[Idr] draft-dong-idr-node-target-ext-comm route distribution procedures

[Jeffrey Haas <[e-mail]>]

Authors,

As noted in other threads, I personally think the use case for targeting a
specific node is clear in terms of community addressing.  I have a number of
concerns about the distribution machinery portion of your draft.  

I would roughly summarize your proposal as an attempt to flood the routes
with this extended community through the AS and have them land only on the
expected device.

In a fully-meshed iBGP network, an implementation can always know what its
attached iBGP BGP Identifiers are.  So, it's possible to target routes to
such nodes immediately.

In a network with route reflectors, given the lack of the BGP Identifier
distribution graph, your strategy is "just pass it through the reflectors".
I would observe that a route reflector may know that it is a route
reflector, but may not know that its peer is also a route reflector.  This
means you can't do selective propagation without this knowledge.

Finally, while you generally place inter-domain "for further study", you do
cover the eBGP case when an adjacent router has a local match for itself and
then it looks for the adjacent eBGP match.

If my summary of the procedures is roughly correct, this is an attempt to
try to do work that would be more easily covered by simply using
RT-Constrain.  For an N node network, you'd have N RT-Constrain routes which
will build the necessary flooding graph as per the usual procedures.  eBGP
routers could signal into that either directly via RT-Constrain eBGP
procedures; alternatively, the ASBRs could emit a RT-Constrain route for
each of their eBGP neighbors by proxy.

Of course, there's one large hold-up with regard to such a methodology:
RT-Constrain is not per-AFI/SAFI for its flooding graph.  In general, this
hasn't been a problem for features deployed for RT-Constrain since the
communities in question tend to be associated with specific AFI/SAFIs.
However, there's no guarantee that a given AFI/SAFI node-targeted route has
an appropriately congruent AFI/SAFI flooding topology along the RT-Constrain
graph.

Solving the RT-Constrain AFI/SAFI congruency issue may be helpful here, and
perhaps has overlapping use with other possible BESS use cases.

-- Jeff