Re: When can NLRI and Withdrawn Route info be modified?

[Paul Traina <[e-mail]>]

Hi Brad...

The protocol does not specify when it can or can't be modified.  Manual
configuration can always modify something (you can lie about your view
of the world if you want to be evil).

Is your question, when can things be aggregated and what do you have to do?
If so, you can always choose to aggregate a route if you have information
about some more specific prefixes of that canidate aggregate.  You can take
this to the extreme of generating what we used to think of as a "default"
route.

When you aggregate, you may, or may not, withdraw the more specific routes.
Your choice again.  Obviously, it makes sense to withdraw more specific
routes in almost all cases.  If you advertise a hunk of NRLI, you should
be prepared to withdraw that same hunk if the conditions that created that
aggregate change.

When do you use AA?  When you create an aggregate of more specific NRLI, the
preferred choice is to combine the AS paths of the more specific NRLI into
an AS set and advertise that set.  If for some reason, you do not create a
set that contains complete information of all ASs in the paths you aggregated,
you add the AA attribute.

*My* reading of the text is, if, for any reason, you lose AS path information
in the process of generating an aggregate, you must set AA to indicate that
information has been suppressed.

Does anyone actually do anything with AA?  Not really as far as I know.

  From: Brad Smith <[e-mail]>
  Subject: When can NLRI and Withdrawn Route info be modified?
  Hello,
  
  I have been attempting to gain an understanding of how BGP works from
  the RFC (1771), and am confused on two related aspects of the protocol
  processing; specifically, when can the NLRI and Withdrawn Route
  information be modified?
  
  A clear example of modifying the NLRI is when route aggregation is
  performed; it is not discussed in the RFC (1771), but I assume that
  the Withdrawn Route information must be also modified at this time
  (corresponding to the changes in the NLRI)..?
  
  Are there other times this can occur?
  
  A related question is under what circumstances are ATOMIC_AGGREGATOR
  path attributes generated?
  
  Thanks in advance,
  
  Brad Smith
  Computer Sciences, UCSC
  [e-mail]