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 ....
-----------------------------------------------------------
------------------------------
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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