Santanu,
On 3/31/15 15:20 , Santanu Kar wrote:
Hi Authors
I think last mail was a bit long to have probably missed the actual
point which I was trying to make.Stating it concisely again.
The PHP Prefix Segment can be advertised by the neighbor as well
as by routers downstream of the neighbor which are connected to it.
correct, typical case is an area boundary, where ABR propagates the
prefix SID between areas.
So to determine a PHP Prefix Segment, should the recipient router
only check for PHP when its advertised from its neighbor(with NP flag
unset), or even when its from other routers downstream of the neighbor.
draft says two things:
1. If the NP-Flag is not set then any upstream neighbor of the Prefix-
SID originator MUST pop the Prefix-SID.
2. When calculating the outgoing label for the prefix, the router MUST
take into account E and P flags advertised by the next-hop router, if
next-hop router advertised the SID for the prefix. This MUST be done
regardless of whether the next-hop router contributes to the best
path to the prefix.
'E and P flags' should be 'E and NP flags' in the above paragraph, will
fix that in the next re-spin.
In summary, you should only follow what NP-bit is telling you, if the
neighbor advertised the SID.
regards,
Peter
The reason for this doubt is this statement in the text
"If the NP-Flag is not set then any*upstream neighbor*of the Prefix-SID
originator MUST pop the Prefix-SID. This is equivalent to
the penultimate hop popping mechanism used in the MPLS dataplane."
Regards
Santanu
-----Original Message-----
From: Santanu Kar [mailto:santanu....@ipinfusion.com]
Sent: Friday, March 27, 2015 7:47 PM
To: 'ospf@ietf.org <mailto:ospf@ietf.org>'; 'ppse...@cisco.com
<mailto:ppse...@cisco.com>'; 'sprev...@cisco.com
<mailto:sprev...@cisco.com>'; 'cfils...@cisco.com
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<mailto:wim.henderi...@alcatel-lucent.com>'
Subject: PHP route determination in
draft-ietf-ospf-segment-routing-extensions-03
Hi
Seeking some more clarification in the draft regarding PHP route
determination. Please consider the following scenario. SID refers to
Prefix Segment Identifier.
Router-A: --------------------- :Router-B: ---------------------(
SID-210) :Router-C
10.0.0.0/24 <http://10.0.0.0/24> 20.0.0.0/24 <http://20.0.0.0/24>
Lets consider the above topology of 3 routers. Router-C has
configured Prefix SID 210 for prefix 20.0.0.0/24 <http://20.0.0.0/24>
prefix. In the context of Router-A, the route 20.0.0.0/24
<http://20.0.0.0/24> is a PHP route, as it belongs to its neighbor
Router-B. Router-B is not configured.
When Ext Prefix LSA with SID 210 originated by Router-C reaches Router-A
, it finds that it's not from its neighbor Router-B and hence it will
end up installing a non-PHP label say 210, instead of doing penultimate
hop pop for the route 20.0.0.0/24 <http://20.0.0.0/24>. But this won't
be correct.
Am interpreting this because as per the following rules for PHP
determination prescribed in the document, "If the NP-Flag is not set
then any upstream neighbor of the Prefix-
SID originator MUST pop the Prefix-SID. This is equivalent to the
penultimate hop popping mechanism used in the MPLS dataplane."
Since this is an intra-area case, NP is not set. However since Router-A
is not upstream neighbor of Router-C it will not apply PHP for this
route, even though its actually a PHP route. Few questions in this
1) Should we just rely on checking that its upstream neighbor and NP
flag and then apply PHP, or Router-A should do a special search its Link
State database to find whether the route actually belongs to its
neighbor, in this case Router-B.
2) Is the above a valid scenario or the administrator is mandated to
configure SID values (which are all same) on all routers connected to
subnet 20.0.0.0/24 <http://20.0.0.0/24>.
3) If we mandate (2) we will have the below scenario
Router-A --------------------- :Router-B :
SID-210--------------------- SID-210:Router-C
10.0.0.0/24 <http://10.0.0.0/24> 20.0.0.0/24 <http://20.0.0.0/24>
In this case, even if we configure SID in Router-C earlier, we will be
needed to configure the same in Router-B also. So Router-A will select
the Ext Prefix LSA coming from Router-B , as its the best path for route
20.0.0.0/24 <http://20.0.0.0/24> than Router-C. So even though we have
non-PHP label installed first, it will be replaced with PHP label as
soon ROuter-B is also configured for same prefix. So we may not choose
to do special search in link state database to find PHP route as
mentioned in (1)
The selection of SID, when both Router-B and Router-C is configured is
based on following the rules mentioned
inhttps://tools.ietf.org/html/draft-ietf-ospf-prefix-link-attr-03#section-2.1
"If this TLV is advertised multiple times for the same prefix in
different OSPFv2 Extended Prefix Opaque LSAs originated by the
different OSPF routers, the application using the information is
required to determine which OSPFv2 Extended Prefix Opaque LSA is
used. For example, the application could prefer the LSA providing
the best path to the prefix."
So should we follow (2) and (3) for considering PHP, or (1). If we don't
follow any, we will end up having non-PHP label even for routes that are
PHP.
Please point me out if in case I am misinterpreting the intent of any
texts mentioned from the drafts.
Regards
Santanu
.
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