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Re: [spring] One question on E-flag of ABR/ASBR in OSPF SR extension

Chao Fu <chao.fu@ericsson.com> Thu, 18 May 2017 09:21 UTC

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