changes. This check (data verification) code is redundant (it adds zero
information to the message) and its algorithm is based on cyclic codes.
2.3. STP Bridge Protocol Data Units (BPDUs)
BPDUs are data messages sent to detect loops in a network topology
(Lammle, 2007). BPDUs include:
• the Root bridge ID;
• the Designated bridge ID;
• the path cost - the distance from the Root to the switch;
• the Designated port ID.
There are two BPDU types:
• Configuration BPDU - used to define loop-free technology;
• Topology Change Notification (TCN) BPDU – used to announce
network topology change. The Designated bridge acknowledges the TCN by
immediately sending back a normal configuration BPDU with the topology
change acknowledgement (TCA) bit set (http://www.cisco.com/en/US/tech/
tk389/tk621/technologies_tech_note09186a0080094797.shtml).
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
99
2.4. STP Path Cost
Each bridge port has an assigned path cost, a parameter that determines
the port’s preference to be included in the active spanning tree topology. During
BPDU exchange, STP sums the path costs along all Designated ports
(Designated path cost). This value is the bridge’s distance from the Root.
The lower the cost is, the closer the switch is to the Root. If two
switches have identical path costs, STP selects the path based on the priorities
and bridge IDs.
3. STP Port States and Timers
STP has five port states (http://www.batm.com/int/index/en/sdoc/type/111,
http://www.cisco.com/en/US/tech/tk389/tk389/tk621/technologies_configuration_
example09186a008009467c.shtml) (Fig. 2 and Table 1).
To ensure a loop-free network during topology changes inactive ports:
− cannot start forwarding prior to the new topology-information
propagating through the switched LAN;
− have to allow frames (that were forwarded using the old topology) to
expire.
3.1. Topology Changes Detection
When a bridge detects a topology change in the network (such as a link
failure or the link changing to Forwarding state), it sends this event to the entire
bridged network.
The process is done in two stages (Xu, 2010):
− The bridge notifies the Root.
− The Root broadcasts the information to the entire network.
When a topology change occurs, the address tables of all switches are
flushed and new paths are learned.
When the Root is aware of a topology change, it sends the configuration
BPDUs with the Topology Change (TC) flag set. As a result, all bridges become
aware of the topology change and reduce the MaxAge timer to the forwarddelay timer (Table 2).
3.2. Calculating the STP Timers
To calculate the STP timers, the following formulas can be used:
MaxAge = 4 x hello + 2 x dia − 2
(1)
Forward-delay = (4 x hello + 3 x dia) / 2
(2)
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− Where hello is the hello-timer and dia is the STP diameter.
Switch/Bridge
Initialization
Blocking State
Listening State
Disabled State
Learning State
Forwarding State
Fig. 2 − STP port states.
Table 1
STP States
State
Description
Blocking
The port does not forward frames. It moves to this
state after the initialization phase, when a different
switch/port was selected as Root.
Listening
The listening state is the first transitional state a port
enters after the Blocking state. STP determines if the
port should forward frames.
Learning
The port is preparing to forward frames. This is the
second transitional state through which a port moves
in anticipation of frame forwarding.
Forwarding
The port forwards frames. The bridge/switch
processes BPDUs and waits for possible new
information that might cause it to return to Blocking
state to prevent a loop.
Disabled
The port does not participate in forwarding frames
and spanning tree.
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
101
Table 2
STP Timers
Timer
Description
Hello timer
The interval (between two consecutive BPDUs) a
switch sends to other switches.
Forward-delay
timer
The time a port is in Listening and Learning states
before the port begins forwarding frames.
Maximum-age
timer (MaxAge)
The time the switch stores protocol information
received on a port.
Message Age
Shows how far a switch is from the Root when it
receives a BDPU.
Based on the above formulas, a low hello-timer value decreases the
other STP parameters. However, it doubles the amount of BPDUs sent/received
by each bridge, causing additional load on the Central Processing Unit (CPU).
STP timers’ settings are based on the STP diameter, the maximum
number of bridges between any two end points on the network. IEEE 802.1D
recommends a maximum network diameter of 7 hops.
4. Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) (http://www.cisco.com/en/US/
tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml) is an
evolution of STP. The protocol provides faster convergence (less than one
second) when a network topology change occurs. This is critical in networks
that carry voice, video, and other delay-sensitive traffic. RSTP automatically
selects the bridge that has the best bridge Identifier (ID) as the Root bridge.
Each bridge has a unique bridge ID that is derived from the bridge address and
from a manageable priority.
4.1. RSTP Algorithm
The RSTP algorithm dynamically creates a tree through the network, used
to efficiently direct packets to their destinations. It reduces the bridged network to
a single spanning tree topology in order to eliminate packet loops (multiple paths
linking one switch to another, resulting in an infinite loop situation).
The RSTP algorithm reactivates redundant connections in the event of a
link or switch failure.
RSTP distinguishes between the port state and the port role
(http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a
0080094cfa.shtml):
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− The port state (Table 3) describes the relationship of that port to the
frame processing (filtering and forwarding) and learning functions.
− The port role (Table 4) describes the role of the port in the spanning
tree function.
In order to create a loop-free environment and to provide rapid
convergence, RSTP selects the switch with the highest priority as the Root
bridge, assigns the port roles, and determines the active topology.
RSTP assigns a role to each bridge port throughout the bridged LAN.
4.2. RSTP Bridge Protocol Data Units (BPDUs)
The RSTP BPDU (http://www.cisco.com/en/US/tech/tk389/tk621/
technologies_white_paper09186a0080094cfa.shtml) has the same format as the
STP BPDU except for the protocol version that is set to 2. The switch proposes
itself to be the designated device by setting (Fig. 3):
− the Proposal flag (bit 1);
− the Port Role flag (bits 2-3) to Designated port.
The receiving device accepts the proposal by setting:
− the Agreement flag (bit 6);
− the Port role flag to Root port.
RSTP uses the Topology Change (TC) flag (http://www.cisco.com/en/US/
tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml) to indicate
topology changes. RSTP does not have a separate topology change notification
(TCN) BPDU as STP. However, for interoperability with STP switches, the
RSTP switch processes and generates TCN BPDUs. The Learning and
Forwarding (bits 4 and 5) flags are determined according to the sending port state.
0
1
2
3
4
5
6
7
Agreement
Forwarding
Learning
Port Role
Proposal
TC
Fig. 3 − RSTP BPDU format.
TC Ack
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
Table 3
STP Port States
State
Description
Learning
As in STP, the port prepares to forward frames. It
learns source addresses from the frames received
and adds them to the filtering database.
Forwarding
As in STP, the port enters this state from the
Learning state. The switch processes BPDUs and
waits for possible new information that may cause it
to move to the Discarding state to prevent a loop.
Discarding
This state contains STP Disabled, Blocking, and
Listening states. In this state, a port that does not
forward traffic in either direction. The port discards
received frames and no learning occurs.
Table 4
STP Port Roles
State
Description
Root port
Port connected to the Root bridge/switch. A
Root port is in Forwarding state.
Designated port
Port
connected
to
the
Designated
bridge/switch - the bridge/switch closest to the
Root bridge/switch. Frames are forwarded to
the
Root
through
the
Designated
bridge/switch. A Designated port is in
Forwarding state.
Alternate port
Port that offers a path to the Root
bridge/switch as an alternative to the path
provided by the Root port. The Alternate Port
can replace the current Root port if a link
failure or a configuration change (such as port
priority change) occurs.
Alternate port is in Discarding state. This role
is equivalent to the STP Blocking state.
Backup port
Backup for the path provided by a Designated
port in the direction of the Spanning Tree
leaves.
Backup port is in Discarding state. This role is
equivalent to the STP Blocking state.
Disabled port
Disabled ports do not forward frames and are
not operational.
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Table 5
RSTP Rapid Convergence
State
Edge ports
Root ports
Point-to-point
links
Description
Edge ports are configured by users. Once
they are configured, these ports transit to
Forwarding state.
When RSTP selects a new Root port, it
blocks the old Root port and puts the new
Root port in Forwarding state.
Point-to-point links are links that directly
connect two switches.
4.3. Line Error Detection
This feature is the same as in STP. For more information, see the Line
Error Detection chapter.
4.4. Edge Ports
All ports that are directly connected to end stations cannot create loops
in the network. Therefore, the edge port directly transitions to the Forwarding
state (see rstp edge-port command).
5. New STP and RSTP Commands
Fig. 4 details the steps required to configure the STP and RSTP parameters
(http://www.batm.com/int/index/en/sdoc/type/111, www.cisco.com/en/US/tech/
tk389/tk621/technologies_configuration_example09186a008009467c.shtml). To
apply and configure the new STP and RSTP commands, the steps displayed in
Fig. 4 must be followed.
5.1. Enable or Disable STP
− stp enable command enables the STP protocol on the switches.
− stp disable command disables the STP protocol.
5.2. Set STP Bridge Priority
• stp priority <priority value> command sets the STP bridge priority.
The default bridge priority is 32768.
◦ <priority value> is bridge priority in the range of <0–65535>. The
bridge with the highest bridge priority (the lowest numerical priority value) is
selected as Root switch.
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
105
5.3. Set STP Timers
− stp hello-timer <hello value> command sets the time interval
between e transmission of BPDUs by a Bridge that is attempting to become the
Root or is the Root.
− <hello value> is the hello-timer interval in the range of <1–9>
seconds. This value must be less than MaxAge/2-1.
− stp maxage <maxage value> command sets the interval the switch
waits for receiving a BPDU before attempting a reconfiguration.
− <maxage value> is the maximum aging time in the range of <6–28>
sec. The MaxAge value must be greater than 2*(hello-time+1) and less than
2*(forward-delay-1).
− stp forward-delay <delay value> command sets the time spent by a
port in the Listening state and the Learning state before moving to the Learning
state or Forwarding state.
− <delay value> is the forward-delay interval in the range of <11–30>
seconds. This value must be greater than MaxAge/2+1. When a topology
change is underway and is detected, this parameter is used to age all dynamic
entries in the Forwarding database.
5.4. Set STP Path Cost
• stp path-cost <cost value> command sets the STP port path cost.
◦ <cost value> is the path cost value in the range of
<1–200000000>. Lower cost values have to be assigned to ports the users want
to select first. If all ports have the same cost value, STP puts the port with the
lowest number in Forwarding state and blocks the other ports.
5.5. Set STP BPDU
− stp bpdu{discard | disable | standard} command defines the STP
reaction when a BPDU is received on the specified port.
− discard: the switch drops received BPDUs (ignores the BPDU
information).
− disable: receiving a BPDU disables the port.
− standard: BPDUs are processed according to standard STP
mechanisms (default).
5.6. Display STP Configuration
− stp display command shows the current STP configuration.
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5.7. Enable or disable RSTP
− rstp enable command enables the RSTP protocol on the switches.
− rstp disable command disables the RSTP protocol on the switches.
5.8. Set RSTP Bridge Priority
• rstp priority <priority value> command sets the RSTP bridge priority.
The default bridge priority is 32768.
◦ <priority value> is the RSTP bridge priority in increments of 4096.
The valid priority values are: 0, 4096, 8192, 12288, 16384, 20480, 24576,
28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
5.9. Set RSTP Timers
− rstp hello-timer <hello value> command sets the time interval
between e transmission of BPDUs by a Bridge that is attempting to become the
Root or is the Root.
− <hello value> is the hello-timer interval in the range of <1–9> sec.
The default hello-timer value is 2.
− rstp maxage <maxage value> command sets the time that learned
RSTP information is kept before being discarded.
− <maxage value> is the maximum aging time in the range of <4–60>
sec. The default MaxAge value is 20 sec (see Calculating the STP timers
chapter).
− rstp forward-delay <delay value> command sets the time spent by a
port in the Listening state and the Learning state before moving to the Learning
or Forwarding state.
− <delay value> is the forward-delay interval in the range of <4–60>
sec. The default forward-delay value is 15 sec (see Calculating the STP timers
chapter).
5.10. Set RSTP Edge Port
• rstp edge-port UU/SS/PP command set the specified port (UU/SS/PP)
as an Edge Port.
◦ Admin EdgePort: configuring a port as an Edge port is known as
Administrative Edge Port; the port has the permission to transition directly to
Forwarding state when it becomes Designated.
• EdgePort: indicates whether the port operates as an Edge Port or not.
When a port that was configured as Administrative Edge Port receives a BPDU,
it automatically changes its operational state to operate as a non-Edge Port, in
order to prevent loops in the network.
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
107
5.11. Set RSTP Path Cost
− rstp path-cost <cost value> command defines the RSTP port path cost.
− <cost value> is the RSTP path cost value, in the range of <1–
200000000>. The path cost value can be used to give priority to preferred links
(for example physical speed and bandwidth). When building the active spanning
tree, the port path-cost determines which port is included in the active topology.
Ports with lower cost values are preferred to ports with higher cost values. If all
ports that provide redundant paths to the root bridge have the same path-cost
value, RSTP puts the port with the lowest number in Forwarding state and
blocks the other ports.
5.12. Display RSTP Configuration
− rstp display command shows the current RSTP configuration.
6. Testing Results
The following is an STP/RSTP configuration in a network:
1) Enable STP/RSTP:
# stp enable
# rstp enable
2) Set STP/RSTP priority to 10:
# stp priority 10
# rstp priority 10
3) Set STP/RSTP MaxAge timer. Max_age = (4 x hello) + (2 x
dia) – 2; hello-timer is 2 (default value) and the diameter is 2:
# stp maxage 10
# rstp maxage 10
4) Set STP/RSTP forward-delay. Forward_delay = ((4 x hello) +
(3 x dia)) / 2; hello-timer is 2 (default value) and the diameter is 2:
# stp forward-delay 7
# rstp forward-delay 7
5) Set path cost:
# stp path-cost 500
# rstp path-cost 100
6) Set port 1/1/1 as edge port:
# rstp edge-port 1/1/1
7) Display STP/RSTP configuration
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Roxana Stănică and Emil Petre
Start
Enable STP/RSTP
No
Is the Bridge
selected as Root?
Yes
Change the priority to
be the lowest in the
network
Set the STP/RSTP timers (hello-time, MaxAge,
forward-delay)
Set the loop free ports as edge ports (for RSTP only)
Change the STP/RSTP path cost of ports
Display STP/RSTP Configuration
End
Fig. 4 - STP/RSTP configuration.
# stp display
Spanning tree
enabled
ProtocolSpecification = ieee8021d
Priority
= 10
TimeSinceTopologyChange = 0 (Sec)
TopChanges
=4
DesignatedRoot
= 04096.00:12:13:14:00:C0
RootPort
= 1/1/1
RootCost
= 500
MaxAge
= 10 (Sec)
HelloTime
= 2 (Sec)
ForwardDelay
= 7 (Sec)
HoldTime
= 1 (Sec)
BridgeMaxAge
= 20 (Sec)
BridgeHelloTime
= 2 (Sec)
BridgeForwardDelay = 15 (Sec)
DetectLineCRCReconfig = disabled
Bul. Inst. Polit. Iaşi, t. LVII (LXI), f. 3, 2011
109
# rstp display
Rapid spanning tree
= enabled
ProtocolSpecification = ieee8021w
Priority
= 10
TimeSinceTopologyChange
= 4 (Sec)
TopChanges
=5
DesignatedRoot
= 04096.00:12:13:14:00:C0
RootPort
= 1/1/1
RootCost
= 100
MaxAge
= 10 (Sec)
HelloTime
= 2 (Sec)
ForwardDelay
= 7 (Sec)
BridgeMaxAge
= 20 (Sec)
BridgeHelloTime
= 2 (Sec)
BridgeForwardDelay
= 15 (Sec)
DetectLineCRCReconfig
= disabled
EdgePort
= 1/1/1
BridgeHelloTime
= 2 (Sec)
BridgeForwardDelay = 15 (Sec)
7. Conclusions
The Spanning Tree Algorithm and Protocol first standardized in
ANSI/IEEE Std. 802.1D-1990 and unchanged in 802.1D-1998, was based on
the work of Radia Perlman (Perlman, 1985; Perlman, 2000).
Spanning Tree Protocol (STP) is a Layer-2 protocol that runs on bridges
and switches that are 802.1D-compliant. STP main purpose is to make sure that
users do not create loops when redundant paths appear in a network. Redundant
links are as important as backups if a failure occurs in a network. A failure of
primary link activates the backup links so that users can continue to use the
network. Without STP enabled on bridges and switches, such a failure can
result in a loop (http://www.cisco.com/en/US/tech/tk389/tk621/technologies_
configuration_example09186a008009467c.shtml).
STP provides fault tolerance on networks by allowing the
implementation of parallel paths for network traffic. The STP algorithm creates
a spanning tree (a loop-free subset of the network topology) that enables a
Learning bridge to dynamically work around loops in a network topology.
Multiple active paths among end stations cause loops in the network. If a loop
exists in the network, end stations might receive duplicate messages. When two
ports on a switch are part of a loop, the STP priority and path cost settings
determine which port is put in the Forwarding state and which is put in the
Blocking state.
RSTP improves the convergence time. RSTP overcomes slow
convergence times by introducing new port roles (edge port, Alternate port and
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Backup port) and new convergence behavior under specific connectivity
conditions (for example, point-to-point or shared medium). These enhancements
enable RSTP to achieve sub-second forwarding and network convergence
(http://www.juniper.net/us/en/local/pdf/implementation-guides/8010002-en.pdf).
In this paper, new commands were added for configuring the STP and RSTP
features, as well as new commands for displaying the changes performed.
REFERENCES
Angelescu S., CCNA® Certification All-In-One for Dummies®. Wiley Publishing Inc.,
385−415, 2010.
http://www.batm.com/int/index/en/sdoc/type/111
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example091
86a008009467c.shtml
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800947
97.shtml
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a008009
4cfa.shtml
http://www.juniper.net/us/en/local/pdf/implementation-guides/8010002-en.pdf.
Lammle T., CCNA®: Cisco® Certified Network Associate: Study Guide, Sixth Edition.
Wiley Publishing Inc., 493−550, 2007.
Perlman R., An Algorithm for Distributed Computation of a Spanning Tree in an
Extended LAN. ACM SIGCOMM Computer Communication Review,
Proceedings of the ninth symposium on Data communications, Vol. 15, 4,
44−53, 1985.
Perlman R., Interconnections, Second Edition: Bridges, Routers, Switches, and
Internetworking Protocols. Addison-Wesley Professional Computing Series,
58−94, 2000.
Wojdak W., Rapid Spanning Tree Protocol: A New Solution from an Old Technology.
Reprinted from CompactPCI Systems, 2003.
Xu Z., Designing and Implementing IP/MPLS-Based Ethernet Layer 2 VPN Services:
An Advanced Guide for VPLS and VLL. Wiley Publishing Inc., 840−895, 2010.
REALIZAREA CONVERGENłEI RAPIDE A PROTOCOLULUI
SPANNING TREE
(Rezumat)
Spanning Tree Protocol (STP) este un protocol Layer-2, care oferă redundanŃă
şi asigură o topologie fără buclă pentru reŃelele locale (Local Area Networks LAN). STP creează un spanning tree ca o reŃea mesh de punŃi Layer-2 conectate (de
obicei, switch-uri Ethernet), şi dezactivează link-uri care nu fac parte din spanning tree,
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lăsând o singură cale activă între oricare două noduri de reŃea. Rapid Spanning Tree
Protocol (RSTP) este o evoluŃie a protocolului STP şi a fost dezvoltat pentrua preveni
întârzierea atunci când gazdele (hosts) sunt conectate la un comutator (switch). RSTP
asigură accelerarea procesului de convergenŃă “spanning tree” după o schimbare a
topologiei. În timp ce lui STP îi poate lua între 30 şi 50 de sec pentru a răspunde la o
schimbare a topologiei, RSTP este capabil să răspundă la schimbări în termen de
3*hello-timer sau în termen de câteva milisecunde de la eşecul unei legături fizice.
RSTP înlătură convergenŃa lentăşi prin introducerea unor roluri noi pentru porturi
(portul edge, portul alternativ şi portul de rezervă). Aceste îmbunătăŃiri permit obŃinerea
unei convergenŃe sub o secundă.
Această lucrare conŃine explicaŃii detaliate despre protocoalele STP şi RSTP,
despre caracteristicile lor principale, precum şi comenzile noi dezvoltate pentru a
asigura o convergenŃă rapidă. Structurile elaborate pentru protocoalele STP şi RSTP
permit configurarea a diferiŃi parametri, cum ar fi hello timer, forward-delay timer şi
MaxAge timer, precum şi a priorităŃii pentru Bridge Protocol Data Units (BPDUs).