IEC 62439-1:2010/AMD1:2012

IEC 62439-1 ®
Edition 1.0 2012-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
AMENDMENT 1
AMENDEMENT 1
Industrial communication networks – High availability automation networks –
Part 1: General concepts and calculation methods

Réseaux de communication industriels – Réseaux de haute disponibilité pour
l'automatisation –
Partie 1: Concepts généraux et méthodes de calcul

IEC 62439-1:2010/A1:2012
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IEC 62439-1 ®
Edition 1.0 2012-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
AMENDMENT 1
AMENDEMENT 1
Industrial communication networks – High availability automation networks –

Part 1: General concepts and calculation methods

Réseaux de communication industriels – Réseaux de haute disponibilité pour

l'automatisation –
Partie 1: Concepts généraux et méthodes de calcul

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX L
ICS 25.040.40; 35.100.01 ISBN 978-2-83220-098-8

– 2 – 62439-1 Amend. 1 © IEC:2012
FOREWORD
This amendment has been prepared by subcommittee 65C: Industrial networks, of IEC
technical committee 65: Industrial-process measurement, control and automation, working
group 15.
The text of this amendment is based on the following documents:
FDIS Report on voting
65C/684/FDIS 65C/691/RVD
Full information on the voting for the approval of this amendment can be found in the report
on voting indicated in the above table.
The committee has decided that the contents of this amendment and the base publication will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
_____________
3.1 Terms and definitions
Add the following new terms and definitions 3.1.67 and 3.1.68:
3.1.67
bridge
device connecting LAN segments at layer 2 according to IEEE 802.1D
NOTE The words “switch” and “bridge” are considered synonyms, the word “bridge” is used in the context of
standards such as RSTP (IEEE 802.1D), PTP (IEC 61588) or IEC 62439-3 (PRP & HSR).
3.1.68
network recovery time
time span from the moment of the first failure of a component or media inside the network to
the moment the network reconfiguration is finished and from which all devices that are still
able to participate in network communication are able to reach all other such devices in the
network again
NOTE When a network redundancy control protocol (like RSTP) reconfigures the network due to a fault, parts of
the network may still be available and communication outages may vary in time and location over the whole
network. In the calculations, only the worst case scenario is considered.

62439-1 Amend. 1 © IEC:2012 – 3 –
3.2 Abbreviations and acronyms
Add, in alphabetical order, in the list of abbreviations the following new abbreviation:
RRP Ring-based Redundancy Protocol, see IEC 62439-7
3.4 Reserved network addresses
Add at the end of the list given in the second paragraph, the following new item:
• RRP (see IEC 62439-7) uses 00-E0-91-02-05-99.
Add at the end of the list given in the third paragraph, the following new item:
• RRP (see IEC 62439-7) uses 0x88FE.
4.1 Conformance to redundancy protocols
Add at the end of the existing list, the following new item:
• compliance to IEC 62439-7 (RRP).
5.1.1 Resilience in case of failure
Add, at the end of the fourth paragraph ("… are met"), the following new sentence:
A network provides a deterministic recovery if it is possible to calculate a finite worst case
recovery time of a given topology when a single failure occurs.
5.1.4 Comparison and indicators
Add, in the existing Table 2, the following new line between the existing lines "BRP" and
"PRP":
RRP IEC 62439-7 Yes In the end Double Single ring 8 ms in 100BASEX,
nodes (switching 4 ms in 1000BASEX
end nodes)
8 RSTP for High Availability Networks: configuration rules, calculation and
measurement method for deterministic recovery time in a ring topology
Replace, in the existing title of this clause, the words "for deterministic recovery time in a ring
topology" by "for predictible recovery time".
Add, between the existing title of this clause and the existing title of 8.1, the following new
note:
NOTE In the context of this Clause, the word “bridge” is used in place of “switch”, respectively “bridging” instead
of “switching”.
Add, at the end of this clause, the following new Subclause 8.5:
8.5 RSTP topology limits and maximum recovery time
NOTE In the next edition of IEC 62439-1, this new Subclause 8.5 will be renumbered as 8.2.
8.5.1 RSTP protocol parameters
This subclause explains the RSTP protocol parameters that impact network recovery times
and shows how a specific topology and protocol configuration influence them. First, RSTP-

– 4 – 62439-1 Amend. 1 © IEC:2012
specific terms are defined. Then, basic guidelines on network design are given and finally a
method to determine an approximation of an upper bond worst case network reconfiguration
time for meshed RSTP networks is given.
This subclause particularly deals with RSTP networks that are composed of more than a
single ring. For a single Ethernet ring running RSTP, the network reconfiguration time can be
determined as 8.2 shows. However, the subsequent statements concerning RSTP parameters
are also applicable in a ring network.
8.5.2 RSTP-specific terms and definitions
NOTE These terms are inherited from IEEE 802.1D.
8.5.2.1 Transmission Hold Count (TxHoldCount)
Each port of an RSTP bridge includes a counter TxHoldCount. This counter starts at zero and
is incremented for each BPDU the port sends. A timer decrements every second the counter.
If TxHoldCount reaches the maximum value, no further BPDU are transmitted over that port
until the counter has been decremented again, regardless of the importance of the BPDU to
network reconfiguration. The default maximum value of TxHoldCount is 6 and the maximum
configurable number is 10.
8.5.2.2 Bridge Max Age
Each RSTP bridge includes a parameter Bridge Max Age that should be configured to the
same value in each bridge. Bridge Max Age defines the maximum total number of “physical
hops” or links between the root bridge and any bridge participating in the same RSTP network.
Its default value is 20 and it can be configured to from 6 to a maximum of 40. In special cases,
Bridge Max Age is configured differently in some bridges.
Because Bridge Max Age defines the maximum extension of an RSTP network, it is
sometimes referred to as “network diameter”. But “Bridge Max Age” and the actually usable
network diameter are not synonymous, see 8.5.2.4.
8.5.2.3 Message Age
Each BPDU includes a parameter Message Age. Upon reception of a BPDU, a bridge
increments Message Age and afterwards compares it to its “Bridge Max Age”. If Message Age
is larger than Bridge Max Age, the bridge discards the BPDU and ignores the information it
carries.
The root bridge starts by sending BPDUs with Message Age = 0. The first bridge after the root
bridge (and subsequent bridges until Message Age reaches Bridge Max Age) receives the
BPDU, increment “Message Age” by 1, compares it to the “Bridge Max Age” and transmit
BPDUs with the updated information.
8.5.2.4 Network diameter and radius
The “diameter” in an RSTP network is the number of bridges on the longest active path in a
network tree between the two bridges that are the farthest away from each other. The
diameter does not necessarily correspond to the RSTP parameter Bridge Max Age (see
Figure 23).
The ”radius” in a RSTP network is the number of bridges from (and including) the active root
bridge to the bridge that is the farthest away from this active root in the topology. This is the
length (in hops) of the longuest path over which the RSTP protocol information needs to be
forwarded (see Figure 23). The maximum supported radius by RSTP can be defined as:
max. radius = Bridge Max Age + 1.

62439-1 Amend. 1 © IEC:2012 – 5 –
The radius is important to determine worst case topologies. In a worst case fault situation
(without an engineered network and consciously placed root bridges), upon failure of a root
bridge, the farthest away leaf might be the backup root bridge, which might become the next
root. In this case, the diameter of the network can become the radius and it becomes the
actual path that the RSTP information to the individual bridges has to travel. (See Figure 23)
NOTE RSTP BPDUs are only transmitted on the link between two directly connected bridges. Each bridge
consumes and produces these BPDUs, but the RSTP information which they carry travels distinct paths through the
network (in a stable network state without reconfiguration).
8.5.3 Example of a small RSTP tree

IEC  953/12
Figure 23 – Diameter and Bridge Max Age
NOTE 1 The RSTP parameter Bridge Max Age has been assigned the value 4 for the sake of this example
although 802.1D does not allow a value lower than 6.
In the example of Figure 23, at first, the network without a failure is in a stable condition with
Bridge Max Age = 4 and because the actual radius is 4 (the RSTP configuration could support
a maximum radius of 5). The diameter is 7, from one leaf in one branch to the other leaf in the
other branch, via the root bridge. Because the root bridge is the root element of a balanced
tree, Bridge Max Age = 4 is sufficient for all bridges to receiv
...