Address-Prefix-Based Outbound Route Filter for BGP-4
Extracted elements (11)
Sequence number assignment is a local matter for the BGP speaker that sends the Address Prefix ORF entries, giving implementations flexibility in how they order and manage ORF entries.
The Address Prefix ORF type builds on the Outbound Route Filtering Capability defined in RFC 5291 (BGP-ORF) and uses the multiprotocol address family support from RFC 4760 (BGP-MP). Implementations must support these base specifications for Address Prefix ORF to function.
The Maxlen value must be no more than the maximum length (in bits) of a host address for the given address family as defined in BGP-MP (RFC 4760).
The values of the Sequence, Length, Minlen, and Maxlen fields must satisfy: 0 <= Length < Minlen <= Maxlen. Tests related to Minlen or Maxlen should be omitted when those fields are unspecified (value 0).
When more than one Address Prefix ORF entry matches the NLRI of a route, the 'first-match' rule applies: the ORF entry with the smallest sequence number among all matching entries is the sole match and determines whether the route should be advertised.
Address Prefix ORF matching works as follows: a route is 'no match' if its NLRI is neither more specific than nor equal to the ORF entry's <Prefix, Length>. When NLRI is more specific or equal, matching depends on Minlen/Maxlen as specified in Table 1: both unspecified means exact length match; only Minlen means NLRI.length >= Minlen; only Maxlen means NLRI.length <= Maxlen; both specified means NLRI.length in [Minlen, Maxlen].
The Address Prefix ORF entry consists of six fields: <Sequence, Match, Length, Prefix, Minlen, Maxlen>. Sequence specifies relative ordering; Match indicates PERMIT (0) or DENY (1); Length is the prefix length in bits; Prefix is the address prefix; Minlen and Maxlen define the minimum and maximum prefix lengths for matching (value 0 means unspecified).
The Prefix field contains the address prefix followed by enough trailing bits to make the field end on an octet boundary. The value of the trailing bits is irrelevant.
This document registers a new Outbound Route Filtering (ORF) type called 'Address Prefix ORF' with the assigned ORF-Type value of 64.
This extension to BGP does not change the underlying security issues in BGP-4 (RFC 4271). No new security considerations are introduced beyond those already present in the base BGP-4 specification.
An Address Prefix ORF entry is encoded with ORF-Type value 64. The type-specific part contains: Sequence (4 octets), Minlen (1 octet), Maxlen (1 octet), Length (1 octet), and Prefix (variable length, padded with trailing bits to an octet boundary). The Match field is encoded in the common ORF part.