Network Working Group Q. Vohra Request for Comments: 4893 Juniper Networks Category: Standards Track E. Chen Cisco Systems May 2007 BGP Support for Four-octet AS Number Space Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007).
AbstractCurrently the Autonomous System (AS) number is encoded as a two-octet entity in BGP. This document describes extensions to BGP to carry the Autonomous System number as a four-octet entity. BGP]. To prepare for the anticipated exhaustion of the two-octet AS numbers, this document describes extensions to BGP to carry the Autonomous System number as a four-octet entity. More specifically, this document defines a new BGP capability, Four- octet AS Number Capability, that can be used by a BGP speaker to indicate its support for the four-octet AS numbers. Two new attributes, AS4_PATH and AS4_AGGREGATOR, are introduced that can be used to propagate four-octet based AS path information across BGP speakers that do not support the four-octet AS numbers. This document also specifies mechanisms for constructing the AS path information from the AS_PATH attribute and the AS4_PATH attribute. The extensions proposed in this document allow a gradual transition from 2-octet AS numbers to 4-octet AS numbers.
RFC2119]. RFC2842] to advertise to its neighbors (either internal or external) that it supports 4-octet AS number extensions, as specified in this document. The Capability that is used by a BGP speaker to convey to its BGP peer the 4-octet Autonomous System number capability, also carries the 4-octet Autonomous System number of the speaker in the Capability Value field of the Capability Optional Parameter. The Capability Length field of the Capability is set to 4. NEW BGP speakers carry AS path information expressed in terms of 4- octet Autonomous Systems numbers by using the existing AS_PATH attribute, except that each AS number in this attribute is encoded not as a 2-octet, but as a 4-octet entity. The same applies to the AGGREGATOR attribute - NEW BGP speakers use the same attribute, except that the AS carried in this attribute is encoded as a 4-octet entity. To preserve AS path information with 4-octet AS numbers across OLD BGP speakers, this document defines a new AS path attribute, called AS4_PATH. This is an optional transitive attribute that contains the AS path encoded with 4-octet AS numbers. The AS4_PATH attribute has the same semantics as the AS_PATH attribute, except that it is optional transitive, and it carries 4-octet AS numbers. To prevent the possible propagation of confederation path segments outside of a confederation, the path segment types AS_CONFED_SEQUENCE and AS_CONFED_SET [RFC3065] are declared invalid for the AS4_PATH attribute.
Similarly, this document defines a new aggregator attribute called AS4_AGGREGATOR, which is optional transitive. The AS4_AGGREGATOR attribute has the same semantics as the AGGREGATOR attribute, except that it carries a 4-octet AS number. Currently assigned 2-octet Autonomous System numbers are converted into 4-octet Autonomous System numbers by setting the two high-order octets of the 4-octet field to zero. Such a 4-octet AS number is said to be mappable to a 2-octet AS number. To represent 4-octet AS numbers (which are not mapped from 2-octets) as 2-octet AS numbers in the AS path information encoded with 2-octet AS numbers, this document reserves a 2-octet AS number. We denote this special AS number as AS_TRANS for ease of description in the rest of this specification. This AS number is also placed in the "My Autonomous System" field of the OPEN message originated by a NEW BGP speaker, if the speaker does not have a (globally unique) 2-octet AS number.
remained unmodified since the route left the last NEW BGP speaker. The trailing AS path information (representing autonomous systems with 2-octet AS numbers and OLD BGP speakers only) is contained only in the current AS_PATH attribute (encoded in the leading part of the AS_PATH attribute). Under certain conditions, it may not be possible to reconstruct the entire AS path information from the AS_PATH and the AS4_PATH attributes of a route. This occurs when two or more routes that carry the AS4_PATH attribute are aggregated by an OLD BGP speaker, and the AS4_PATH attribute of at least one of these routes carries at least one 4-octet AS number (as oppose to a 2-octet AS number that is encoded in 4 octets). Depending on the implementation, either the AS4_PATH attribute would be lost during route aggregation, or both the AS_PATH attribute and the AS4_PATH attribute would contain valid, partial information that cannot be combined seamlessly, resulting in incomplete AS path information in these cases. A NEW BGP speaker should also be prepared to receive the AS4_AGGREGATOR attribute along with the AGGREGATOR attribute from an OLD BGP speaker. When both the attributes are received, if the AS number in the AGGREGATOR attribute is not AS_TRANS, then: - the AS4_AGGREGATOR attribute and the AS4_PATH attribute SHALL be ignored, - the AGGREGATOR attribute SHALL be taken as the information about the aggregating node, and - the AS_PATH attribute SHALL be taken as the AS path information. Otherwise, - the AGGREGATOR attribute SHALL be ignored, - the AS4_AGGREGATOR attribute SHALL be taken as the information about the aggregating node, and - the AS path information would need to be constructed, as in all other cases. In order to construct the AS path information, it would be necessary to first calculate the number of AS numbers in the AS_PATH and AS4_PATH attributes using the method specified in Section 126.96.36.199 [BGP] and [RFC3065] for route selection.
If the number of AS numbers in the AS_PATH attribute is less than the number of AS numbers in the AS4_PATH attribute, then the AS4_PATH attribute SHALL be ignored, and the AS_PATH attribute SHALL be taken as the AS path information. If the number of AS numbers in the AS_PATH attribute is larger than or equal to the number of AS numbers in the AS4_PATH attribute, then the AS path information SHALL be constructed by taking as many AS numbers and path segments as necessary from the leading part of the AS_PATH attribute, and then prepending them to the AS4_PATH attribute so that the AS path information has an identical number of AS numbers as the AS_PATH attribute. Note that a valid AS_CONFED_SEQUENCE or AS_CONFED_SET path segment SHALL be prepended if it is either the leading path segment or adjacent to a path segment that is prepended. RFC1997], when the high-order two-octets of the community attribute is neither 0x0000 nor 0xffff, these two octets encode the Autonomous System number. Quite clearly this would not work for BGP speakers that use 4-octets Autonomous System numbers. Such BGP speakers should use the Four-octet AS Specific Extended Communities [AS-EXT-COM] instead.
with the OLD speakers may result in a situation where Multi-Exit Discriminator will influence route selection among the routes that were received from different neighboring Autonomous Systems. Under certain conditions, it may not be possible to reconstruct the entire AS path information from the AS_PATH and the AS4_PATH attributes of a route. This occurs when two or more routes that carry the AS4_PATH attribute are aggregated by an OLD BGP speaker, and the AS4_PATH attribute of at least one of these routes carries at least one 4-octet AS number (as oppose to a 2-octet AS number that is encoded in 4 octets). When such aggregation results in creating a route that is less specific than any of the component routes (route whose Network Layer Reachability Information (NLRI) covers NLRI of all the component routes), loss of the AS path information does not create a risk of a routing loop. In all other cases, loss of the AS path information does create a risk of a routing loop. RFC 2842. In addition, this document introduces two new BGP optional transitive attributes, and their type codes have been assigned by the IANA. The first one is the AS4_PATH attribute, value 17, which preserves the AS path information with 4-octet AS numbers across old BGP speakers. The second one is the AS4_AGGREGATOR attribute, value 18, which is similar in use to the current AGGREGATOR attribute, but it carries a 4-octet AS number. Finally, this document introduces a reserved 2-octet AS number -- AS_TRANS. The AS number 23456 has been assigned by the IANA for AS_TRANS.
[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, August 1996. [RFC3392] Chandra, R. and J. Scudder, "Capabilities Advertisement with BGP-4", RFC 3392, November 2002. [RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 3065, February 2001. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [AS-EXT-COM] Rekhter, Y., Ramachandra, S., and D. Tappan, "Four-octet AS Specific BGP Extended Community", Work in Progress, April 2007.
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