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Network Working Group Z. Chen
Internet-Draft X. Xu
Intended status: Standards Track Huawei Technologies
Expires: September 14, 2017 March 13, 2017
Avoiding Traffic Black-Holes for Route Aggregation in IS-IS
draft-chen-isis-black-hole-avoid-00
Abstract
When the Intermediate System to Intermediate System (IS-IS) routing
protocol is adopted by a highly symmetric network such as the Leaf-
Spine or Fat-Tree network, the Leaf nodes (e.g., Top of Rack switches
in datacenters) are recommended to be prevented from receiving other
nodes' explicit routes in order to achieve scalability. However,
such a setup would cause traffic black-holes or suboptimal routing if
link failure happens in the network. This document extends IS-IS to
solve this problem.
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].
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 http://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 September 14, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Description . . . . . . . . . . . . . . . . . . . . . 3
3. IS-IS Extensions . . . . . . . . . . . . . . . . . . . . . . 4
3.1. TLV Encoding . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Unreachable Prefixes Advertisement . . . . . . . . . . . 5
4. Alternative Solution . . . . . . . . . . . . . . . . . . . . 6
5. IPv6 Support . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
When running the Intermediate System to Intermediate System (IS-IS)
routing protocol in a highly symmetric network such as the Leaf-Spine
or Fat-Tree network, the Leaf nodes (e.g., Top of Rack switches in
datacenters) are recommended to be prevented from receiving other
nodes' explicit routes in order to achieve scalability, as proposed
in [IS-IS-SL-Extension], [IS-IS-Overhead-Reduction], [RIFT], and
[OpenFabric]. In particular, each Leaf node SHOULD simply maintain a
default (or aggregated) route (e.g., 0.0.0.0/0) in its routing table,
of which the next hop SHOULD be an Equal Cost Multi Path (ECMP) group
including all Spines nodes that the Leaf node connects to. However,
such a setup would cause traffic black-holes or suboptimal routing if
link failure happens in the network, since the Leaf nodes are not
aware of any topology information.
To solve this problem, this document extends IS-IS to advertise
unreachable prefixes, which are defined as the prefixes that a
default (or aggregated) route's next hop can no longer reach. When
link failure happens between a Spine node and a Leaf node, the Spine
node SHOULD advertise all prefixes attached to the Leaf node (i.e.,
the unreachable prefixes) to every other Leaf node it connects to.
On receiving the unreachable prefixes, each Leaf node SHOULD add the
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unreachable prefixes to its routing table, thus avoiding traffic
black-holes and suboptimal routing.
2. Problem Description
This section illustrates why link failure would cause traffic black-
hole or suboptimal routing when Leaf nodes only maintain default (or
aggregated) routes.
+--------+ +--------+ +--------+
| Spine1 | | Spine2 | | Spine3 |
+-+-+-+-++ +-+-+-+-++ +-+-+-+-++
+------+ | | | | | | | | | | |
| +------|-|-|-------------+ | | | | | | X
| | +----|-|-|---------------|-|-|-------------+ | | X
| | | | | | +------+ | | | | X
| | | | | | | +------|-|---------------+ | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | | | +-------+ +-----+
| | | | | | | | | +---------|-------------+ |
| | | | | | | | +---------+ | | |
| | | | | +--------|-|----------------|-|-----------+ | |
| | | | +----------|-|--------------+ | | | | |
| | | +----------+ | | | | | | | |
+-+-+-+-+ +-+-+-+-+ +-+-+-+-+ +-+-+-+-+
| Leaf1 | | Leaf2 | | Leaf3 | | Leaf4 |
+-------+ +-------+ +-------+ +-------+
| |
--- ---
prefixA prefixB
Figure 1: Topology Example
Figure 1 shows a Spine-Leaf topology example where Leaf1 to Leaf4 are
connected to Spine1 to Spine3, and prefixA and prefixB are attached
to Leaf4. To achieve scalability, as proposed in [IS-IS-SL-
Extension], [IS-IS-Overhead-Reduction], [RIFT], and [OpenFabric],
Leaf1 to Leaf4 SHOULD NOT receive explict routes from each other nor
the Spine nodes. Instead, each of them maintains a default (or
aggregated) route (e.g., 0.0.0.0/0) in the routing table, of which
the next hop is an ECMP group including Spine1, Spine2, and Spine3.
Flows from one Leaf node to another are shared among Spine1, Spine2,
and Spine3 based on the well known 5-tuple hashing.
However, such a setup would cause traffic black-hole or suboptimal
routing when link failure happens in the network. For example, if
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the link between Spine3 and Leaf4 is broken, Leaf1, Leaf2, and Leaf3
could not get aware of the failure. As a result, these Leaf nodes
will still send a portion of traffic destined for prefixA or prefixB
toward Spine3, which makes the traffic be discarded at Spine3,
causing traffic black-hole. On the other hand, if there is a higher
tier of switches interconnecting Spine1, Spine2, and Spine3, the
traffic will be steered up to the higher-tier switches by Spine3,
causing suboptimal routing.
Therefore, this document extends IS-IS to advertise unreachable
prefixes thus solving this problem.
3. IS-IS Extensions
3.1. TLV Encoding
This document introduces one IS-IS TLV to advertise unreachable
prefixes, called the IP Unreachability TLV, which SHOULD be carried
in the IS-IS Link State Packet (LSP). The format of the IP
Unreachability TLV is shown as follow:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (1 or 2 or 3 or 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Sub-TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (1 or 2 or 3 or 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Sub-TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of this TLV are defined as follows:
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Type: TBD.
Length: Length of the Value field of the TLV.
Reserved: Bits reserved for future usage.
Prefix Length: The value can be 0 to 32, indicating the number of
effective bits in the Prefix field.
Prefix: Encoding the unreachable prefix in the minimal number of
octets for the given number of effective bits (i.e., the Prefix
Length field). The remaining bits of prefix SHOULD be set zero
and ignored upon receipt.
Sub-TLV Length: Length of Sub-TLVs.
Sub-TLVs: Optional Sub-TLVs for future extension.
Note that the last four fields can appear repeatedly.
3.2. Unreachable Prefixes Advertisement
When link failure happens between a Spine node and a Leaf node, the
Spine node SHOULD 1) encode all prefixes attached to the Leaf node
(i.e., the unreachable prefixes) into the IP Unreachability TLV, 2)
append the IP Unreachability TLV to the IS-IS LSP, and 3) send the
LSP to every other Leaf node it connects to.
When a Leaf node receives unreachable prefixes (contained in a LSP)
advertised by a Spine node, it SHOULD install each of the unreachable
prefixes into its routing table, of which the next hop SHOULD be set
an ECMP group including all Spine nodes it connects to except the one
who advertises the unreachable prefix.
For example, if the link between Spine3 and Leaf4 in Figure 1 is
broken, Spine3 SHOULD advertise prefixA and prefixB to Leaf1, Leaf2,
and Leaf3, by sending them an IS-IS LSP containing the IP
Unreachability TLV. On receiving the LSP, Leaf1, Leaf2, and Leaf3
SHOULD install prefixA and prefixB into their routing tables, and the
next hop of prefixA or prefixB SHOULD be set an ECMP group including
Spine1 and Spine2. For instance, the routing table of Leaf1 before
and after the link failure is shown in Figure 2 and Figure 3,
respectively.
Note that the mechanism described above could achieve minimal
signaling latency, which helps to avoid black-hole or suboptimal
routing rapidly when link failure happens.
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+-----------+-----+---+----+-----+-------+--------------+
|Destination|Proto|Pre|Cost|Flags|NextHop|Interface |
+-----------+-----+---+----+-----+-------+--------------+
|0.0.0.0/0 |ISIS |15 |20 |D |Spine1 |Ethernet0/0/0 |
| |ISIS |15 |20 |D |Spine2 |Ethernet0/0/1 |
| |ISIS |15 |20 |D |Spine3 |Ethernet0/0/2 |
+-----------+-----+---+----+-----+-------+--------------+
Figure 2: Routing Table of Leaf1 before link failure
+-----------+-----+---+----+-----+-------+--------------+
|Destination|Proto|Pre|Cost|Flags|NextHop|Interface |
+-----------+-----+---+----+-----+-------+--------------+
|0.0.0.0/0 |ISIS |15 |20 |D |Spine1 |Ethernet0/0/0 |
| |ISIS |15 |20 |D |Spine2 |Ethernet0/0/1 |
| |ISIS |15 |20 |D |Spine3 |Ethernet0/0/2 |
+-----------+-----+---+----+-----+-------+--------------+
|prefixA |ISIS |15 |20 |D |Spine1 |Ethernet0/0/0 |
| |ISIS |15 |20 |D |Spine2 |Ethernet0/0/1 |
+-----------+-----+---+----+-----+-------+--------------+
|prefixB |ISIS |15 |20 |D |Spine1 |Ethernet0/0/0 |
| |ISIS |15 |20 |D |Spine2 |Ethernet0/0/1 |
+-----------+-----+---+----+-----+-------+--------------+
Figure 3: Routing Table of Leaf1 after link failure
4. Alternative Solution
The unreachable prefixes can alternatively be encoded as a new Sub-
TLV of the Extended IP Reachability TLV defined in [RFC 5305]. The
format of the Sub-TLV is shown as follow:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (1 or 2 or 3 or 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length (1 octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (1 or 2 or 3 or 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields of this Sub-TLV are defined as follows:
Type: TBD.
Length: Length of the Value field of the Sub-TLV.
Reserved: Bits reserved for future usage.
Prefix Length: The value can be 0 to 32, indicating the number of
effective bits in the Prefix field.
Prefix: Encoding the unreachable prefix in the minimal number of
octets for the given number of effective bits (i.e., the Prefix
Length field). The remaining bits of prefix SHOULD be set zero
and ignored upon receipt.
Note that the last two fields can appear repeatedly.
When link failure happens between a Spine node and a Leaf node, the
Spine node SHOULD 1) encode all prefixes attached to the Leaf node
(i.e., the unreachable prefixes) into the Sub-TLV described above, 2)
encode the Sub-TLV into the Extended IP Reachability TLV, 3) append
the Extended IP Reachability TLV to the IS-IS LSP, and 4) send the
LSP to every other Leaf node it connects to. The Prefix field of the
Extended IP Reachability TLV SHOULD be set the default (or
aggregated) route that each of the Leaf nodes already maintains.
When a Leaf node receives unreachable prefixes (contained in a LSP)
advertised by a Spine node, it SHOULD install each of the unreachable
prefixes into its routing table, of which the next hop SHOULD be set
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an ECMP group including all Spine nodes it connects to except the one
who advertises the unreachable prefix.
5. IPv6 Support
Will be completed in the next version of the document.
6. IANA Considerations
TBD.
7. Security Considerations
TBD.
8. Acknowledgements
TBD.
9. References
[IS-IS-Overhead-Reduction]
Chen, Z. and X. Xu, "Overheads Reduction for IS-IS Enabled
Spine-Leaf Networks", draft-chen-isis-sl-overheads-
reduction-00 (work in progress) , January 2017.
[IS-IS-SL-Extension]
Shen, N. and S. Thyamagundalu, "IS-IS Routing for Spine-
Leaf Topology", draft-shen-isis-spine-leaf-ext-02 (work in
progress) , October 2016.
[OpenFabric]
White, R. and S. Zandi, "OpenFabric", draft-white-
openfabric-00 (work in progress) , March 2017.
[RFC1195] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
Dual Environments", RFC 1195 , December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305 , October 2008.
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[RIFT] Przygienda, T., Drake, J., and A. Atlas, "RIFT: Routing in
Fat Trees", draft-przygienda-rift-01 (work in progress) ,
January 2017.
Authors' Addresses
Zhe Chen
Huawei Technologies
No. 156 Beiqing Rd
Beijing 100095
China
Email: chenzhe17@huawei.com
Xiaohu Xu
Huawei Technologies
No. 156 Beiqing Rd
Beijing 100095
China
Email: xuxiaohu@huawei.com
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