On 19/05/17 05:40 , Chao Fu wrote:
Hi Peter,

Yes I think it is better to add the text that would say that ABR MUST NOT set 
the E-bit.
If not, when ABR receives the intra prefix with SID, it would be thought to 
inherit the SID value including its flags (NP and E) and then flood the SID to 
other areas. I guess that's why NP is described expressly on ABR not to do so. 
Then I think E-flag should be clarified also in case the non-directly attached 
prefix sets it. It could avoid any misunderstanding.
Anyway, the E-flag will not be set in this case, right? Thanks.

right.

Peter


Regards,
Chao Fu

-----Original Message-----
From: Peter Psenak [mailto:ppse...@cisco.com]
Sent: Thursday, May 18, 2017 17:47
To: Chao Fu <chao...@ericsson.com>; spr...@ietf.org
Subject: Re: [spring] One question on E-flag of ABR/ASBR in OSPF SR extension

Hi Chao,

On 18/05/17 11:15 , Chao Fu wrote:
Hi Peter,

It's right if the NP-flag is not set then the received E-flag is
ignored. But, if the NP-flag is SET because the prefix is not directly
attached to the ABR, E-flag will not be ignored

why would an E-bit be set in such case? It's up to the originator of the SID to 
decide when to set the E-bit. If the ABR does not set the E-bit no PHP would be 
done and advertised SID will be preserved.

Are you trying to add the text that would say that ABR MUST NOT set the E-bit 
either? I don't think it's necessary.

thanks,
Peter

and the upstream neighbor will replace the Prefix-SID with the Explicit-NULL 
label 0. I guess actually the packet should be forwarded with the original 
prefix-SID, and no need to pop the 0 label and look up path again.

Regards,
Chao Fu


-----Original Message-----
From: Peter Psenak [mailto:ppse...@cisco.com]
Sent: Thursday, May 18, 2017 16:54
To: Chao Fu <mailto:chao...@ericsson.com>; mailto:spr...@ietf.org
Subject: Re: [spring] One question on E-flag of ABR/ASBR in OSPF SR
extension

Hi Chao,

On 18/05/17 09:44 , Chao Fu wrote:
Hi,

Should we clarify how to set E-flag for ABR/ASBR in OSPF SR extension?

In
https://www.ietf.org/id/draft-ietf-ospf-segment-routing-extensions-14.
txt, the draft describes how to set NP-flag on ABR and ASBR (Section
5 [Page
14]):

      The NP-Flag (No-PHP) MUST be set for Prefix-SIDs allocated to
inter-

      area prefixes that are originated by the ABR based on intra-area
or

      inter-area reachability between areas.  When the inter-area
prefix is

      generated based on a prefix which is directly attached to the
ABR,

      the NP-Flag SHOULD NOT be set.

      The NP-Flag (No-PHP) MUST be be set for Prefix-SIDs allocated to

      redistributed prefixes, unless the redistributed prefix is
directly

      attached to the ASBR, in which case the NP-flag SHOULD NOT be set.

However, the E-flag (Explicit-Null Flag) is not described. Should we
clarify it also? I think E-flag SHOULD NOT be set if the prefix is
not directly attached to the ABR or ASBR, and if necessary, it SHOULD
be set if the prefix is directly attached to the ABR or ASBR.

The existing draft says:

"If the NP-flag is not set then the received E-flag is ignored."

Given that the draft clearly states when the NP-flag is set on ABR/ASBR above 
statement should be sufficient.

thanks,
Peter



Regards,

Chao Fu



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