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

"Saikat Ray (sairay)" <sairay@cisco.com> Tue, 25 November 2014 00:21 UTC

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From: "Saikat Ray (sairay)" <sairay@cisco.com>
To: "Jakob Heitz (jheitz)" <jheitz@cisco.com>, 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: Tue, 25 Nov 2014 00:21:23 +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> <075DE01702BBC249BE1357EFD20DCFE5141ADB69@xmb-aln-x02.cisco.com>
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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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Also the neighbor might not support SR. I.e., if there is a topology
A--B--C, where A and C support SR, but B doesn't. The attribute is needed
for C to know what label-index A is using (for A's loopback interface).

On 11/24/14, 1:25 PM, "Jakob Heitz (jheitz)" <jheitz@cisco.com> wrote:

>> -----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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