[Isis-wg] Handling same SID mapped to different prefixes and vice versa cases

[Imtiyaz Mohammad <[e-mail]>]

Hello experts !

The draft-ietf-isis-segment-routing-extensions draft states that:

 "The 'Prefix SID' MUST
   be unique within a given IGP domain (when the L-flag is not set)."


Makes sense from usage point of view. What if some implementation does
not enforce a configuration time check and allows such possibilities
in the network wherein the following three situations arise:


(1) Same SID is received from one device and mapped to multiple prefixes.

(2) Same SID is received from two devices and mapped to multiple prefixes.

(3) Same prefix is mapped with two different SID on two different devices.


Let me go through each case with examples.


(1) Same SID is received from one device and mapped to multiple prefixes.


devA-----ISIS------devB


devB has:

     node segment 100 for 30.30.30.30/32

     node segment 100 for 40.40.40.40/32


It sends prefix segment subTLV for both these entries.


devA receives these subTLVs and since they have arrived from the very
same device, the LFIB entries would be same calculated through
30.30.30.30/32:100 or 40.40.40.40/32:100 because your next operation
and nexthops would be identical.


(2) Same SID is received from two devices and mapped to multiple prefixes.


devA-----ISIS---cost10---devB

          |

          |_____cost20___devC


devB has:

     node segment 100 for 30.30.30.30/32


devC has:

     node segment 100 for 40.40.40.40/32


This cause would clearly not work as devA would need to know SID 100
represents devB or devC. So, on receipt on such subTLV entries, we
would syslog an error.


(3) Same prefix is mapped with two different SID on two different devices.


devA-----ISIS---cost10---devB

          |

          |_____cost20___devC


devB has:

     node segment 100 for 30.30.30.30/32


devC has:

     node segment 200 for 30.30.30.30/32


Even in this case, things would fall apart as the 30.30.30.30/32 fec
has two paths, one of them may be the best path or there might be an
ecmp but one would not know when using incoming label 100 or 200 that
the packet would go to devB or devC.


Is my understanding correct for these three scenarios ?


I am thinking that in scenario (2) and (3), on receipt of duplicate
subTLV entries ( same SID different prefixes or vice versa from
different systemIDs ), we should store all of them, mark one of them
as 'active' based on same criteria such highest systemID and use that
for LFIB generation. The inactive subTLV entry would just lie there in
case the active one ceases to exist for some reason later in time at
which point, the inactive one could be made active and used for LFIB
generation.


Comments / Opinions ?




-- 
Imtiyaz