Re: BGP-4+

[Yakov Rekhter <[e-mail]>]

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       |  Withdrawn Routes (variable)                        |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.