Supporting leaves without northbound neighbors connecting to a fat-tree network using RIFT
draft-zwx-rift-leaf-ring-01
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| Authors | Zheng Zhang , Yuehua Wei , BenChong Xu | ||
| Last updated | 2022-07-03 | ||
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draft-zwx-rift-leaf-ring-01
Internet Engineering Task Force Z. Zhang
Internet-Draft Y. Wei
Intended status: Standards Track B. Xu
Expires: 4 January 2023 ZTE Corporation
3 July 2022
Supporting leaves without northbound neighbors connecting to a fat-tree
network using RIFT
draft-zwx-rift-leaf-ring-01
Abstract
This document discusses the usage and solution for leaf nodes without
northbound neighbors connecting to a fat-tree network by leaf nodes
having direct northbound neighbors in RIFT.
Status of This Memo
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This Internet-Draft will expire on 4 January 2023.
Copyright Notice
Copyright (c) 2022 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Capability advertisement . . . . . . . . . . . . . . . . 6
3.2. Prefix transferring . . . . . . . . . . . . . . . . . . . 6
3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[I-D.ietf-rift-rift] specifies a dynamic routing protocol for Clos
and fat-tree network topology. It suits most of the deployments. In
some situations, the leaves are connected by ring or chain topology,
and some of them may have no northbound link, so these nodes may not
be able to connecting the network. This document discusses the usage
and proposes a solution.
1.1. 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 RFC 2119 [RFC2119].
2. Problem Statement
[I-D.ietf-rift-rift] specifies a dynamic routing protocol for Clos
and fat-tree network topology. The leaf part of a traditional Clos
and fat-tree network topology is shown as:
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+ + +
| N1 | N2 | N3
| | |
+--+----+ +--+----+ +--+-----+
| | | | | |
| S01 +----------+ S02 +----------+ S03 | Level 1
++-+-+--+ ++--+--++ +---+-+-++
| | | | | | | | |
| | +----------------------------------+ | | |
| | | | | | | | |
| +-------------+ | | | +--------------+ |
| | | | | | | | |
| +----------------+ | +-----------------+ |
| | | | | | | | |
| | +------------------------------------+ | |
| | | | | | | | |
++-+-+--+ | +---+---+ | +-+---+-++
| | +-+ +-+ | |
| L01 |----------| L02 |----------| L03 | Level 0
+-------+ +-------+ +--------+
Figure 1
In most of the cases, each leaf node has north connections with at
least one spine, and there may be east-west links between leaf nodes.
In case a leaf node lost all the north connections, it can still
access the network through the east-west link between leaves. For
example in Figure 1, there is an east-west link between L01 and L02,
and there is an east-west link between L02 and L03. When the
northbound connections for the leaves are all work well, L01, L02 and
L03 may generate a Prefix South TIE with default route and advertise
it through the east-west links according to the definition in section
4.2.3.4 in [I-D.ietf-rift-rift]. In case L01 lost all the northbound
links with S01, S02 and S03, according to Northbound SPF algorithm
defined in section 4.2.4.1 in [I-D.ietf-rift-rift], L01 can compute
the next hop L02 for the default route. On the other hand, the
prefix of L01 can be flood northbound by L02. Then L01 can still
access the network through the east-west link between L01 and L02.
But in some deployments, the leaves may connect with each other by
ring topology (sometimes, a chain topology), and not all of them have
northbound connection with spine nodes.
For example, in the IP Radio Access Network (IP RAN, mobile backhaul
network), the 4G eNB or the 5G gNB connect to an IP access network of
a ring topology. The access network attaches to an aggregation
network through two aggregation nodes. In 5G era, the aggregation
network and the metro network is envolving to Spine-and-Leaf
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architecture to take the advantage of Spine-and-Leaf. Figure 2
depicts an digram of an IPRAN network with Spine-and-Leaf
architecture. If the aggregation network runs RIFT, using the
proposal of this draft, the access network ring does not need to
deploy other IGP protocol to enable the routing.
+----+
|gNB |
+--+-+
|
+--+-+
+-----+CSG4+---- --+
| +----+ | Aggregation
+---+ +--+-+ +-+--+ Network +-----+ xxxxx
|eNB+---+CSG1| | +-----------+Spine+--xxxx xxxx
+---+ +--+-+ |Leaf| +----++ xxx
| | | | xx
| +----+------------+ | x
| Access Network | | Metro Network x
| +----+------------+---+ x
| | | | xx
+---+ +--+-+ |Leaf| ++----+ xxx
|gNB+---+CSG2| | +-----------+Spine+--xxxx xxxx
+---+ +--+-+ +-+--+ +-----+ xxxxx
| +----+ |
+-----+CSG3+---- --+
+--+-+
|
+--+-+
|eBN |
+----+
Figure 2: IPRAN network with Spine-and-Leaf architecture
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+ + +
| N1 | N2 | N3
| | |
+--+----+ +--+----+ +--+-----+
| | | | | |
| S01 +----------+ S02 +----------+ S03 | Level 1
++-+-+--+ ++--+--++ +---+-+-++
| | | | | | | | |
| | +----------------------------------+ | | |
| | | | | | | | |
| +-------------+ | | | +--------------+ |
| | | | | | | | |
| +----------------+ | +-----------------+ |
| | | | | | | | |
| | +------------------------------------+ | |
| | | | | | | | |
++-+-+--+ | +---+---+ | +-+---+-++
| | +-+ +-+ | |
+--| L01 |--------- | L02 |----------| L03 |--+
| +-------+ +-------+ +--------+ |
| |
| | Level 0
| ++-+-+--+ ++-+-+--+ ++-+-+--+ |
| | | | | | | |
+--| L04 |---------| L05 |-----------| L06 |---+
+-------+ +-------+ +-------+
Figure 3
Figure 3 is an abstract of Figure 2, several leaves connect to the
fat-tree network by ring topology, each leaf has two east-west
connections with other leaves, but only L01, L02 and L03 have
northbound connection with spine nodes. L01, L02 and L03 advertise a
Prefix South TIE with default route through the east-west links. L04
and L06 can access the network through L01 and L03. But L04 and L06
do not generate or flood the Prefix South TIE with default route
because they have no northbound link. So L05 cannot receive the
Prefix South TIE with default route through the link between L04 and
L05, or the link between L05 and L06. On the other hand, the prefix
of L05 cannot be flooded east-west by L04 and L05. So L05 cannot
send flow to other leaf, and other leaf cannot send flow to L05 also.
L05 cannot access the network.
This document discuss the extension that can be used to solve the
problem when some leaves without northbound neighbors connecting to a
fat-tree network.
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3. Solution
3.1. Capability advertisement
A new link capability which is named leaf-transport is set in LIE and
node TIE. The new capability indicates that the leaf node can
transfer the Prefix TIE received from the east-west link to the other
leaf node.
The LIE FSM will not be affected if only one leaf supports the
capability and the neighbor does not support.
The capability can be used for diagnosis in case there is something
wrong when computing route.
3.2. Prefix transferring
When a leaf node which has no northbound connection receives Prefix
TIE from a neighbor by an east-west link, it can transfer the Prefix
TIE to the other neighbor by the other east-west link, with increased
metric. The increased metric should be the sum of the received
metric and the metric of the east-west link.
The Prefix TIE can be the default route and the prefix of the leaf
node, other prefixes can also be transferred. The network
administrator can control the prefix transferring by policy.
Prefix South TIE transferring
The leaf node without northbound neighbors which supports the
leaf transfer capability, MUST transfer the Prefix South TIE
received from an east-west neighbor to the other east-west
neighbor which also has no northbound connection.
Prefix North TIE transferring
The leaf node without northbound neighbors which supports the
leaf transfer capability, MUST transfer the Prefix North TIE
received from an east-west neighbor which has no northbound
connection to the other east-west neighbor.
External Prefix North TIE transferring
The leaf node without northbound neighbors which supports the
leaf transfer capability, MUST transfer the External Prefix
North TIE received from an east-west neighbor which has no
northbound connection to the other east-west neighbor.
The leaf node without northbound neighbors which supports the leaf
transfer capability, MAY transfer the Prefix TIE from an east-west
neighbor to the other east-west neighbor which has northbound
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connection. According to Northbound SPF algorithm defined in section
4.2.4.1 in [I-D.ietf-rift-rift], the transferring does not affect the
routing calculating result of the neighbors which has northbound
connection.
3.3. Example
In figure 2, either L04 or L06, or both of them advertise the leaf-
transport capability in LIE to L05 when they are forming an
adjacency. And the leaf-transport capability is also set in node TIE
when L04 or L06 advertise the TIE.
L04/L06 receives Prefix South TIE with default route from L01/L03,
L04/L06 transfers the route through Prefix South TIE with increased
metric to L05.
L04/L06 receives Prefix North TIE of L05 and transfers the route
through Prefix North TIE with increased metric to L01/L03.
L05 receives the Prefix South TIE from L04/L06, after N-SPF
calculation, L05 calculates the default route with next hop L04/L06.
L01/L03 receives prefix of L05 from L04/L06, and L01/03 floods the
Prefix North TIE northbound. Then L05 can send flow to other leaf
through L04/L06. The flow sent by other leaf with destination set to
the prefix of L05 can also be routed to L05. Then L05 can access the
network.
4. IANA Considerations
A new type for 'leaf-transfer' is requested in link capability.
5. Security Considerations
When the node without northbound neighbors supports the function
defined in this document, there may be unnecessary Prefix TIE
advertisement, and unnecessary prefix may be leaked.
6. References
6.1. Normative References
[I-D.ietf-rift-rift]
Sharma, A., Thubert, P., Rijsman, B., Afanasiev, D., and
A. Przygienda, "RIFT: Routing in Fat Trees", Work in
Progress, Internet-Draft, draft-ietf-rift-rift-15, 3
January 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rift-rift-15>.
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[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>.
Authors' Addresses
Zheng Zhang
ZTE Corporation
China
Email: zhang.zheng@zte.com.cn
Yuehua Wei
ZTE Corporation
China
Email: wei.yuehua@zte.com.cn
Benchong Xu
ZTE Corporation
China
Email: xu.benchong@zte.com.cn
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