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3. Extension of AFI/SAFI Definitions for the IPv4 Address Family

As mentioned earlier, MP-BGP specifies that the set of usable next-hop address families is determined by the AFI and the SAFI. The following AFI/SAFI definitions for the IPv4 NLRI or VPN-IPv4 NLRI (<1/1>, <1/2>, <1/4>, <1/128>, and <1/129>) only have provisions for advertising a next-hop address that belongs to the IPv4 protocol. This document extends the set of usable next-hop address families to include IPv6 in addition to IPv4 when advertising an IPv4 or VPN-IPv4 NLRI.

Specifically, this document allows advertising the MP_REACH_NLRI attribute [RFC4760] with this content:

  • AFI = 1

  • SAFI = 1, 2, or 4

  • Length of Next Hop Address = 16 or 32

  • Next Hop Address = IPv6 address of a next hop (potentially followed by the link-local IPv6 address of the next hop). This field is to be constructed as per Section 3 of [RFC2545].

  • NLRI = NLRI as per the AFI/SAFI definition

It also allows advertising the MP_REACH_NLRI attribute [RFC4760] with this content:

  • AFI = 1

  • SAFI = 128 or 129

  • Length of Next Hop Address = 24 or 48

  • Next Hop Address = VPN-IPv6 address of a next hop with an 8-octet RD set to zero (potentially followed by the link-local VPN-IPv6 address of the next hop with an 8-octet RD set to zero).

  • NLRI = NLRI as per the AFI/SAFI definition

This is in addition to the existing mode of operation allowing advertisement of NLRI for <AFI/SAFI> of <1/1>, <1/2>, and <1/4> with a next-hop address of an IPv4 type and advertisement of NLRI for an <AFI/SAFI> of <1/128> and <1/129> with a next-hop address of a VPN-IPv4 type.

The BGP speaker receiving the advertisement MUST use the Length of Next Hop Address field to determine which network-layer protocol the next-hop address belongs to.

  • When the AFI/SAFI is <1/1>, <1/2>, or <1/4> and when the Length of Next Hop Address field is equal to 16 or 32, the next-hop address is of type IPv6.

  • When the AFI/SAFI is <1/128> or <1/129> and when the Length of Next Hop Address field is equal to 24 or 48, the next-hop address is of type VPN-IPv6.

Note that this method of using the Length of Next Hop Address field to determine which network-layer protocol the next-hop address belongs to (out of the set of protocols allowed by the AFI/SAFI definition) is the same as that used in [RFC4684] and [RFC6074].

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