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Notes:
This format is used when BGP carries IP v4 routing information. With the MultiProtocol extensions, BGP can be used to carry different types of addresses instead the same BGP session (e.g. IP v6, RFC2547 VPNs, MPLS labels, or IP Multicast routing information). See e.g. :
P. Marques, F. Dupont, "Use of BGP-4 Multiprotocol Extensions for
IPv6 Inter-Domain Routing", RFC 2545, March 1999.
In this case, the capabilities optional parameter is used inside the OPEN message to negotiate the utilization of other addresses formats. Those non-IPv4 addresses are carried inside optional path attributes (MP_REACH_NLRI and MP_UNREACH_NLRI). Those attributes are encoded as described in :
T. Bates, R. Chandra, D. Katz, Y. Rekhter, Multiprotocol Extensions for BGP-4, Internet draft, draft-ietf-idr-rfc2858bis-02.txt, October 2002, work in progress
Being able to pack multiple route announcements and withdrawals in the same BGP message is very important for performance reasons, since a good packing of the BGP messages can significantly reduce the number of BGP messages exchanged. In this tutorial, for simplicity, we will only utilize BGP messages carrying an advertisement or a withdrawal for a single IP prefix. We will utilize the word “UPDATE” for a BGP UPDATE message containing a single advertised prefix and the word “WITDRAW” for a BGP UPDATE message containing a single withdrawn prefix.