802.1aq Shortest Path Bridging Design and Evolution: The Architect's Perspective

215 Pages·2012·3.255 MB·English

by  David Allan, Nigel Bragg

802.1aq Shortest
Path Bridging
Design and Evolution
802.1aq Shortest
Path Bridging
Design and Evolution
The Architect’s Perspective
David Allan
Nigel Bragg
Published by
Standards Information Network
IEEE Press
A John Wiley & Sons, Inc., Publication
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10 9 8 7 6 5 4 3 2 1
Contents
Figures vii
Acknowledgments ix
Introduction xi
Abbreviations xvii
1. IEEE 802.1aq in a Nutshell: Antecedents
and Technology 1
2. Why SPB Looks as It Does 36
3. Why the SPB Control Plane Looks as It Does 74
4. Practical Deployment Considerations 130
5. Applications of SPB 150
6. Futures 158
Conclusion 186
References 188
Index 190
v
Figures
Figure 1.1 A simple spanning tree 2
Figure 1.2 The evolution of Ethernet stacking 5
Figure 1.3 IEEE 802.1aq technology — data plane connectivity
for BEB “ A ” 9
Figure 1.4 New information items in IS - IS for SPBM 17
Figure 2.1 Metro network architecture: layer view 39
Figure 2.2 Hop - by - hop load spreading — ECMP 48
Figure 2.3 End - to - end load spreading — ECMT 49
Figure 2.4 Port - based interface structures (from [PBB]) 54
Figure 2.5 Different sources may not exhibit congruent trees 68
Figure 2.6 The condition for downstream congruency 69
Figure 3.1 The rise of computing power 77
Figure 3.2 Signaling to support multipoint - to - multipoint
tree setup 79
Figure 3.3 Minimum - cost MDT 81
Figure 3.4 Shortest path tree with maximum diversity 81
Figure 3.5 Minimum - cost shortest path tree 82
Figure 3.6 A multicast reconvergence scenario 83
Figure 3.7 New information items for IS - IS for SPBM 89
Figure 3.8 New information items for IS - IS for SPBV 90
Figure 3.9 Use of multiple I - SIDs for resilient E - TREE
structures 99
Figure 3.10 The routed restoration cycle 106
Figure 3.11 The importance of consistent tiebreaking 108
Figure 3.12 A looping example 112
Figure 3.13 The SPB topology digest 119
Figure 3.14 “ Fat tree ” switching structure 121
vii
viii Figures
Figure 3.15 Network partitioning from the perspective
of node 2 128
Figure 3.16 Network coalescence from the perspective
of node 2 128
Figure 4.1 Announcement of a dual - homed UNI
into SPBM 135
Figure 4.2 Announcement of a UNI fault into SPBM 136
Figure 4.3 SPBM delivering “ multipoint link aggregation ” 138
Figure 4.4 Multipoint LAG endpoints: structure and
advertisements 139
Figure 4.5 Multipoint LAG: failure of an access link 141
Figure 4.6 IS - IS use of the pseudonode to model broadcast
segments 142
Figure 4.7 The ease of loop formation on LAN segments 144
Figure 4.8 SPBM overlay of bridged Ethernet emulation 145
Figure 4.9 LAN segments and frame duplication 146
Figure 4.10 Use of multiple Ethernet segments
by SPBM overlay 148
Figure 6.1 A multiarea model potentially used by SPB 174
Figure 6.2 Multiarea forwarding and the common (unary)
FDB model 176
Figure 6.3 SPBM ABB structures 177
Figure 6.4 PCI: PBB - TE trunk between individual
endpoints 184
Acknowledgments
This document is an amalgam of practical wisdom of Dave Allan, Peter
Ashwood- Smith, Nigel Bragg, Janos Farkas, Don Fedyk, J é r ô me
Chiabaut, Dinesh Mohan, Mick Seaman, and Paul Unbehagen.
Thanks also to Anne Bragg for her persistence in reducing our
wayward capitalization, punctuation, and other stylistic idiosyncracies
to more conventional forms.
Technical Reviewers
Simon Parry, Ciena Limited
Joel M. Halpern, D istinguished Engineer, Ericsson
ix
Introduction
Ethernet is a diffi cult and demanding taskmaster.
We start from the position that for any networking technology of
suffi cient power, an elegant and self - consistent solution to a given con-
nectivity problem exists. Switched Ethernet is the product of 30 years
on Occam’ s razor, and although the attributes and scale of the applica-
tion domains covered by this book were until recently inconceivable,
we have concluded that it remains a technology of suffi cient power and
self - consistency.
The success and longevity of Ethernet can be put down to the fact
that it has been able to evolve to accommodate new requirements, both
in its original LAN application space and in the increasing proportion
of Provider networking space. Shortest path bridging (SPB) is one of
the most recent of these evolutionary steps, and we would like to estab-
lish at this early point both what is the fundamental problem it solves
and why the solution is useful.
The short and suffi cient answer is, “ elimination of the Spanning
Tree Protocol and its shortcomings, and its replacement by a superior
routed technology, and without changing the service model. ” This
answer is “ suffi cient ” for now because it is generally accepted in the
industry that Spanning Tree Protocol presents problems and limits the
applications accessible to Ethernet, and we therefore defer further dis-
cussion on the origins and root cause of this problem to the beginning
of the next section.
Replacement of Spanning Tree Protocol by something substantially
superior is a general “ good ” that applies to Ethernet networking in both
Enterprise and Provider space. The other key requirement of Ethernet
networking, which is increasingly shared by Enterprise applications as
well as Providers, is virtualization, which is the ability to support multiple
independent LAN segments on the same physical infrastructure. SPB did
not originate the technology to do this, but directly supports earlier IEEE
Standards (Provider Bridging and Provider Backbone Bridging) that
defi ned the hierarchical data path constructs to support virtualization.
xi
xii Introduction
There are two variants of SPB, one using the 802.1ad Q - in -
Q datapath — shortest path bridging VID (SPBV) — and one using the
hierarchical 802.1ah MAC - in - MAC datapath — shortest path bridging
MAC (SPBM). SPBV and SPBM share a control plane, algorithms, and
common routing mechanisms; where the term “ SPB ” is used, this
describes aspects common to both variants.
The authors embarked on their journey motivated by the issues of
highly scalable networks intended for deployment by Service Provid-
ers, a path that lead to a precursor of SPBM, known as Provider Link
State Bridging, or PLSB. A signifi cant number of the topics discussed
are more relevant to a technology supporting virtualization, and the
reader should therefore expect a signifi cant focus on SPBM, both
because of its highly scalable support of virtualization and, as a corol-
lary, because of from where the authors came.
The fi rst, and substantially shorter, part of this book summarizes
succinctly and informally what SPB “ is ” today, with the aim of offer-
ing a reader new to the technology a consistent mental model of what
it does and how it does it. The second part provides the rationale for
why SPB is as it is, and has to be so. This therefore not only includes
a post hoc rationalization of SPB with the 20:20 vision of hindsight,
but also describes some of the blind alleys explored in getting there,
because these alleys give additional insight into why SPB has to be
what it is.
We start with a short history of SPB and its antecedents, with only
the briefest allusions to the motivations for SPB at this stage (Chapter
1 , “ IEEE 802.1aq in a Nutshell; Antecedents and Technology ” ). We
then offer a short description of SPB as it is now (three sections on
“ SPB technology: The Control Plane, ” starting on p. 15). These are
succinct, but capture the key principles and attributes of SPB. We
nonetheless anticipate that readers will fi nish this with more questions
than answers, such as:
• Why is congruence so important?
• How big a network can you really make?
• Why is this really so different from other network
technologies?
The rest of the book sets out to answer these questions.
We start by considering the key requirements that the different
networking scenarios present (the section on “ The Problem Space, ”
Introduction xiii
p. 37). As a way of introducing the constraints and degrees of freedom
offered by the Ethernet baseline, we follow this with a summary of our
progress toward SPB as documented here (“ H istory,” p. 52). Unlike
subsequent chapters which organize material by topic, this is a
chronological record that talks through some of the twists on the
journey.
W e then revisit some fundamental principles of Ethernet, showing
why we decided to stick with some key bridging constructs even though
the adoption of a control plane meant that they were no longer strictly
mandatory. We also reinterpret the use of some other key Ethernet
concepts, such as the Virtual LAN (VLAN), always within their strict
specifi cations, but in ways possibly contrary to received wisdom on
their usage.
In the section on “ Rounding Out Design Details ” (p. 69), we focus
on overall networking challenges beyond basic functionality that have
to be addressed to make a technology deployable. There is a discussion
of data plane instrumentation, the OAM (deliberately short, to make
an important point), followed by more extensive discussion on dual-
homing for resiliency, always a thorny issue for Ethernet.
The section in Chapter 3 on “ The Control Plane Is as Simple as It
Can Be, but No Simpler” (p. 74) shows that essentially all SPB func-
tionality can be delivered by the routing system. We fi rst discuss SPB’ s
most radical departure from previous received wisdom, the complete
elimination of signaling from both unicast and multicast state installa-
tion. We then provide a factual introduction to the extensions to IS - IS
required by SPB, showing how modest these are. Finally, we explain
some of the algorithmic innovations required by SPB over previous
link state routing practice.
S o far, the exposition has assumed point- t o- p oint connectivity
between bridges in the SPB domain, and ignored the traditional shared
segment. Chapter 4 , “ Practical Deployment Considerations ” (p. 130)
considers this and other topics, because an SPB overlay of anemulated
LAN segment is a real deployment scenario. Although a solution is
described, this is not quite a “ d one deal,” because it needs modest
extensions to the Ethernet forwarding path.
Next, in Chapter 5 , we explore applications of SPB (p. 150), pro-
viding walk- through examples of operation in various deployment sce-
narios covering the delivery of Metro Ethernet Forum defi ned services
by carriers, and the use of SPB in enterprise applications. Because
Ethernet has been very widely deployed in these applications in the