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Re: [Idr] draft-keyupate-idr-bgp-prefix-sid-00

"Jakob Heitz (jheitz)" <jheitz@cisco.com> Mon, 24 November 2014 21:25 UTC

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From: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
To: Eric C Rosen <erosen@juniper.net>, "idr@ietf.org" <idr@ietf.org>
Thread-Topic: [Idr] draft-keyupate-idr-bgp-prefix-sid-00
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Date: Mon, 24 Nov 2014 21:25:12 +0000
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References: <CA+b+ER=Ja5B7qMU7MDVDZPjbyxmMP+CuS9Gh4T5L=7OdfniO8g@mail.gmail.com> <546617D1.2070606@juniper.net> <075DE01702BBC249BE1357EFD20DCFE5141A9FBB@xmb-aln-x02.cisco.com> <54739E3B.2050307@juniper.net>
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Cc: "Keyur Patel (keyupate)" <keyupate@cisco.com>, Robert Raszuk <robert@raszuk.net>
Subject: Re: [Idr] draft-keyupate-idr-bgp-prefix-sid-00
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> -----Original Message-----
> From: Eric C Rosen [mailto:erosen@juniper.net]
> Sent: Monday, November 24, 2014 1:08 PM
> 
> See question in-line.
> 
> On 11/22/2014 6:52 PM, Jakob Heitz (jheitz) wrote:
> > If only one segment is required to forward a packet, then we use
> > regular 3107 and only L1 is needed. L2 is redundant.
> >
> > However...
> >
> > when we use multiple segments, for which the whole concept of
> segment
> > routing is useful, the ingress router must know the labels used by
> > subsequent segment routers in the segment list. Those labels are
> NOT
> > advertised by RFC3107. The label index is used to derive them.
> 
> If the ingress router knows the label advertised by the next hop
> (L1), and if it knows the SRGB of the next hop, then why can't it
> just  figure out the "label index" by doing a subtraction?  Why the
> need for a separate attribute that specifies the label index?

Because a neighbor may advertise 3 (implicit null)

--Jakob

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