I have a couple of questions/comments in association
with draft-venkatachalam-interarea-mpls-te-00.txt about LSP setup across
IGP areas.
1. In section 4.4 (Routing algorithm):
6. If the destination is in this area, calculate an
intra-area route
from the originating node to the
destination that satisfies the
^^^^^^^^^^^^^^^^
constraints, and splice the egress
of the original LSP and the
^^^^^^^^^^^^
ingress of the new LSP.
Shouldn't it be transit ABR instead of originating node, and also upstream
LSP instead of original LSP.
2. Is there some special software entity besides the signaling entity
(RSVP-TE or CR-LDP) at ABR that splices upstream LSP section to downstream
LSP section. Is this entity what you call routing process in step 4 of
section 4.4. Is this entity responsible for intercepting LABEL_REQUEST
and PATH (as well as other CR-LDP and RSVP) messages at ABRs.
3. About crankback. It is stated that crankback is to the previous nearest
ABR from the point of failure. At that ABR the routing algorithm by routing
process is repeated. Is there any mechanism that prevents the selection
of the same ABR as was used previously. For example, refering to the following
example
Area 1
Area 0
Area 2
-------------------- ------------------
------------------------
|
| |
| |
|
| |N1 PPS1 -------
PPS3 --------
PPS5 |N2 |
| |-------------- |ABR1 |--------------|ABR 2 |--------X-------|
|
| |-------| -------
-------- --|PPS7 |------| |
| PSS2 |
------- PSS4 --------
|-----| PSS6 |
| --------|ABR
3 |--------------|ABR 4 |---------
|
|
-------
--------
|
|
| |
| |
|
-------------------- -------------------
-------------------------
Let's assume PPS5 failed and we initially cranked back to ABR 2 but
there was no intra-area route to the destination so we cranked back to
ABR1. At ABR1 when performing the algorithm of section 4.4 we should not
consider ABR2 as this is from where we cranked back and we know that ABR2
already tried unsuccessfully to route to the destination.
Also, routing in response to crankback really depends on whether the
failure causing the crankback was within the current area or another downstream
area. If it was within the current area then same downstream ABR can be
selected. If it was outside of the current area then a different ABR should
be selected. For example, if PPS5 fails and we crank back to ABR2 then
to ABR1 (as I eluded to above) then ABR2 should not be considered when
routing at ABR1. On the other hand, if PPS3 fails and we crank back to
ABR1 then ABR2 should be considered.
4. Have you given some thought to the idea that an LSP section failure
in one area should not have an effect on surrounding LSP sections in other
areas. For example, refering to the above picture, if PPS3 fails then another
intra-area route through Area 0 between ABR1 and ABR2 is computed and LSP
section reestablished along that route without affecting LSP sections PPS1
and PPS5. This would be similar to "local repair" where local = area.
Cheers,
Darek