INTERNET-DRAFT Enke Chen Tony Bates MCI July 1995 Destination Preference Attribute for BGP Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft. Abstract The Border Gateway Protocol [1] is an inter-autonomous system routing protocol designed for TCP/IP internets. This document describes a new BGP path attribute termed "Destination Preference Attribute" (DPA) which can be used by a single autonomous system (AS) to specify globally transitive metrics in its routing announcement via BGP. The metric can then be used by upstream BGP speakers to favor certain path for return traffic. The application of this attribute includes facilitating the implementation of symmetric routing and load sharing in the multi-provider Internet. Introduction In certain cases there is a need for an autonomous system (AS) to specify a globally transitive preference in its routing announcement Chen & Bates [Page 1] INTERNET-DRAFT DPA Attribute for BGP July 1995 via BGP so that the upstream BGP speakers can use the preference to favor certain path for return traffic. For instance, as discussed in [3], currently it is difficult to implement symmetric routing and load sharing in the multi-provider Internet due to the lack of this preference in BGP. In this paper, we propose a new BGP attribute termed "Destination Preference Attribute" (DPA) to address such a need. More specifically, the DPA is a globally transitive metric that can be used by an AS to specify preference in its routing announcement so that the return traffic favors certain path. As illustrated in [4] through several examples, this metric, combined with AS-based "local_pref" offers much greater flexibility and manageability in implementing symmetric inter-domain routing and load sharing in the multi-provider Internet. Destination Preference Attribute (DPA) This document proposes the DPA path attribute, which is an optional transitive attribute of fixed length. The attribute is represented by a pair . The AS# is a two octet non-negative integer, which denotes the AS that specifies the preference. The DPA value is a four octet non-negative integer. The DPA attribute has Type Code 11. Route Selection Process The DPA attributes are considered comparable only if the DPA attributes are present in all the routes being compared and are set by the same AS. The comparable DPA attributes shall be used as a selection criteria, after the "local_pref" attribute is evaluated, and before the evaluation of the AS path length and the multi-exit-discriminator (MED). Non-comparable DPA attributes shall not be used in the route selection process. The higher the DPA attribute value, the more preferred the route. Operation The DPA attribute should not be used as a replacement for MED. MED should still be used when an AS has multiple connections to a single neighboring AS. Chen & Bates [Page 2] INTERNET-DRAFT DPA Attribute for BGP July 1995 The DPA attribute should only be set when needed. The AS that sets this preference must include its AS number in the attribute. A BGP speaker may use the "local_pref" attribute to prefer a different path other than the one specified by the DPA attribute value. This does not preclude an AS from re-setting this attribute. However, coordination with the upstream and/or downstream neighbors is strongly recommended. Aggregation If aggregation is done, the resultant aggregate shall be treated as a new NLRI. No DPA attribute shall be derived from more specific NLRIs which formed the aggregate. The resultant aggregate is free to have the DPA attribute set if so desired. Remarks It is noted that this new BGP attribute is simple and requires little change to the current practice and operation of BGP4. Nevertheless, the new attribute would offer the flexibility of shifting more influence on route selection to where the route originates, which has become increasingly meaningful as the Internet becomes more complex and dynamic. At the same time, the autonomy of an AS is preserved as the "local_pref" feature remains unchanged. A typical application of this attribute is illustrated in [4] where the DPA attribute is used to simplify the implementation of symmetric inter-domain routing and load-sharing. Applicability The DPA path attribute may be used with BGP version 4 and all subsequent versions of BGP unless specifically noted otherwise. Security Considerations Security considerations are not discussed in this memo. Acknowledgments The authors would like to thank Yakov Rekhter of Cisco for his insightful comments and suggestions. Chen & Bates [Page 3] INTERNET-DRAFT DPA Attribute for BGP July 1995 References [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", RFC1771, March 1995. [2] Y. Rekhter, and P. Gross, "Application of the Border Gateway Protocol in the Internet", RFC1772, March 1995. [3] Chen, E., and Bates, T., "Current Practice of Implementing Symmetric Routing and Load Sharing in the Multi-Provider Internet", INTERNET-DRAFT, , June 1995. [4] Chen, E., and Bates, T., "Application of the BGP Destination Preference Attribute in Implementing Symmetric Routing", INTERNET- DRAFT, , June 1995. [5] Antonov, V., "BGP AS Path Metrics", INTERNET DRAFT, , March 1995. [6] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787, April 1995. Author's Addresses Enke Chen MCI 2100 Reston Parkway Reston, VA 22091 phone: +1 703 715 7087 email: enke@mci.net Tony Bates MCI 2100 Reston Parkway Reston, VA 22091 phone: +1 703 715 7521 email: Tony.Bates@mci.net Chen & Bates [Page 4]