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draft-ietf-lsvr-applicability-15_241211.xml
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<?xml version="1.0" encoding="US-ASCII"?>
<?rfc toc="yes"?>
<?rfc tocompact="yes"?>
<?rfc tocdepth="3"?>
<?rfc tocindent="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<rfc category="info" docName="draft-ietf-lsvr-applicability-15"
ipr="trust200902" submissionType="IETF" tocDepth="5" version="2">
<front>
<title abbrev="BGP-SPF Applicability">Usage and Applicability of Link
State Vector Routing in Data Centers</title>
<author fullname="Keyur Patel" initials="K" surname="Patel">
<organization>Arrcus, Inc.</organization>
<address>
<postal>
<street>2077 Gateway Pl</street>
<city>San Jose, CA</city>
<country>USA</country>
<code>95110</code>
</postal>
<phone/>
<email>[email protected]</email>
</address>
</author>
<author fullname="Acee Lindem" initials="A" surname="Lindem">
<organization>LabN Consulting, L.L.C.</organization>
<address>
<postal>
<street>301 Midenhall Way</street>
<city>Cary, NC</city>
<country>USA</country>
<code>95110</code>
</postal>
<phone/>
<email>[email protected]</email>
</address>
</author>
<author fullname="Shawn Zandi" initials="S" surname="Zandi">
<organization>Linkedin</organization>
<address>
<postal>
<street>222 2nd Street</street>
<city>San Francisco</city>
<region>CA</region>
<code>94105</code>
<country>USA</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author fullname="Gaurav Dawra" initials="G" surname="Dawra">
<organization>Linkedin</organization>
<address>
<postal>
<street>222 2nd Street</street>
<city>San Francisco</city>
<region>CA</region>
<code>94105</code>
<country>USA</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author fullname="Jie Dong" initials="J." surname="Dong">
<organization>Huawei Technologies</organization>
<address>
<postal>
<street>No. 156 Beiqing Road</street>
<city>Beijing</city>
<region/>
<code/>
<country>China</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<date/>
<abstract>
<t>This document discusses the usage and applicability of BGP Link-State
Shortest Path First (BGP-SPF) extensions in data center networks
utilizing Clos or Fat-Tree topologies. The document is intended to
provide a simplified guide for the deployment of BGP-SPF extensions.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document complements <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/> by discussing the applicability of the
BGP-SPF technology in a simple and fairly common deployment scenario,
which is described in <xref format="default" target="usecases"/>.</t>
<t>After describing the deployment scenario, <xref format="default"
target="motivation"/> describes the reasons for BGP modifications for
such deployments.</t>
<t>After the control plane routing protocol requirements are described,
<xref format="default" target="bgpspf"/> covers the BGP-SPF protocol
enhancements to BGP to meet these requirements and their applicability
to Data Center <xref format="default" target="Clos"/> networks.</t>
</section>
<section anchor="reco" title="Recommended Reading">
<t>This document assumes knowledge of existing data center networks and
data center network topologies <xref format="default" target="Clos"/>.
This document also assumes knowledge of data center routing protocols
such as BGP <xref format="default" target="RFC4271"/>, BGP-SPF <xref
format="default" target="I-D.ietf-lsvr-bgp-spf"/>, OSPF <xref
format="default" target="RFC2328"/>, as well as data center OAM
protocols like LLDP <xref format="default" target="RFC4957"/> and BFD
<xref format="default" target="RFC5580"/>.</t>
</section>
<section anchor="usecases" title="Common Deployment Scenario">
<t>Within a Data Center, servers are commonly interconnected using the
Clos topology <xref format="default" target="Clos"/>. The Clos topology
is fully non-blocking and the topology is realized using Equal Cost
Multi-Path (ECMP). In a multi-stage Clos topology, the minimum number of
parallel paths in each tier is determined by the width of the stage as
shown in the figure 1.</t>
<t>
<figure anchor="fig1">
<name>Illustration of the basic Clos</name>
<artwork align="left" alt="" name="" type=""><![CDATA[
Tier-1
+-----+
|NODE |
+->| 12 |--+
| +-----+ |
Tier-2 | | Tier-2
+-----+ | +-----+ | +-----+
+------------>|NODE |--+->|NODE |--+--|NODE |-------------+
| +-----| 9 |--+ | 10 | +--| 11 |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->|NODE |--+ |NODE | +--|NODE |-----+-----+ |
| | | +---| 6 |--+->| 7 |--+--| 8 |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
|NODE | |NODE | Tier-3 +->|NODE |--+ Tier-3 |NODE | |NODE |
| 1 | | 2 | | 3 | | 4 | | 5 |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
A O B O <- Servers -> Z O O O
]]></artwork>
</figure>
</t>
</section>
<section anchor="motivation" title="Justification for BGP-SPF Extension">
<t>To simplify L3 routing and operations <xref format="default"
target="RFC7938"/>, many data centers use BGP as a routing protocol to
create both an underlay and an overlay network for their Clos
Topologies. However, BGP is a path-vector routing protocol. Since it
does not create a fabric topology, it uses hop-by-hop External BGP
(EBGP) peering to facilitate hop-by-hop routing to create the underlay
network and to resolve any overlay next hops. The hop-by-hop BGP peering
paradigm imposes several restrictions within a Clos. It severely
prohibits a deployment of Route Reflectors/Route Controllers as the EBGP
sessions are congruent with the data path. The BGP best-path algorithm
is prefix-based and it prevents announcements of prefixes to other BGP
speakers until the best-path decision process has been performed for the
prefix at each intermediate hop. These restrictions significantly delay
the overall convergence of the underlay network within a Clos
network.</t>
<t>The BGP-SPF modifications allow BGP to overcome these limitations.
Furthermore, using the BGP-LS Network Layer Reachability Information
(NLRI) format <xref target="RFC9552"/> allows the BGP-SPF data to be
advertised for nodes, links, and prefixes in the BGP routing domain and
used for SPF computations.</t>
</section>
<section anchor="bgpspf" title="BGP-SPF Applicability to Clos Networks">
<t>With the BGP-SPF extensions <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/>, the BGP best-path computation and
route computation are replaced with link-state algorithms such as those
used by <xref format="default" target="RFC2328"/>, both to determine
whether an BGP-LS SPF NLRI has changed and needs to be re-advertised and
to compute the BGP routes. These modifications will significantly
improve convergence of the underlay while affording the operational
benefits of a single routing protocol <xref format="default"
target="RFC7938"/>.</t>
<t>Data center controllers typically require visibility to the BGP
topology to compute traffic-engineered paths. These controllers learn
the topology and other relevant information via the BGP-LS address
family <xref format="default" target="RFC9552"/> which is totally
independent of the underlay address families (usually IPv4/IPv6
unicast). Furthermore, in traditional BGP underlays, all the BGP routers
will need to advertise their BGP-LS information independently. With the
BGP-SPF extensions, controllers can learn the topology using the same
BGP advertisements used to compute the underlay routes. Furthermore,
these data center controllers can avail the convergence advantages of
the BGP-SPF extensions. The placement of controllers can be outside of
the forwarding path or within the forwarding path.</t>
<t>Alternatively, as each and every router in the BGP-SPF domain will
have a complete view of the topology, the operator can also choose to
configure BGP sessions in hop-by-hop peering model described in <xref
format="default" target="RFC7938"/> along with BFD <xref
format="default" target="RFC5580"/>. In doing so, while the hop-by-hop
peering model lacks the inherent benefits of the controller-based model,
BGP updates need not be serialized by BGP best-path algorithm in either
of these models. This helps overall network convergence.</t>
<section anchor="lsvrsafi" title="Usage of BGP-LS SPF SAFI">
<t>The Section 5.1 of <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/> defines a new BGP-LS-SPF SAFI for
announcement of BGP-SPF link-state. The NLRI format and its associated
attributes follow the format of BGP-LS for node, link, and prefix
announcements. Whether the peering model within a Clos follows
hop-by-hop peering described in <xref format="default"
target="RFC7938"/> or any controller-based or route-reflector peering,
an operator can exchange BGP-LS SPF SAFI routes over the BGP peering
by simply configuring BGP-LS SPF SAFI between the necessary BGP
speakers.</t>
<t>The BGP-LS SPF SAFI can also co-exist with BGP IP Unicast SAFI
<xref target="RFC4760"/> which could exchange overlapping IP routes.
The reasons for enabling both SAFIs at the same time is out of the
scope of this document. The routes received by these SAFIs are
evaluated, stored, and announced independently according to the rules
of <xref format="default" target="RFC4760"/>. The tie-breaking of
route installation is a matter of the local policies and preferences
of the network operator.</t>
<t>Finally, as the BGP-SPF peering is done following the procedures
described in <xref format="default" target="RFC4271"/>, all the
existing transport security mechanisms including <xref
format="default" target="RFC5925"/> are available for the BGP-LS SPF
SAFI.</t>
<section anchor="other-safi"
title="Relationship to Other BGP AFI/SAFI Tuples">
<t>Normally, the BGP-LS SPF AFI/SAFI is used solely to compute the
underlay and is given preference over other AFI/SAFIs. Other BGP
SAFIs, e.g., IPv6/IPv6 Unicast VPN would use the BGP-SPF computed
routes for next hop resolution.</t>
</section>
</section>
<section anchor="peering" title="Peering Models">
<t>As previously stated, BGP-SPF can be deployed using the existing
peering model where there is a single-hop BGP session on each and
every link in the data center fabric <xref format="default"
target="RFC7938"/>. This provides for both the advertisement of routes
and the determination of link and neighboring switch availability.
With BGP-SPF, the underlay will converge faster due to changes to the
decision process that will allow NLRI changes to be advertised faster
after detecting a change.</t>
<section anchor="sparse-peering" title="Sparse Peering Model">
<t>Alternately, BFD <xref format="default" target="RFC5580"/> can be
used to swiftly determine the availability of links and the BGP
peering model can be significantly sparser than the data center
fabric. BGP-SPF sessions only need to be established with enough
peers to provide a bi-connected graph. If IBGP is used, then the BGP
routers at tier N-1 will act as route-reflectors for the routers at
tier N.</t>
<t>The obvious usage of sparse peering is to avoid parallel BGP
sessions on links between the same two switches in the data center
fabric. However, this use case is not very useful since parallel L3
links between the same two BGP routers are rare in Clos or Fat-Tree
topologies. Additionally, when there are multiple links, they are
often aggregated at the link layer rather than the IP layer. Two
more interesting scenarios are described below.</t>
<t>In current data center topologies, there is often a very dense
mesh of links between levels, e.g., leaf and spine, providing
32-way, 64-way, or more Equal-Cost Multi-Path (ECMP) paths. In these
topologies, it is desirable not to have a BGP session on every link
and techniques such as the one described in <xref format="default"
target="bi-connected"/> can be used to establish sessions on some
subset of northbound links. For example, in a Spine-Leaf topology,
each leaf switch would only peer with a subset of the spines
dependent on the flooding redundancy required to be reasonably
certain that every node within the BGP-LS SPF routing domain has the
complete topology.</t>
<t>Alternately, controller-based data center topologies are
envisioned where BGP speakers within the data center only establish
BGP sessions with two or more controllers. In these topologies,
fabric nodes below the first tier (using <xref format="default"
target="RFC7938"/> hierarchy) will establish BGP multi-hop sessions
with the controllers. For the multi-hop sessions, determining the
route to the controllers without depending on BGP would need to be
through some other means beyond the scope of this document. However,
the BGP discovery mechanisms described in <xref format="default"
target="bgp-discovery"/> would be one possibility.</t>
</section>
<section anchor="bi-connected" title="Bi-Connected Graph Heuristic">
<t>With this heuristic, discovery of BGP peers is assumed, e.g., as
described in <xref format="default" target="bgp-discovery"/>.
Additionally, it assumed that the direction of the peering can be
ascertained. In the context of a data center fabric, direction is
either northbound (toward the spine), southbound (toward the
Top-Of-Rack (TOR) switches) or east-west (same level in hierarchy).
The determination of the direction is beyond the scope of this
document. However, it would be reasonable to assume a technique
where the TOR switches can be identified and the number of hops to
the TOR is used to determine the direction.</t>
<t>In this heuristic, BGP speakers allow passive session
establishment for southbound BGP sessions. For northbound sessions,
BGP speakers will attempt to maintain two northbound BGP sessions
with different switches (in data center fabrics there is normally a
single layer-3 connection anyway). For east-west sessions, passive
BGP session establishment is allowed. However, BGP speaker will
never actively establish an east-west BGP session unless it cannot
establish two northbound BGP sessions.</t>
</section>
</section>
<section anchor="bgp-policy" title="BGP Spine/Leaf Topology Policy">
<t>One of the advantages of using BGP-SPF as the underlay protocol is
that BGP policy can be applied at any level. For example, depending on
the topology, it may be possible to aggregate prefix advertisements
using existing BGP policy. In Spine/Leaf topologies, it is not
necessary to advertise BGP-LS Prefix NLRI received by leaves
northbound to the spine nodes. An aggregate route or a default route
could suffice. If a common AS is used for the spine nodes, this can
easily be accomplished with EBGP and a simple policy to filter
advertisements from the leaves to the spine if the first AS in the AS
path is the spine AS.</t>
<t>In the figure below, the leaves would not advertise any NLRI with
AS 64512 as the first AS in the AS path.</t>
<t>
<figure anchor="fig2">
<name>Spine/Leaf Topology Policy</name>
<artwork align="left" alt="" name="" type=""><![CDATA[
+--------+ +--------+ +--------+
AS 64512 | | | | | |
for Spine | Spine 1+----+ Spine 2+- ......... -+ Spine N|
Nodes at | | | | | |
this Level +-+-+-+-++ ++-+-+-+-+ +-+-+-+-++
+------+ | | | | | | | | | | |
| +-----|-|-|------+ | | | | | | |
| | +--|-|-|--------+-|-|-----------------+ | | |
| | | | | | +---+ | | | | |
| | | | | | | +--|-|-------------------+ | |
| | | | | | | | | | +------+ +----+
| | | | | | | | | +--------------|----------+ |
| | | | | | | | +-------------+ | | |
| | | | | +----|--|----------------|--|--------+ | |
| | | | +------|--|--------------+ | | | | |
| | | +------+ | | | | | | | |
++--+--++ +-+-+--++ ++-+--+-+ ++-+--+-+
| Leaf 1|~~~~~~| Leaf 2| ........ | Leaf X| | Leaf Y|
+-------+ +-------+ +-------+ +-------+
]]></artwork>
</figure>
</t>
</section>
<section anchor="bgp-discovery-req"
title="BGP Peer Discovery Requirements">
<t>The basic requirement is to be able to discover the address of a
single-hop peer in case where the peer address is not pre-configured.
This is being accomplished today with using IPv6 Router Advertisements
(RA) <xref format="default" target="RFC4861"/> and assuming that a BGP
session is desired with any discovered peer. Beyond the basic
requirement, it is useful to have to following information relating to
the BGP session:</t>
<t><list style="symbols">
<t>Autonomous System (AS) and BGP Identifier of a potential peer.
The latter can be used for debugging and to decrease the
likelihood of BGP session establishment collisions.</t>
<t>Security capabilities supported and for cryptographic
authentication, the security capabilities and possibly a key-chain
<xref format="default" target="RFC8177"/> to be used.</t>
<t>Session Policy Identifier - A group number or name used to
associate common session parameters with the peer. For example, in
a data center, BGP sessions with a ToR device could have
parameters than BGP sessions between leaf and spine.</t>
</list>In a data center fabric, it is often useful to know whether a
peer is southbound (towards the servers) or northbound (towards the
spine or super-spine), e.g., <xref format="default"
target="bi-connected"/>. A potential requirement would be to determine
this dynamically. One mechanism, without specifying all the details,
might be for the ToR switches to be identified when installed and for
the others switches in the fabric to determine their level based on
the distance from the closest ToR switch.</t>
<t>If there are multiple links between BGP speakers or the links
between BGP speakers are unnumbered, it is also useful to be able to
establish multi-hop sessions using the loopback addresses. This will
often require the discovery protocol to install route(s) toward the
potential peer loopback addresses prior to BGP session
establishment.</t>
<t>Finally, a simple BGP discovery protocol could also be used to
establish a multi-hop session with one or more controllers by
advertising connectivity to one or more controllers. However, once the
multi-hop session traverses multiple nodes, it is bordering a
distance-vector routing protocol and possibly this is not a good
requirement for the discovery protocol.</t>
</section>
<section anchor="bgp-discovery" title="BGP Peer Discovery">
<section anchor="bgp-ipv6-peering" title="BGP IPv6 Simplified Peering">
<t>To conserve IPv4 address space and simplify operations, BGP-LS
SPF routers in Clos/Fat Tree deployments can use IPv6 addresses as
peer address. For IPv4 address families, IPv6 peering as specified
in <xref format="default" target="RFC8950"/> can be deployed to
avoid configuring IPv4 addresses on BGP- LS SPF router interfaces.
When this is done, dynamic discovery mechanisms, as described in
<xref format="default" target="bgp-discovery"/>, can used to learn
the global or link-local IPv6 peer addresses and IPv4 addresses need
not be configured on these interfaces. If IPv6 link-local peering is
used, then configuration of IPv6 global addresses is also not
required and these IPv6 link-local addresses must then be advertised
in the BGP-LS Link Descriptor IPv6 Address TLV (262) <xref
format="default" target="RFC9552"/>.</t>
</section>
<section anchor="config-checking"
title="BGP-LS SPF Topology Visibility for Management">
<t>Irrespective of whether or not BGP-LS SPF is used for route
calculation, the BGP-LS SPF route advertisements can be used to
periodically construct the Clos/Fat Tree topology. This is
especially useful in deployments where an IGP is not used and the
base BGP-LS routes <xref format="default" target="RFC9552"/> are not
available. The resultant topology visibility can then be used for
troubleshooting and consistency checking. This would normally be
done on a central controller or other management tool which could
also be used for fabric data path verification. The precise
algorithms and heuristics, as well as the complete set of management
applications is beyond the scope of this document.</t>
</section>
<section anchor="dci"
title="Data Center Interconnect (DCI) Applicability">
<t>Since BGP-SPF is to be used for the routing underlay and DCI
gateway boxes typically have direct or very simple connectivity, BGP
external sessions would typically not include the BGP-LS SPF
SAFI.</t>
</section>
</section>
</section>
<section anchor="other-env"
title="Non-CLOS/FAT Tree Topology Applicability">
<t>The BGP-SPF extensions <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/> can be used in other topologies and
avail the inherent convergence improvements. Additionally, sparse
peering techniques may be utilized <xref format="default"
target="peering"/>. However, determining whether to establish a BGP
session is more complex and the heuristic described in <xref
format="default" target="bi-connected"/> cannot be used. In such
topologies, other techniques such as those described in <xref
format="default" target="RFC9667"/> may be employed. One potential
deployment would be the underlay for a Service Provider (SP) backbone
where usage of a single protocol, i.e., BGP, is desired.</t>
</section>
<section anchor="non-transit-node" title="Non-Transit Node Capability">
<t>In certain scenarios, a BGP node wishes to participate in the BGP-SPF
topology but never be used for transit traffic. These include situations
where a server wants to make application services available to clients
homed at subnets throughout the BGP-SPF domain but does not ever want to
be used as a router (i.e., carry transit traffic). Another specific
instance is where a controller is resident on a server and direct
connectivity to the controller is required throughout the entire domain.
This can readily be accomplished using the BGP-LS Node NLRI Attribute
SPF Status TLV as described in <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/>.</t>
</section>
<section anchor="policy" title="BGP Policy Applicability">
<t>Existing BGP policy including aggregation and prefix filtering may be
used in conjunction with the BGP-LS SPF SAFI. When aggregation policy is
used, BGP-LS SPF prefix NLRI will be originated for the aggregate prefix
and BGP-LS SPF prefix NLRI for components will be filtered.
Additionally, link and node NLRI may be filtered and the abstracted by
the prefix NLRI.</t>
<t>When BGP policy is used with the BGP-LS SPF SAFI, BGP speakers in the
BGP-LS SPF routing domain will not all have the same set of NLRI and
will compute a different BGP local routing table. Consequently, care
must be taken to assure routing is consistent and blackholes or routing
loops do not ensue. However, this is no different than if tradition BGP
routing using the IPv4 and IPv6 address families were used.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>No IANA updates are requested by this document.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>This document introduces no new security considerations above and
beyond those already specified in the <xref format="default"
target="RFC4271"/> and <xref format="default"
target="I-D.ietf-lsvr-bgp-spf"/>.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors would like to thank Alvaro Retana, Yan Filyurin, Boris
Hassanov, Stig Venaas, and Ron Bonica for their review and comments.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.I-D.ietf-lsvr-bgp-spf.xml'?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.2328.xml'?>
<?rfc include='reference.RFC.4271.xml'?>
<?rfc include='reference.RFC.4760.xml'?>
<?rfc include='reference.RFC.4861.xml'?>
<?rfc include='reference.RFC.4957.xml'?>
<?rfc include='reference.RFC.5580.xml'?>
<?rfc include='reference.RFC.5925.xml'?>
<?rfc include='reference.RFC.7938.xml'?>
<?rfc include='reference.RFC.8177.xml'?>
<?rfc include='reference.RFC.8950.xml'?>
<?rfc include='reference.RFC.9552.xml'?>
<?rfc include='reference.RFC.9667.xml'?>
<reference anchor="Clos" target="">
<front>
<title>A Study of Non-Blocking Switching Networks</title>
<author initials="" surname="">
<organization/>
</author>
<date month="March" year="1953"/>
</front>
<seriesInfo name=""
value="The Bell System Technical Journal, Vol. 32(2), DOI 10.1002/j.1538-7305.1953.tb01433.x"/>
</reference>
</references>
</back>
</rfc>