Internet DRAFT - draft-ct-mt-considerations-dc-fabrics
draft-ct-mt-considerations-dc-fabrics
Network Working Group U. Chunduri
Internet-Draft Huawei Technologies
Intended status: Informational J. Tantsura
Expires: May 3, 2018 Individual
October 30, 2017
Multi Topology Considerations for DC Fabrics
draft-ct-mt-considerations-dc-fabrics-00
Abstract
This document analyzes IS-IS Multi Topology (MT) considerations in
general and specific to layer-3 Data Center (DC) proposals based on
IS-IS protocol. This document explores various IS-IS address
families, topologies and considerations while choosing the right
combination for a specific DC deployment.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 3, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
Chunduri & Tantsura Expires May 3, 2018 [Page 1]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Need for multiple topologies in fabric . . . . . . . . . . . 3
3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Topologies and Address Families . . . . . . . . . . . . . . . 3
4.1. Single Topology Mode and Multiple Address Families . . . 3
4.2. Multiple Topology Mode and Multiple Address Families . . 5
4.2.1. Transition Mode . . . . . . . . . . . . . . . . . . . 5
4.3. IPv6 Only Topology . . . . . . . . . . . . . . . . . . . 6
5. Openfabric . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
ISIS originally developed for OSI [ISO10589], extensions have been
made available to support IPv4 [RFC1195]. A method for exchanging
IPv6 routing information using the IS-IS routing protocol is
specified in [RFC5308]. How to run a set of independent IP
topologies with topology specific adjacencies, within a single IS-IS
domain has been defined in IS-IS MT [RFC5120].
Various layer-3 DC fabric routing options (refs: openfabric, spine-
leaf, controller-based) by changing or optimizing some aspects w.r.t
adjacency formation, flooding optimizations, or/and mechanisms to
automatically compute the location of the node in the fat tree
topology are proposed recently and this document brings some of the
multi topology deployment aspects relevant to these networks. Please
note part of the discussion around IS-IS MT is not specific to DC or
CLOS fabrics and generally applicable to any IS-IS deployment but
discussed here because of multiple proposals to use various forms of
IS-IS in this context.
Chunduri & Tantsura Expires May 3, 2018 [Page 2]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
2. Need for multiple topologies in fabric
In the spirit of DC fabric just provide reachability, only one
address family (either IPv4 or IPv6) SHOULD be sufficient. However
if either only IPv6 address family is needed in the underlay or
deploying both IPv4 and IPv6 address families are desired discussion
in Section 4 is relevant.
It is an unlikely requirement, where DC fabric to be partitioned
logically to have different topologies in the underlay. If one does
to meet a particular requirement, this does introduce manageability
complexity of these logical topologies. IS-IS MT [RFC 5120] also
designed to address the above need and discussion in Section 4.2 is
relevant. It is worth noting, majority of the IS-IS deployments (non
DC) use MT primarily to have a separate logical topology for IPv6
address family.
3. Acronyms
IIH : IS-IS Hello Protocol Data Unit
LSP : Link State PDU
MT : Multi Topology
SPF : Shortest Path First
4. Topologies and Address Families
Terminology around IS-IS topologies and address families is somewhat
confusing at best. Just to give an example, MT ID #2 defined in [RFC
5120] says, it is "Reserved for IPv6 routing topology". While
multiple MT ID's can be deployed in a network with IPv6 topologies,
MT ID #2, perhaps referring to a first such topology with IPv6 only
address family. This section details various topology and address
family options possible with currently available IS-IS specifications
with respective defined TLVs.
4.1. Single Topology Mode and Multiple Address Families
IS-IS with IPv4 address family and with wide-metrics [RFC 5305] is
widely deployed, with TLV 22 defined for IS Reachability and TLV 135
for IP (IPv4) reachability information . This is essentially a single
topology for the entire IS-IS area/domain with a single address
family (IPv4 unicast).
IS-IS can also be enabled with IPv6 unicast address family in a
single topology mode along with IPv4 unicast address family. Here
Chunduri & Tantsura Expires May 3, 2018 [Page 3]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
IPv6 uses the same underlying topology that is used for IPv4 and this
can be done as specified in IS-IS IPv6 [RFC5308] which introduces TLV
236, an IPv6 reachability TLV. It is important to note same IS-IS
adjacency is used for both address families and with a single SPF
(decision process) both IPv4 and IPv6 reachability would be computed.
However, for the above to work effectively, both IPv4 and IPv6
address families MUST share a common network topology. That is to
use IS-IS for IPv4 and IPv6 routing, any interface configured for
IPv4 IS-IS MUST also be configured for IPv6 IS-IS, and vice versa.
All routers within an IS-IS area (Level 1 routing) or domain (Level 2
routing) MUST also support the same set of address families: IPv4
only, IPv6 only, or both IPv4 and IPv6. Any discrepancy in the
configuration w.r.t above can cause routing black holes and one such
scenario is discussed below.
| / \|
...Rx Ry...
| \ /
| \ / |
| \ / |
| /\ |
| / \ |
| / \|
... R1 R2...
| \ / |
Figure 1: IS-IS with multiple address families
As shown, in the above diagram all routers in the network enabled
with both IPv4 and IPv6 unicast address families at the IS level and
single topology would be built. However, at a link level all but
except one link, say if IPv6 is not configured on the link between
the routers Rx and R2; due to a single IS-IS topology, the shortest
path between Rx and R2 is the direct link and since IPv6 is not
enabled on that link, Rx and R2 cannot exchange IPv6 data traffic
even though there's an alternate path between them in the topology
through Rx, R1, Ry and R2.
Hence to summarize the restrictions, as laid out above: all routers
in the topology MUST support only IPv4, only IPv6 or both IPv4 and
IPv6 address families on all links and node. In other words, network
MUST be congruent. While this model is to simpler to operate, might
not be flexible enough for some IS-IS deployments.
Chunduri & Tantsura Expires May 3, 2018 [Page 4]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
4.2. Multiple Topology Mode and Multiple Address Families
Multi-topology IS-IS uses multiple SPFs to compute routes and removes
the restriction that all interfaces MUST support all configured
address families and that all routers in an IS-IS area or domain MUST
support the same set of address families. This introduces the
concept of topology specific adjacency with MT IS Reachability TLV
222 and MT capable IPv4 Reachability with TLV 235 and MT capable IPv6
Reachability with TLV 237.
When MT IS-IS is enabled with IPv4 and IPv6 address families, the
routers build two topologies, one for each address family (IPv4 and
IPv6) and can find the optimum path for each address family even when
some links in the network support only one of them. IS-IS MT [RFC
5120] defines MT ID #0 for backward compatibility, as the "standard"
topology and this essentially operate as IS-IS single topology mode
as specified in Section 4.1 and supports both IPv4 and IPv6 address
families. MT ID #2 [RFC 5120] is defined for IPv6 address family in
MT mode.
4.2.1. Transition Mode
Most of the vendors supported MT transition feature (though some
vendors disabled to avoid confusion around this) in the IS-IS
networks to facilitate MT deployments without disrupting the single
topology mode. The MT transition mode allows a network operating in
single topology IS-IS IPv6 [RFC 5308] to continue to work while
upgrading routers to include MT IS-IS IPv6 support i.e., MT ID #2
with [RFC 5120] . While in transition mode, both types of TLVs
(single-topology with TLV 236 and MT with TLV 237) are sent in LSPs
for all configured IPv6 addresses, nodes can continue to operate in
single topology mode though being in MT mode ("standard" IS-IS
topology with MT ID #0). After all routers in the area or domain
have been upgraded to support MT IPv6 transition mode can be removed
from the configuration. Once all routers in the area or domain are
operating in MT IPv6 mode, the topological restrictions of single-
topology mode are no longer in effect.
When transition mode is enabled, the router advertises both MT TLVs
and the old style IS-IS IPv6 TLVs but the topological restrictions of
the single topology mode discussed above are in effect with MT
transition mode. However, there were instances while this is enabled
and folks expect a different result in the actual deployments.
Chunduri & Tantsura Expires May 3, 2018 [Page 5]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
4.3. IPv6 Only Topology
Though it is theoretically possible to build IPv6 only underlay (with
TLV 236 for IPv6 reachability prefixes) in single topology mode as
discussed in Section 4.1, lot of legacy implementations require IPv4
address families too be configured in single topology mode (ingrained
code structures for IPv4 address family). IPv6 only DC underlay
network can be built with multi topology adjacencies (TLV 222) and
reachability prefixes (TLV 237) with MT ID #2 as discussed above in
Section 4.2. With this any other address family can be introduced
including "standard" topology MT ID #0 (Single topology mode with
both address families) and there are no restrictions on which address
family has to enable on which link as specified in Section 4.1.
5. Openfabric
What is needed for Openfabric (TBD). Depends on Section 2; however
following apply -
o Topology specific modified adjacency formation process (if both
IPv4 and IPv6 address families are used with MT).
o Mechanisms for determining which tier within a spine and leaf
fabric in which the IS is located MUST be specific to MT ID #
(TBD).
o Advertisement of Reachability Information as specified in the
above section is specific to if MT is enabled or not.
o TBD.
6. Acknowledgements
Thanks to .. TBD.
7. IANA Considerations
This document has no actions for IANA.
8. Security Considerations
This document does not introduce any change in any of the IS-IS
protocol or IS-IS protocol extensions. This document also does not
introduce any new security issues other than as noted in the
referenced IS-IS protocol extensions.
Chunduri & Tantsura Expires May 3, 2018 [Page 6]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
Authors' Addresses
Uma Chunduri
Huawei Technologies
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: uma.chunduri@huawei.com
Chunduri & Tantsura Expires May 3, 2018 [Page 7]
Internet-DraftMulti Topology Considerations for DC Fabrics October 2017
Jeff Tantsura
Individual
USA
Email: jefftant.ietf@gmail.con
Chunduri & Tantsura Expires May 3, 2018 [Page 8]
