[Lsr] Question on RFC-8667 section 2.1.1.2 and 2.1.2 related PrefixSID Propagation and L1L2 leaks ...

[Waman Nawathe <[e-mail]>]

Hello All,

New to this IETF list and posting ..

Regarding this section RFC-8667 2.1.1.2, It should ONLY apply to
ISIS L1L2 (or ABR) router and not L1 only or L2 only and here is my
reasoning ....

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Reference Diagram (A):
-----------------------

                                   L1L2 (ABR)

Grunt-54 (L1) -------------- (L1) Grunt-104 (L2) ------------- (L2) Grunt
106

                                  L1 --> L2 Route Leaks or
                                  L1 <-- L2



-----------------------
Reference Diagram (B):  Showing Flat L1 or Flat L2 not using any ABRs:
-----------------------



    Grunt-54 --- G100 --- G101 --- G103       Grunt-104     Grunt-106 ----
G200 ---- G201 ----- G201

    L1           L1       L1       L1            L1L2          L2
L2        L2         L2

                                            *NOT* Connected
                                              to L1 OR L2



  Please refer to RFC-8667 Section 2.1.1.2 Page 7 and Section 2.1.2 Page 8
wrt ISIS
  ISIS route/prefix leaks. It mentioned 3 types:

      (a) L1L2
      (b) L2L1 and
      (c) redistribution from another protocol.

   Cases (a) and (b) are fine wrt to my understanding ... but (c) is NOT
clear.

   NOTE: each prefix space (tlv-135) in LSP is approx 10 bytes (prefix
length based)
         and this Prefix-SID TLV is an additional 8 bytes. So if we do this
for ALL
         Leaked routes then we reduce the total route capacity in
         LSP by 40-50% which is "not" needed really as these routes are
associated
         with Prefix-SID from the same ISIS node.

   1) I can understand if this adding of prefix-sid "sub-tlv" is is done
"only" at
      the L1L2 (ABR). as that would maintain correct Prefix-SID association
with
      the route accross ABR when it could have been lost.

      I do understand this is not for local routes ie static/connected BUT
      only for ospf/bgp into ISIS, no issues with that - on the L1L2 (ABR).
      This part is fine by me.


   2) Consider reference diagram (B) where L1 or L2 are flat networks with
no
      L1L2 (ABR) but have redistributed ospf/bgp under router isis, so why
      should each leaked route be EXPLICITLY associated with Prefix-SID
sub-tlv
      when there is ISIS node based Prefix-SID association which is
available
      for all Flat network members ?

      The only advantage to this is to reset Prefix-SID flags but we would
reduce LSP
      space by 40% wrt leaked routes., which is not clear why such an
expensive
      penalty for leaked redistributed routes.


   4) I could not see ANY good examples of leaks on the web to clarify this
issue.

      This is the ONLY reference I could see ...

      check Slide #14


https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2019/pdf/BRKRST-3009.pdf

      Side #64


https://www.segment-routing.net/tutorials/2016-09-27-segment-routing-igp-control-plane/

Comments and feedback welcome,
Thanks,

-Waman Nawathe
Boston Area, SR Learner

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