RE: PHP route determination in draft-ietf-ospf-segment-routing-extensions-03
Hi Peter Please find my responses inline. Regards Santanu -----Original Message----- From: Peter Psenak [mailto:ppse...@cisco.com <ppse...@cisco.com>] Sent: Tuesday, March 31, 2015 8:43 PM To: Santanu Kar; ospf@ietf.org; sprev...@cisco.com; cfils...@cisco.com; han...@juniper.net; rob.sha...@bt.com; wim.henderi...@alcatel-lucent.com Subject: Re: PHP route determination in draft-ietf-ospf-segment-routing-extensions-03 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. SANTANU> I actually wanted to highlight the non-ABR cases here. Consider the 3 routers below, in same area. A -----10.1.1.0/24----- B ------20.1.1.0/24 -----C In the context of A, the route of 20.1.1.0/24 is a PHP route. Now the Prefix Segment for prefix 20.1.1.0/24 can be advertised by both B, as well as by C towards A. The case I am considering here is, C has advertised the prefix segment of 20.1.1.0/24 to A first. Still when A is calculating label for 20.1.1.0/24, it should take it as PHP. However the text in draft states " upstream neighbor of the Prefix-SID originator MUST pop the Prefix-SID". Here A is not the upstream neighbor of C. > > 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 <santanu....@ipinfusion.com>] > Sent: Friday, March 27, 2015 7:47 PM > To: 'ospf@ietf.org <mailto:ospf@ietf.org <ospf@ietf.org>>'; ' ppse...@cisco.com > <mailto:ppse...@cisco.com <ppse...@cisco.com>>'; 'sprev...@cisco.com > <mailto:sprev...@cisco.com <sprev...@cisco.com>>'; 'cfils...@cisco.com > <mailto:cfils...@cisco.com <cfils...@cisco.com>>'; 'han...@juniper.net > <mailto:han...@juniper.net <han...@juniper.net>>'; 'rob.sha...@bt.com > <mailto:rob.sha...@bt.com <rob.sha...@bt.com>>'; ' wim.henderi...@alcatel-lucent.com > <mailto:wim.henderi...@alcatel-lucent.com <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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