Network Working Group Z. Chen Internet-Draft X. Xu Intended status: Standards Track Huawei Technologies Expires: March 17, 2018 September 13, 2017 Avoiding Traffic Black-Holes for Route Aggregation in IS-IS draft-chen-isis-black-hole-avoid-01 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 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 March 17, 2018. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. Chen & Xu Expires March 17, 2018 [Page 1] Internet-Draft IS-IS Black-Hole Avoiding September 2017 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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. 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 Chen & Xu Expires March 17, 2018 [Page 2] Internet-Draft IS-IS Black-Hole Avoiding September 2017 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 Chen & Xu Expires March 17, 2018 [Page 3] Internet-Draft IS-IS Black-Hole Avoiding September 2017 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: Chen & Xu Expires March 17, 2018 [Page 4] Internet-Draft IS-IS Black-Hole Avoiding September 2017 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. Chen & Xu Expires March 17, 2018 [Page 5] Internet-Draft IS-IS Black-Hole Avoiding September 2017 +-----------+-----+---+----+-----+-------+--------------+ |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: Chen & Xu Expires March 17, 2018 [Page 6] Internet-Draft IS-IS Black-Hole Avoiding September 2017 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 Chen & Xu Expires March 17, 2018 [Page 7] Internet-Draft IS-IS Black-Hole Avoiding September 2017 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-01 (work in progress) , July 2017. [IS-IS-SL-Extension] Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS Routing for Spine-Leaf Topology", draft-shen-isis-spine- leaf-ext-04 (work in progress) , June 2017. [OpenFabric] White, R. and S. Zandi, "OpenFabric", draft-white- openfabric-02 (work in progress) , April 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, . [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305 , October 2008. Chen & Xu Expires March 17, 2018 [Page 8] Internet-Draft IS-IS Black-Hole Avoiding September 2017 [RIFT] Przygienda, T., Sharma, A., Drake, J., and A. Atlas, "RIFT: Routing in Fat Trees", draft-przygienda-rift-02 (work in progress) , June 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 Chen & Xu Expires March 17, 2018 [Page 9]