RIFT WG Yuehua. Wei
Internet-Draft Zheng. Zhang
Intended status: Standards Track ZTE Corporation
Expires: May 6, 2020 Dmitry. Afanasiev
Yandex
Tom. Verhaeg
Interconnect Services B.V.
Jaroslaw. Kowalczyk
Orange Polska
November 3, 2019
RIFT Applicability
draft-wei-rift-applicability-02
Abstract
This document discusses the properties, applicability and operational
considerations of RIFT in different network scenarios. It intends to
provide a rough guide how RIFT can be deployed to simplify routing
operations in Clos topologies and their variations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement of Routing in Modern IP Fabric Fat Tree
Networks . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability of RIFT to Clos IP Fabrics . . . . . . . . . . 3
3.1. Overview of RIFT . . . . . . . . . . . . . . . . . . . . 3
3.2. Applicable Topologies . . . . . . . . . . . . . . . . . . 5
3.2.1. Horizontal Links . . . . . . . . . . . . . . . . . . 6
3.2.2. Vertical Shortcuts . . . . . . . . . . . . . . . . . 6
3.3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. DC Fabrics . . . . . . . . . . . . . . . . . . . . . 6
3.3.2. Metro Fabrics . . . . . . . . . . . . . . . . . . . . 7
3.3.3. Building Cabling . . . . . . . . . . . . . . . . . . 7
3.3.4. Internal Router Switching Fabrics . . . . . . . . . . 7
3.3.5. CloudCO . . . . . . . . . . . . . . . . . . . . . . . 7
4. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
4.1. South Reflection . . . . . . . . . . . . . . . . . . . . 10
4.2. Suboptimal Routing on Link Failures . . . . . . . . . . . 10
4.3. Black-Holing on Link Failures . . . . . . . . . . . . . . 12
4.4. Zero Touch Provisioning (ZTP) . . . . . . . . . . . . . . 13
4.5. Miscabling Examples . . . . . . . . . . . . . . . . . . . 13
4.6. IPv4 over IPv6 . . . . . . . . . . . . . . . . . . . . . 16
4.7. In-Band Reachability of Nodes . . . . . . . . . . . . . . 17
4.7.1. Reachability of Leafs . . . . . . . . . . . . . . . . 17
4.7.2. Reachability of Spines . . . . . . . . . . . . . . . 17
4.8. Dual Homing Servers . . . . . . . . . . . . . . . . . . . 17
4.9. Fabric With A Controller . . . . . . . . . . . . . . . . 18
4.9.1. Controller Attached to ToFs . . . . . . . . . . . . . 19
4.9.2. Controller Attached to Leaf . . . . . . . . . . . . . 19
4.10. Internet Connectivity Without Underlay . . . . . . . . . 19
4.10.1. Internet Default on the Leafs . . . . . . . . . . . 19
4.10.2. Internet Default on the ToFs . . . . . . . . . . . . 20
4.11. Subnet Mismatch and Address Families . . . . . . . . . . 20
4.12. Anycast Considerations . . . . . . . . . . . . . . . . . 20
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
7. Normative References . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
This document intends to explain the properties and applicability of
RIFT [I-D.ietf-rift-rift] in different deployment scenarios and
highlight the operational simplicity of the technology compared to
traditional routing solutions. It also documents special
considerations when RIFT is used with or without overlays,
controllers and corrects topology miscablings and/or node and link
failures.
2. Problem Statement of Routing in Modern IP Fabric Fat Tree Networks
Clos and Fat-Tree topologies have gained prominence in today's
networking, primarily as result of the paradigm shift towards a
centralized data-center based architecture that is poised to deliver
a majority of computation and storage services in the future.
Today's current routing protocols were geared towards a network with
an irregular topology and low degree of connectivity originally.
When they are applied to Fat-Tree topologies:
o they tend to need extensive configuration or provisioning during
bring up and re-dimensioning.
o spine and leaf nodes have the entire network topology and routing
information, which is in fact, not needed on the leaf nodes during
normal operation.
o significant Link State PDUs (LSPs) flooding duplication between
spine nodes and leaf nodes occurs during network bring up and
topology updates. It consumes both spine and leaf nodes' CPU and
link bandwidth resources and with that limits protocol
scalability.
3. Applicability of RIFT to Clos IP Fabrics
Further content of this document assumes that the reader is familiar
with the terms and concepts used in OSPF [RFC2328] and IS-IS
[ISO10589-Second-Edition] link-state protocols and at least the
sections of RIFT [I-D.ietf-rift-rift] outlining the requirement of
routing in IP fabrics and RIFT protocol concepts.
3.1. Overview of RIFT
RIFT is a dynamic routing protocol for Clos and fat-tree network
topologies. It defines a link-state protocol when "pointing north"
and path-vector protocol when "pointing south".
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It floods flat link-state information northbound only so that each
level obtains the full topology of levels south of it. That
information is never flooded East-West or back South again. So a top
tier node has full set of prefixes from the SPF calculation.
In the southbound direction the protocol operates like a "fully
summarizing, unidirectional" path vector protocol or rather a
distance vector with implicit split horizon whereas the information
propagates one hop south and is 're-advertised' by nodes at next
lower level, normally just the default route.
+-----------+ +-----------+
| ToF | | ToF | LEVEL 2
+ +-----+--+--+ +-+--+------+
| | | | | | | | | ^
+ | | | +-------------------------+ |
Distance | +-------------------+ | | | | |
Vector | | | | | | | | +
South | | | | +--------+ | | | Link+State
+ | | | | | | | | Flooding
| | | +-------------+ | | | North
v | | | | | | | | +
+-+--+-+ +------+ +-------+ +--+--+-+ |
|SPINE | |SPINE | | SPINE | | SPINE | | LEVEL 1
+ ++----++ ++---+-+ +--+--+-+ ++----+-+ |
+ | | | | | | | | | ^ N
Distance | +-------+ | | +--------+ | | | E
Vector | | | | | | | | | +------>
South | +-------+ | | | +-------+ | | | |
+ | | | | | | | | | +
v ++--++ +-+-++ ++-+-+ +-+--++ +
|LEAF| |LEAF| |LEAF| |LEAF | LEVEL 0
+----+ +----+ +----+ +-----+
Figure 1: Rift overview
A middle tier node has only information necessary for its level,
which are all destinations south of the node based on SPF
calculation, default route and potential disaggregated routes.
RIFT combines the advantage of both Link-State and Distance Vector:
o Fastest Possible Convergence
o Automatic Detection of Topology
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o Minimal Routes/Info on TORs
o High Degree of ECMP
o Fast De-commissioning of Nodes
o Maximum Propagation Speed with Flexible Prefixes in an Update
And RIFT eliminates the disadvantages of Link-State or Distance
Vector:
o Reduced and Balanced Flooding
o Automatic Neighbor Detection
So there are two types of link state database which are "north
representation" N-TIEs and "south representation" S-TIEs. The N-TIEs
contain a link state topology description of lower levels and S-TIEs
carry simply default routes for the lower levels.
There are a bunch of more advantages unique to RIFT listed below
which could be understood if you read the details of RIFT
[I-D.ietf-rift-rift].
o True ZTP
o Minimal Blast Radius on Failures
o Can Utilize All Paths Through Fabric Without Looping
o Automatic Disaggregation on Failures
o Simple Leaf Implementation that Can Scale Down to Servers
o Key-Value Store
o Horizontal Links Used for Protection Only
o Supports Non-Equal Cost Multipath and Can Replace MC-LAG
o Optimal Flooding Reduction and Load-Balancing
3.2. Applicable Topologies
Albeit RIFT is specified primarily for "proper" Clos or "fat-tree"
structures, it already supports PoD concepts which are strictly
speaking not found in original Clos concepts.
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Further, the specification explains and supports operations of multi-
plane Clos variants where the protocol relies on set of rings to
allow the reconciliation of topology view of different planes as most
desirable solution making proper disaggregation viable in case of
failures. This observations hold not only in case of RIFT but in the
generic case of dynamic routing on Clos variants with multiple planes
and failures in bi-sectional bandwidth, especially on the leafs.
3.2.1. Horizontal Links
RIFT is not limited to pure Clos divided into PoD and multi-planes
but supports horizontal links below the top of fabric level. Those
links are used however only as routes of last resort northbound when
a spine loses all northbound links or cannot compute a default route
through them.
A possible configuration is a "ring" of horizontal links at a level.
In presence of such a "ring" in any level (except ToF level) neither
N-SPF nor S-SPF will provide a "ring-based protection" scheme since
such a computation would have to deal necessarily with breaking of
"loops" in Dijkstra sense; an application for which RIFT is not
intended.
A full-mesh connectivity between nodes on the same level can be
employed and that allows N-SPF to provide for any node loosing all
its northbound adjacencies (as long as any of the other nodes in the
level are northbound connected) to still participate in northbound
forwarding.
3.2.2. Vertical Shortcuts
Through relaxations of the specified adjacency forming rules RIFT
implementations can be extended to support vertical "shortcuts" as
proposed by e.g. [I-D.white-distoptflood]. The RIFT specification
itself does not provide the exact details since the resulting
solution suffers from either much larger blast radii with increased
flooding volumes or in case of maximum aggregation routing bow-tie
problems.
3.3. Use Cases
3.3.1. DC Fabrics
RIFT is largely driven by demands and hence ideally suited for
application in underlay of data center IP fabrics, vast majority of
which seem to be currently (and for the foreseeable future) Clos
architectures. It significantly simplifies operation and deployment
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of such fabrics as described in Section 4 for environments compared
to extensive proprietary provisioning and operational solutions.
3.3.2. Metro Fabrics
The demand for bandwidth is increasing steadily, driven primarily by
environments close to content producers (server farms connection via
DC fabrics) but in proximity to content consumers as well. Consumers
are often clustered in metro areas with their own network
architectures that can benefit from simplified, regular Clos
structures and hence RIFT.
3.3.3. Building Cabling
Commercial edifices are often cabled in topologies that are either
Clos or its isomorphic equivalents. With many floors the Clos can
grow rather high and with that present a challenge for traditional
routing protocols (except BGP and by now largely phased-out PNNI)
which do not support an arbitrary number of levels which RIFT does
naturally. Moreover, due to limited sizes of forwarding tables in
active elements of building cabling the minimum FIB size RIFT
maintains under normal conditions can prove particularly cost-
effective in terms of hardware and operational costs.
3.3.4. Internal Router Switching Fabrics
It is common in high-speed communications switching and routing
devices to use fabrics when a crossbar is not feasible due to cost,
head-of-line blocking or size trade-offs. Normally such fabrics are
not self-healing or rely on 1:/+1 protection schemes but it is
conceivable to use RIFT to operate Clos fabrics that can deal
effectively with interconnections or subsystem failures in such
module. RIFT is neither IP specific and hence any link addressing
connecting internal device subnets is conceivable.
3.3.5. CloudCO
The Cloud Central Office (CloudCO) is a new stage of telecom Central
Office. It takes the advantage of Software Defined Networking (SDN)
and Network Function Virtualization (NFV) in conjunction with general
purpose hardware to optimize current networks. The following figure
illustrates this architecture at a high level. It describes a single
instance or macro-node of cloud CO. An Access I/O module faces a
Cloud CO Access Node, and the CPEs behind it. A Network I/O module
is facing the core network. The two I/O modules are interconnected
by a leaf and spine fabric. [TR-384]
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+---------------------+ +----------------------+
| Spine | | Spine |
| Switch | | Switch |
+------+---+------+-+-+ +--+-+-+-+-----+-------+
| | | | | | | | | | | |
| | | | | +-------------------------------+ |
| | | | | | | | | | | |
| | | | +-------------------------+ | | |
| | | | | | | | | | | |
| | +----------------------+ | | | | | | | |
| | | | | | | | | | | |
| +---------------------------------+ | | | | | | |
| | | | | | | | | | | |
| | | +-----------------------------+ | | | | |
| | | | | | | | | | | |
| | | | | +--------------------+ | | | |
| | | | | | | | | | | |
+--+ +-+---+--+ +-+---+--+ +--+----+--+ +-+--+--+ +--+
|L | | Leaf | | Leaf | | Leaf | | Leaf | |L |
|S | | Switch | | Switch | | Switch | | Switch| |S |
++-+ +-+-+-+--+ +-+-+-+--+ +--+-+--+--+ ++-+--+-+ +-++
| | | | | | | | | | | | | |
| +-+-+-+--+ +-+-+-+--+ +--+-+--+--+ ++-+--+-+ |
| |Compute | |Compute | | Compute | |Compute| |
| |Node | |Node | | Node | |Node | |
| +--------+ +--------+ +----------+ +-------+ |
| || VAS5 || || vDHCP|| || vRouter|| ||VAS1 || |
| |--------| |--------| |----------| |-------| |
| |--------| |--------| |----------| |-------| |
| || VAS6 || || VAS3 || || v802.1x|| ||VAS2 || |
| |--------| |--------| |----------| |-------| |
| |--------| |--------| |----------| |-------| |
| || VAS7 || || VAS4 || || vIGMP || ||BAA || |
| |--------| |--------| |----------| |-------| |
| +--------+ +--------+ +----------+ +-------+ |
| |
++-----------+ +---------++
|Network I/O | |Access I/O|
+------------+ +----------+
Figure 2: An example of CloudCO architecture
The Spine-Leaf architectures deployed inside CloudCO meets the
network requirements of adaptable, agile, scalable and dynamic.
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4. Deployment Considerations
RIFT presents the opportunity for organizations building and
operating IP fabrics to simplify their operation and deployments
while achieving many desirable properties of a dynamic routing on
such a substrate:
o RIFT design follows minimum blast radius and minimum necessary
epistemological scope philosophy which leads to very good scaling
properties while delivering maximum reactiveness.
o RIFT allows for extensive Zero Touch Provisioning within the
protocol. In its most extreme version RIFT does not rely on any
specific addressing and for IP fabric can operate using IPv6 ND
[RFC4861] only.
o RIFT has provisions to detect common IP fabric mis-cabling
scenarios.
o RIFT negotiates automatically BFD per link allowing this way for
IP and micro-BFD [RFC7130] to replace LAGs which do hide bandwidth
imbalances in case of constituent failures. Further automatic
link validation techniques similar to [RFC5357] could be supported
as well.
o RIFT inherently solves many difficult problems associated with the
use of traditional routing topologies with dense meshes and high
degrees of ECMP by including automatic bandwidth balancing, flood
reduction and automatic disaggregation on failures while providing
maximum aggregation of prefixes in default scenarios.
o RIFT reduces FIB size towards the bottom of the IP fabric where
most nodes reside and allows with that for cheaper hardware on the
edges and introduction of modern IP fabric architectures that
encompass e.g. server multi-homing.
o RIFT provides valley-free routing and with that is loop free.
This allows the use of any such valley-free path in bi-sectional
fabric bandwidth between two destination irrespective of their
metrics which can be used to balance load on the fabric in
different ways.
o RIFT includes a key-value distribution mechanism which allows for
many future applications such as automatic provisioning of basic
overlay services or automatic key roll-overs over whole fabrics.
o RIFT is designed for minimum delay in case of prefix mobility on
the fabric.
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o Many further operational and design points collected over many
years of routing protocol deployments have been incorporated in
RIFT such as fast flooding rates, protection of information
lifetimes and operationally easily recognizable remote ends of
links and node names.
4.1. South Reflection
South reflection is a mechanism that South Node TIEs are "reflected"
back up north to allow nodes in same level without E-W links to "see"
each other.
For example, Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs
from ToF21 to ToF22 separately. Respectively,
Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs from ToF22
to ToF21 separately. So ToF22 and ToF21 see each other's node
information as level 2 nodes.
In an equivalent fashion, as the result of the south reflection
between Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122,
Spine121 and Spine 122 knows each other at level 1.
4.2. Suboptimal Routing on Link Failures
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+--------+ +--------+
| ToF21 | | ToF22 | LEVEL 2
++--+-+-++ ++-+--+-++
| | | | | | | +
| | | | | | | linkTS8
+-------------+ | +-+linkTS3+-+ | | | +--------------+
| | | | | | + |
| +----------------------------+ | linkTS7 |
| | | | + + + |
| | | +-------+linkTS4+------------+ |
| | | + + | | |
| | | +------------+--+ | |
| | | | | linkTS6 | |
+-+----++ ++-----++ ++------+ ++-----++
|Spin111| |Spin112| |Spin121| |Spin122| LEVEL 1
+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
| | | | | | | |
| +--------------+ | + ++XX+linkSL6+---+ +
| | | | linkSL5 | | linkSL8
| +------------+ | | + +---+linkSL7+-+ | +
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+-+--+
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ ++------+ +-----+-+ +-+-----+
+ + + +
Prefix111 Prefix112 Prefix121 Prefix122
Figure 3: Suboptimal routing upon link failure use case
As shown in Figure 3, as the result of the south reflection between
Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122, Spine121 and
Spine 122 knows each other at level 1.
Without disaggregation mechanism, when linkSL6 fails, the packet from
leaf121 to prefix122 will probably go up through linkSL5 to linkTS3
then go down through linkTS4 to linkSL8 to Leaf122 or go up through
linkSL5 to linkTS6 then go down through linkTS4 and linkSL8 to
Leaf122 based on pure default route. It's the case of suboptimal
routing or bow-tieing.
With disaggregation mechanism, when linkSL6 fails, Spine122 will
detect the failure according to the reflected node S-TIE from
Spine121. Based on the disaggregation algorithm provided by RIFT,
Spine122 will explicitly advertise prefix122 in Disaggregated Prefix
S-TIE PrefixesElement(prefix122, cost 1). The packet from leaf121 to
prefix122 will only be sent to linkSL7 following a longest-prefix
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match to prefix 122 directly then go down through linkSL8 to Leaf122
.
4.3. Black-Holing on Link Failures
+--------+ +--------+
| ToF 21 | | ToF 22 | LEVEL 2
++-+--+-++ ++-+--+-++
| | | | | | | |
| | | | | | | linkTS8
+--------------+ | +--linkTS3-X+ | | | +--------------+
linkTS1 | | | | | | |
| +-----------------------------+ | linkTS7 |
| | | | | | | |
| | linkTS2 +--------linkTS4-X-----------+ |
| | | | | | | |
| linkTS5 +-+ +---------------+ | |
| | | | | linkTS6 | |
+-+----++ +-+-----+ ++----+-+ ++-----++
|Spin111| |Spin112| |Spin121| |Spin122| LEVEL 1
+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
| | | | | | | |
| +---------------+ | | +----linkSL6----+ |
linkSL1 | | | linkSL5 | | linkSL8
| +---linkSL3---+ | | | +----linkSL7--+ | |
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+-+--+
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ ++------+ +-----+-+ +-+-----+
+ + + +
Prefix111 Prefix112 Prefix121 Prefix122
Figure 4: Black-holing upon link failure use case
This scenario illustrates a case when double link failure occurs and
with that black-holing can happen.
Without disaggregation mechanism, when linkTS3 and linkTS4 both fail,
the packet from leaf111 to prefix122 would suffer 50% black-holing
based on pure default route. The packet supposed to go up through
linkSL1 to linkTS1 then go down through linkTS3 or linkTS4 will be
dropped. The packet supposed to go up through linkSL3 to linkTS2
then go down through linkTS3 or linkTS4 will be dropped as well.
It's the case of black-holing.
With disaggregation mechanism, when linkTS3 and linkTS4 both fail,
ToF22 will detect the failure according to the reflected node S-TIE
of ToF21 from Spine111\Spine112\Spine121\Spine122. Based on the
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disaggregation algorithm provided by RITF, ToF22 will explicitly
originate an S-TIE with prefix 121 and prefix 122, that is flooded to
spines 111, 112, 121 and 122.
The packet from leaf111 to prefix122 will not be routed to linkTS1 or
linkTS2. The packet from leaf111 to prefix122 will only be routed to
linkTS5 or linkTS7 following a longest-prefix match to prefix122.
4.4. Zero Touch Provisioning (ZTP)
Each RIFT node may operate in zero touch provisioning (ZTP) mode. It
has no configuration (unless it is a Top-of-Fabric at the top of the
topology or it is desired to confine it to leaf role w/o leaf-2-leaf
procedures). In such case RIFT will fully configure the node's level
after it is attached to the topology.
The most import component for ZTP is the automatic level derivation
procedure. All the Top-of-Fabric nodes are explicitly marked with
TOP_OF_FABRIC flag which are initial 'seeds' needed for other ZTP
nodes to derive their level in the topology. The derivation of the
level of each node happens then based on LIEs received from its
neighbors whereas each node (with possibly exceptions of configured
leafs) tries to attach at the highest possible point in the fabric.
This guarantees that even if the diffusion front reaches a node from
"below" faster than from "above", it will greedily abandon already
negotiated level derived from nodes topologically below it and
properly peer with nodes above.
4.5. Miscabling Examples
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+----------------+ +-----------------+
| ToF21 | +------+ ToF22 | LEVEL 2
+-------+----+---+ | +----+---+--------+
| | | | | | | | |
| | | +----------------------------+ |
| +---------------------------+ | | | |
| | | | | | | | |
| | | | +-----------------------+ | |
| | +------------------------+ | | |
| | | | | | | | |
+-+---+-+ +-+---+-+ | +-+---+-+ +-+---+-+
|Spin111| |Spin112| | |Spin121| |Spin122| LEVEL 1
+-+---+-+ ++----+-+ | +-+---+-+ ++----+-+
| | | | | | | | |
| +---------+ | link-M | +---------+ |
| | | | | | | | |
| +-------+ | | | | +-------+ | |
| | | | | | | | |
+-+---+-+ +--+--+-+ | +-+---+-+ +--+--+-+
|Leaf111| |Leaf112+-----+ |Leaf121| |Leaf122| LEVEL 0
+-------+ +-------+ +-------+ +-------+
Figure 5: A single plane miscabling example
Figure Figure 5 shows a single plane miscabling example. It's a
perfect fat-tree fabric except link-M connecting Leaf112 to ToF22.
The RIFT control protocol can discover the physical links
automatically and be able to detect cabling that violates fat-tree
topology constraints. It react accordingly to such mis-cabling
attempts, at a minimum preventing adjacencies between nodes from
being formed and traffic from being forwarded on those mis-cabled
links. Leaf112 will in such scenario use link-M to derive its level
(unless it is leaf) and can report links to spines 111 and 112 as
miscabled unless the implementations allows horizontal links.
Figure Figure 6 shows a multiple plane miscabling example. Since
Leaf112 and Spine121 belong to two different PoDs, the adjacency
between Leaf112 and Spine121 can not be formed. link-W would be
detected and prevented.
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+-------+ +-------+ +-------+ +-------+
|ToF A1| |ToF A2| |ToF B1| |ToF B2| LEVEL 2
+-------+ +-------+ +-------+ +-------+
| | | | | | | |
| | | +-----------------+ | | |
| +--------------------------+ | | | |
| | | | | | | |
| +------+ | | | +------+ |
| | +-----------------+ | | | | |
| | | +--------------------------+ | |
| A | | B | | A | | B |
+-----+-+ +-+---+-+ +-+---+-+ +-+-----+
|Spin111| |Spin112| +----+Spin121| |Spin122| LEVEL 1
+-+---+-+ ++----+-+ | +-+---+-+ ++----+-+
| | | | | | | | |
| +---------+ | | | +---------+ |
| | | | link-W | | | |
| +-------+ | | | | +-------+ | |
| | | | | | | | |
+-+---+-+ +--+--+-+ | +-+---+-+ +--+--+-+
|Leaf111| |Leaf112+------+ |Leaf121| |Leaf122| LEVEL 0
+-------+ +-------+ +-------+ +-------+
+--------PoD#1----------+ +---------PoD#2---------+
Figure 6: A multiple plane miscabling example
RIFT provides an optional level determination procedure in its Zero
Touch Provisioning mode. Nodes in the fabric without their level
configured determine it automatically. This can have possibly
counter-intuitive consequences however. One extreme failure scenario
is depicted in Figure 7 and it shows that if all northbound links of
spine11 fail at the same time, spine11 negotiates a lower level than
Leaf11 and Leaf12.
To prevent such scenario where leafs are expected to act as switches,
LEAF_ONLY flag can be set for Leaf111 and Leaf112. Since level -1 is
invalid, Spine11 would not derive a valid level from the topology in
Figure 7. It will be isolated from the whole fabric and it would be
up to the leafs to declare the links towards such spine as miscabled.
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+-------+ +-------+ +-------+ +-------+
|ToF A1| |ToF A2| |ToF A1| |ToF A2|
+-------+ +-------+ +-------+ +-------+
| | | | | |
| +-------+ | | |
+ + | | ====> | |
X X +------+ | +------+ |
+ + | | | |
+----+--+ +-+-----+ +-+-----+
|Spine11| |Spine12| |Spine12|
+-+---+-+ ++----+-+ ++----+-+
| | | | | |
| +---------+ | | |
| | | | | |
| +-------+ | | +-------+ |
| | | | | |
+-+---+-+ +--+--+-+ +-----+-+ +-----+-+
|Leaf111| |Leaf112| |Leaf111| |Leaf112|
+-------+ +-------+ +-+-----+ +-+-----+
| |
| +--------+
| |
+-+---+-+
|Spine11|
+-------+
Figure 7: Fallen spine
4.6. IPv4 over IPv6
RIFT allows advertising IPv4 prefixes over IPv6 RIFT network. IPv6
AF configures via the usual ND mechanisms and then V4 can use V6
nexthops analogous to RFC5549. It is expected that the whole fabric
supports the same type of forwarding of address families on all the
links. RIFT provides an indication whether a node is v4 forwarding
capable and implementations are possible where different routing
tables are computed per address family as long as the computation
remains loop-free.
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+-----+ +-----+
+---+---+ | ToF | | ToF |
^ +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
V6 +-----+ +-+---+
Forwarding |SPINE| |SPINE|
| +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
v +-----+ +-+---+
+---+---+ |LEAF | | LEAF|
+--+--+ +--+--+
| |
IPv4 prefixes| |IPv4 prefixes
| |
+---+----+ +---+----+
| V4 | | V4 |
| subnet | | subnet |
+--------+ +--------+
Figure 8: IPv4 over IPv6
4.7. In-Band Reachability of Nodes
4.7.1. Reachability of Leafs
TODO
4.7.2. Reachability of Spines
TODO
4.8. Dual Homing Servers
Each RIFT node may operate in zero touch provisioning (ZTP) mode. It
has no configuration (unless it is a Top-of-Fabric at the top of the
topology or the must operate in the topology as leaf and/or support
leaf-2-leaf procedures) and it will fully configure itself after
being attached to the topology.
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+---+ +---+ +---+
|ToF| |ToF| |ToF|
+---+ +---+ +---+
| | | | | |
| +----------------+ | |
| | | | | |
| +----------------+ |
| | | | | |
+----------+--+ +--+----------+
| Spine|ToR1 | | Spine|ToR2 |
+--+------+---+ +--+-------+--+
+---+ | | | | | | +---+
| | | | | | | |
| +-----------------+ | | |
| | | +-------------+ | |
+ | + | | |-----------------+ |
X | X | +--------x-----+ | X |
+ | + | | | + |
+---+ +---+ +---+ +---+
| | | | | | | |
+---+ +---+ ...............+---+ +---+
SV(1) SV(2) SV(n+1) SV(n)
Figure 9: Dual-homing servers
In the single plane, the worst condition is disaggregation of every
other servers at the same level. Suppose the links from ToR1 to all
the leaves become not available. All the servers' routes are
disaggregated and the FIB of the servers will be expanded with n-1
more spicific routes.
Sometimes, pleople may prefer to disaggregate from ToR to servers
from start on, i.e. the servers have couple tens of routes in FIB
from start on beside default routes to avoid breakages at rack level.
Full disaggregation of the fabric could be achieved by configuration
supported by RIFT.
4.9. Fabric With A Controller
There are many different ways to deploy the controller. One
possibility is attaching a controller to the RIFT domain from ToF and
another possibility is attaching a controller from the leaf.
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+------------+
| Controller |
++----------++
| |
| |
+----++ ++----+
---------- | ToF | | ToF |
| +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
+-----+ +-+---+
RIFT domain |SPINE| |SPINE|
+--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
| +-----+ +-+---+
---------- |LEAF | | LEAF|
+-----+ +-----+
Figure 10: Fabric with a controller
4.9.1. Controller Attached to ToFs
If a controller is attaching to the RIFT domain from ToF, it usually
uses dual-homing connections. The loopback prefix of the controller
should be advertised down by the ToF and spine to leaves. If the
controller loses link to ToF, make sure the ToF withdraw the prefix
of the controller(use different mechanisms).
4.9.2. Controller Attached to Leaf
If the controller is attaching from a leaf to the fabric, no special
provisions are needed.
4.10. Internet Connectivity Without Underlay
4.10.1. Internet Default on the Leafs
TODO
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4.10.2. Internet Default on the ToFs
TODO
4.11. Subnet Mismatch and Address Families
+--------+ +--------+
| | LIE LIE | |
| A | +----> <----+ | B |
| +---------------------+ |
+--------+ +--------+
X/24 Y/24
Figure 11: subnet mismatch
LIEs are exchanged over all links running RIFT to perform Link
(Neighbor) Discovery. A node MUST NOT originate LIEs on an address
family if it does not process received LIEs on that family. LIEs on
same link are considered part of the same negotiation independent on
the address family they arrive on. An implementation MUST be ready
to accept TIEs on all addresses it used as source of LIE frames.
As shown in the above figure, without further checks adjacency of
node A and B may form, but the forwarding between node A and node B
may fail because subnet X mismatches with subnet Y.
To prevent this a RIFT implementation should check for subnet
mismatch just like e.g. ISIS does. This can lead to scenarios where
an adjacency, despite exchange of LIEs in both address families may
end up having an adjacency in a single AF only. This is a
consideration especially in Section 4.6 scenarios.
4.12. Anycast Considerations
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+ traffic
|
v
+------+------+
| ToF |
+---+-----+---+
| | | |
+------------+ | | +------------+
| | | |
+---+---+ +-------+ +-------+ +---+---+
| | | | | | | |
|Spine11| |Spine12| |Spine21| |Spine22| LEVEL 1
+-+---+-+ ++----+-+ +-+---+-+ ++----+-+
| | | | | | | |
| +---------+ | | +---------+ |
| | | | | | | |
| +-------+ | | | +-------+ | |
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+--+-+
| | | | | | | |
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ ++------+ +-----+-+ +-----+-+
+ + + ^ |
PrefixA PrefixB PrefixA | PrefixC
|
+ traffic
Figure 12: Anycast
If the traffic comes from ToF to Leaf111 or Leaf121 which has anycast
prefix PrefixA. RIFT can deal with this case well. But if the
traffic comes from Leaf122, it will always get to Leaf121 and never
get to Leaf111. If the intension is that the traffic should been
offloaded to Leaf111, then use policy guided prefixes [PGP
reference].
5. Acknowledgements
6. Contributors
The following people (listed in alphabetical order) contributed
significantly to the content of this document and should be
considered co-authors:
Tony Przygienda
Juniper Networks
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1194 N. Mathilda Ave
Sunnyvale, CA 94089
US
Email: prz@juniper.net
7. Normative References
[I-D.ietf-rift-rift]
Przygienda, T., Sharma, A., Thubert, P., and D. Afanasiev,
"RIFT: Routing in Fat Trees", draft-ietf-rift-rift-08
(work in progress), September 2019.
[I-D.white-distoptflood]
White, R., Hegde, S., and S. Zandi, "IS-IS Optimal
Distributed Flooding for Dense Topologies", draft-white-
distoptflood-01 (work in progress), September 2019.
[ISO10589-Second-Edition]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", Nov 2002.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
[RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
Forwarding Detection (BFD) on Link Aggregation Group (LAG)
Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
2014, <https://www.rfc-editor.org/info/rfc7130>.
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[TR-384] Broadband Forum Technical Report, "TR-384 Cloud Central
Office Reference Architectural Framework", Jan 2018.
Authors' Addresses
Yuehua Wei
ZTE Corporation
No.50, Software Avenue
Nanjing 210012
P. R. China
Email: wei.yuehua@zte.com.cn
Zheng Zhang
ZTE Corporation
No.50, Software Avenue
Nanjing 210012
P. R. China
Email: zzhang_ietf@hotmail.com
Dmitry Afanasiev
Yandex
Email: fl0w@yandex-team.ru
Tom Verhaeg
Interconnect Services B.V.
Email: t.verhaeg@interconnect.nl
Jaroslaw Kowalczyk
Orange Polska
Email: jaroslaw.kowalczyk2@orange.com
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