| Internet-Draft | RT-Constrain in Hierarchical RR Scenario | March 2024 |
| Dong, et al. | Expires 5 September 2024 | [Page] |
- Workgroup:
- Network Working Group
- Internet-Draft:
- draft-ietf-idr-rtc-hierarchical-rr-04
- Published:
- Intended Status:
- Standards Track
- Expires:
Extensions to RT-Constrain in Hierarchical Route Reflection Scenarios
Abstract
The Route Target (RT) Constrain mechanism specified in RFC 4684 is used to build a route distribution graph in order to restrict the propagation of Virtual Private Network (VPN) routes. In network scenarios where hierarchical route reflection (RR) is used, the existing RT-Constrain mechanism cannot guarantee a correct route distribution graph. This document describes the problem scenario and proposes a solution to address the RT-Constrain issue in hierarchical RR scenarios.¶
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/.¶
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This Internet-Draft will expire on 5 September 2024.¶
Copyright Notice
Copyright (c) 2024 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 (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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Introduction
The Route Target (RT) Constrain mechanism specified in [RFC4684] is used to build a route distribution graph in order to restrict the propagation of Virtual Private Network (VPN) routes. In network scenarios where hierarchical route reflection (RR) is used, the existing advertisment rules of RT membership information as defined in section 3.2 of [RFC4684] cannot guarantee a correct route distribution graph.¶
This document describes the problem scenario and proposes a solution to address the RT-Constrain issue in hierarchical RR scenarios.¶
2. Problem Statement
+---------------------------------+
| +----+ |
| Clu-1 |RR-1| |
| /+----+\ |
| / \ |
| +----+ +----+ |
| Clu-2 |RR-2| |RR-3| Clu-3 |
| +-/--+ +/--\+ |
| / / \ |
| +----+ +----+ +----+ |
| |PE-1| |PE-2| |PE-3| |
| +----+ +----+ +----+ |
| | | | |
+-------|----------|---------|----+
RT-1 | RT-1 | | RT-1
+--------+ +--------+ +--------+
| VPN-1 | | VPN-1 | | VPN-1 |
+--------+ +--------+ +--------+
Figure 1. RT-Constrain with Hierarchical RR
¶
As shown in Figure 1, hierarchical RRs are deployed in the network, RR-2 and RR-3 are route-reflectors of their connecting PEs, and are also the clients of RR-1. If each PE advertises RT membership information of RT-1 to the upstream RR, after the best path selection, both RR-2 and RR-3 would create the CLUSTER_LIST attribute, prepend their local CLUSTER_ID and then advertise the best path to RR-1 and their clients respectively.¶
On receipt of the RT-Constrain routes from RR-2 and RR-3, RR-1 will select one of the received routes as the best route, here assume the route received from RR-2 is selected by RR-1 as the best route. Then RR-1 needs to advertise the best RT-Constrain route to both RR-2 and RR-3 to create the route distribution graph of VPN-1. RR-1 would prepend its CLUSTER_ID to the CLUSTER_LIST of the path, and according to the rules in Section 3.2 of [RFC4684], it sets the ORIGINATOR_ID to its own router-id, and the NEXT_HOP to the local address for the session. Then RR-1 would advertise this route to both RR-2 and RR-3. On receipt of the RT-Constrain route from RR-1, RR-2 checks the CLUSTER_LIST and find its own CLUSTER_ID in the list, so this route will be ignored by RR-2. As a result, RR-2 will not form the outbound filter of RT-1 towards RR-1, hence it will not advertise the VPN routes of VPN-1 to RR-1.¶
3. Potential Solutions
This document specifies 2 potential solutions for the RTC issue in hierarchical RR scenario. In a later revision, one solution will to be selected based on the decision of the IDR working group.¶
3.1. Add-path Based Solution
This section provides one possible solution which is based on the add-path mechanism defined in [RFC7911]. It makes use of the add-path mechanism for RTC route advertisement between the hierarchical RRs. The solution is summerized as follows:¶
-
The route-reflector clients which themselves are also route-reflectors SHOULD be identified, then BGP add-paths [RFC7911] SHOULD be enabled for RT membership NLRI on the BGP sessions between the higher layer RR and the lower layer RRs to ensure that sufficient RT-Constrain routes can be advertised by the higher layer RR to the lower layer RRs to pass BGP loop detection. In this case normal BGP path advertisement rules as defined in [RFC4271] SHOULD be applied. The number of RT-Constrain routes to be advertised with add-path mechanism is a local decision of operators. To ensure that sufficient RT-Constrain routes are advertised to build the distribution graph, the recommended add-path number is the maximum number of the BGP client sessions in the same cluster plus 1.¶
-
When advertising an RT membership NLRI to a route-reflector client which is not a lower layer RR, the advertisement rule as defined in section 3.2 of [RFC4684] SHOULD be applied.¶
With the above advertisement rule, RR-1 in figure 1 SHOULD advertise to RR-2 the RT-Constrain routes received from both RR-2 and RR-3, then the RTC route from RR-3 will pass the BGP loop detection on RR-2, thus the route distribution graph can be set up correctly.¶
3.2. Disjoint Alternate Path Solution
This section specifies another possible solution which proposes some modification to the intra-AS advertisement rule of RTC route.¶
Since the advertisement of RT-Constrain route is to set up a route distribution graph and not to guide the data packet forwarding, actually all the available RT-Constrain routes should be considered in setting up the route distribution graph, not just the best one. Thus the following advertisment rule for RT membership information is proposed to replace the rule i and ii in section 3.2 of [RFC4684]:¶
-
When advertising an RT membership NLRI to a route-reflector peer (either client or non-client), if the best path as selected by the path selection procedure described in Section 9.1 of [RFC4271] is the path received from this peer, and there are alternative paths received from other peers, then the most disjoint alternative route SHOULD be advertised to this peer. The most disjoint alternative path is the path whose CLUSTER_LIST and ORIGINATOR_ID attributes are diverse from the attributes of the best path.¶
With the above advertisement rule, RR-1 in figure 1 would advertise to RR-2 the RT-Constrain route received from RR-3, which is the most disjoint alternative route compared with the best route received from RR-2. In this way, RR-2 will not discard the RT-constrain route received from RR-1, and the route distribution graph can be set up correctly.¶
4. IANA Considerations
This document makes no request of IANA.¶
5. Security Considerations
This document does not change the security properties of BGP based VPNs and [RFC4684].¶
6. Acknowledgements
The authors would like to thank Yaqun Xiao for the discussion of RT-Constrain issue in hierarchical RR scenario. Many people have made valuable comments and suggestions, including Susan Hares, Jeffrey Haas, Stephane Litkowski, Vitkovský Adam, Xiaohu Xu, Uttaro James, Shyam Sethuram, Saikat Ray and Bruno Decraene.¶
7. References
7.1. Normative References
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- [RFC4271]
- Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, , <https://www.rfc-editor.org/info/rfc4271>.
- [RFC4684]
- Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk, R., Patel, K., and J. Guichard, "Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684, , <https://www.rfc-editor.org/info/rfc4684>.
7.2. Informative References
- [RFC7911]
- Walton, D., Retana, A., Chen, E., and J. Scudder, "Advertisement of Multiple Paths in BGP", RFC 7911, DOI 10.17487/RFC7911, , <https://www.rfc-editor.org/info/rfc7911>.
