A possible solution for source-based site-exit routing

["Aldrin Isaac" <aisaac@bloomberg.com>]

I would like to present a method for operationally simple source-based
site-exit routing that may or may not have been presented before.
Those who are familiar with MPLS VPNs will find analagies.

There are four new "things" defined by this method.

Source-constrained Route Table (SRT): is a routing table instance used
to distinguish destinations route entries that are reachable for a
specific Source Prefix (SP) from the same destination route entries
that are reachable for other SPs.  Route metrics for routes within an
SRT are processed independently of route metrics for routes in other
SRTs and the Base Routing Table (BRT).  ** There is one SRT per every
unique SP. **

Base Route Table (BRT): This one isn't new.  This route table carries
all routes that are not constrained by the source address.  Generally
this would be all internal routes.

Source-constrained Route Distinguisher (SRD): is an attribute given to
an external route (external-destination-prefix +
border-address/border-nexthop) that can be used for routing packets
whose source address matches a particular SP.  The value of the SRD is
simply the SP.  A destination route prefix with a particular SRD value
must not be considered by a routing protocol to be equivalent to the
same destination route prefix with another SRD value.  Border routers
import routes from an external peer directly into SRTs for each SP
routable via that peer.  The border routers export routes from the
BRT/SRTs into IGP/IBGP as normal, but routes from the SRTs are
additionally exported with the associated SRD.  Other routers within
the site import routes with an SRD into the SRT associated to the SP.

SRT Lookup Table (SRTLT): is the SP table whose input is a source
address and output is a reference to the SRT associated to the source
address.

Tying it all together is the...

Three Point Lookup (TPL): When a packet arrives at a router, the
router will first perform a lookup of the destination in the BRT.  If
the route does not exist in the BRT, it does a lookup in the SRTLT to
find the SRT whose SP best matches the packets source address.  If
SRTLT lookup fails for the source address or a destination route does
not exist in the SRT, then an ICMP Destination Unreachable is sent to
the source address.

If used, this feature would need to be enabled on all routers
participating in the sites IGP.  An SRD attribute would need to be
defined for all IGPs including IBGP.

In a simplified configuration, each border router would import a
static ::/0 route pointed towards each of it's external peers into the
SRTs whose SPs can be routed via those peers.  In which case ::/0
route should not exist in the BRT.  More precisely, a route should not
exist in the BRT if it exists in an SRT.

Comments?

-- aldrin