router-to-router NHRP (moderately long)

[Yakov Rekhter <[e-mail]>]

Folks,

Appended is a proposal on how to provide shortcuts among routers.
Please comment.

Yakov.
--------------------------------cut here--------------------------
Router-to-router (R2R) NHRP

To simplify the solution for the R2R NHRP case we constrain the problem by
only allowing shortcuts within a single routing domain. If a path spans
multiple routing domains (all on a common NBMA network), then the
shortcuts will be established within each such domain. However, at the
domains boundaries forwarding will be provided via IP routers, so that
the path across the NBMA network will have more than one IP hop.

A router that originates NHRP Request  annotates the Request with
sufficient information (as described below), to allow an unambiguous
determination of routing domain boundaries.

The annotated information consists of two components, protocol
independent and protocol specific. The protocol independent component
includes the address prefix (NLRI) for which the shortcut is requested,
and the protocol type that identifies the protocol that was used to
construct the prefix at the originator of the Request. The protocol
specific component carries information specific to a particular
protocol (as defined below) that was used to construct the prefix.

When an NHS receives an R2R NHRP Request, the NHS (using the
information carried in the protocol independent and protocol specific
components of the Request) checks whether it is in the same routing
domain as the originator of the Request (using procedures described
below), and if yes, then whether it is a border router for that domain
(using the procedures described below).

If the NHS is within the same routing domain as the originator of the
Request, and is not a border router for that domain, then the NHS just
forwards the Request (using the NLRI information carried in the
Request).

If the NHS is a border router for the routing domain that the
originator of the Request is in, then the NHS can either (a) annotate
the request with its own IP and NBMA addresses and forward the Request,
or (b) send back an NHRP reply (and thus terminate the query).  The
choice between (a) and (b) is a local to the NHS matter.

If an NHS receives an R2R NHRP Request that is annotated with the
border router information, then the originator of the Request and the
NHS are in different routing domains (in this case NHRP can not
guarantee loop-free routing information). In this case the NHS uses
only the NLRI information from the protocol independent component to
handle the Request.


RIP - Protocol Type TBD

For RIP  the protocol specific component is empty.

When an NHS receives an NHRP R2R Request that is not annotated with the
border router information, and the protocol independent component
specifies RIP, the NHS checks whether its Routing Information Base
(RIB) contains a RIP-learned route with NLRI identical to the NLRI
carried in the Request. If such a route is present,  then the NHS and
the originator of the Request are in the same routing domain, and the
NHS is not a border router for that domain. If no such route is
present, then the NHS and the originator of the Request are in the same
routing domain, and the NHS is a border router for that domain.

RIP-2  - Protocol Type TBD

For RIP-2 the protocol specific component contains the Route Tag of the
route.

When an NHS receives an NHRP R2R Request that is not annotated with the
border router information, and the protocol independent component
specifies RIP-2,  the NHS checks whether its Routing Information Base
(RIB) contains a RIP-learned route with NLRI identical to the NLRI
carried in the Request, and the Route Tag identical to the Route Tag
carried in the Request. If such a route is present,  then the NHS and
the originator of the Request are in the same routing domain, and the
NHS is not a border router for that domain. If no such route is
present, then the NHS and the originator of the Request are in the same
routing domain, and the NHS is a border router for that domain.

OSPF - Protocol Type TBD

For OSPF the protocol specific component is empty.

When an NHS receives an NHRP R2R Request, and the protocol specific
component specifies OSPF, if the NHS is is neither an Area Border
Router (ABR), nor AS Boundary Router (ASBR), the NHS just forwards the
Request.

If the NHS is either an ABR or ASBR, and the Request is not annotated,
then the NHS checks its Forwarding Data Base (FIB) for a presence of an
OSPF learned route that has the same NLRI as carried in the Request. If
such a route is found, if the next hop for this route is an OSPF
neighbor, then the NHS just forwards the Request; otherwise (the next
hop for this route is not an OSPF neighbor), the originator and the NHS
are in the same routing domain, and the NHS is a border router.

If no such route is found (this could happen when you either exiting an
area or entering an area), then the NHS uses the Destination IP Address
field from the Request to determine its local route (from its FIB) to
that destination.  Once the route is determined, the NHS replaces NLRI
carried in the Request with NLRI of the route. If the found route is
not an OSPF learned route, then the NHS and the originator of the
Request are in the same domain, and the NHS is a border route.  If the
found route is an OSPF learned route, if the next hop for the found
route is an OSPF neighbor, the NHS just forwards the Request; otherwise
the originator and the NHS are in the same routing domain, and the NHS
is a border router.

Dual IS-IS (Protocol Type TBD)

For Dual IS-IS the protocol specific component is empty.

When an NHS receives an NHRP R2R Request, and the protocol specific
component specifies Dual IS-IS, if the NHS is a Level 1 router, the NHS
just forwards the Request. If the NHS is a Level 2 router, and the
Request is not annotated, then the NHS checks its Forwarding Data Base
(FIB) for a presence of an IS-IS learned route that has the same NLRI
as carried in the Request.

If such a route is found, if the next hop for this route is an IS-IS
neighbor, then the NHS just forwards the Request; otherwise (the next
hop for this route is not an IS-IS neighbor), the originator and the
NHS are in the same routing domain, and the NHS is a border router.

If no such route is found (this could happen when you either exiting an
area or entering an area), then the NHS uses the Destination IP Address
field from the Request to determine its local route (from its FIB) to
that destination.  Once the route is determined, the NHS replaces NLRI
carried in the Request with NLRI of the route. If the found route is
not an IS-IS learned route, then the NHS and the originator of the
Request are in the same domain, and the NHS is a border route.  If the
found route is an IS-IS learned route, if the next hop for the found
route is either a Level 1 or a Level 2 neighbor, the NHS just forwards
the Request; otherwise the originator and the NHS are in the same
routing domain, and the NHS is a border router.