BGP-4+

[John W. Stewart III]

i guess i don't understand the spec completely, then.
how do you specify no NLRI, then?  a prefix-length of
0 doesn't do it because that means 0.0.0.0/0, right?

/jws

 > John,
 >  
 > > i have a follow-up question to one of the things dennis asked,
 > > but i have a general question as well
 > > 
 > > the UPDATE message looks like:
 > > 
 > >       +-----------------------------------------------------+
 > >       |   Unfeasible Routes Length (2 octets)               |
 > >       +-----------------------------------------------------+
 > >       |  Withdrawn Routes (variable)                        |
 > >       +-----------------------------------------------------+
 > >       |   Total Path Attribute Length (2 octets)            |
 > >       +-----------------------------------------------------+
 > >       |    Path Attributes (variable)                       |
 > >       +-----------------------------------------------------+
 > >       |   Network Layer Reachability Information (variable) |
 > >       +-----------------------------------------------------+
 > > 
 > > let's say that i'm an implementation with these extensions and
 > > i want to advertise an IPv6 route over an ebgp session with
 > > just the mandatory well-known attributes ORIGIN="IGP",
 > > AS-PATH="3561" and NEXT-HOP="10.1.1.1".  what do i put in the
 > > "NLRI" field of the UPDATE message (not the NLRI component of
 > > the attribute)?  as i read it, the proposed extension doesn't
 > > include the ability to specify attributes within the
 > > MP_REACH_NLRI attribute, so the "Total Path Attribute Length"
 > > and "Path Attributes" fields of the UPDATE message need to be
 > > used.  the spec says:
 > > 
 > >  >>
 > >  >>   Total Path Attribute Length:
 > >  >>
 > >  >>      [...]
 > >  >>
 > >  >>      A value of 0 indicates that no Network Layer Reachability
 > >  >>      Information field is present in this UPDATE message.
 > >  >>
 > > 
 > > so conversely a non-zero value in "Total Path Attribute Length"
 > > means that NLRI *is* present.  since i need to associate
 > > attributes with the IPv6 route, "Total Path Attribute Length"
 > > needs to be non-zero, so what goes in NLRI if i don't have any
 > > IP4-related thing to do in this message?
 > 
 > The converse is not true (and perhaps this should be clarified in the
 > BGP-4 spec). That is, "Total Path Attribute Length" may be non-zero *and* 
 > no NLRI may be present. This way one could have NLRI carried only in the
 > MP_REACH_NLRI and have all the other necessary attributes as well.
 > 
 > Yakov.