Re: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)

["Ketan Talaulikar (ketant)" <[e-mail]>]

Hi Boris,

Using your original email to respond instead of top-posting so I do not miss anything in your original query. This builds on top of the responses from Robert and Jeff with some more details.

Please check inline below.

From: Idr <[e-mail]> On Behalf Of Boris Hassanov
Sent: 21 July 2021 15:25
To: IDR List <[e-mail]>
Subject: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)

Hi all,

1) Need an advice, I might be missing something but I see that Anycasi SID (3333 in the example below)  is in BGP-LS update as regular Prefix SID TLV:

 Path Attribute - BGP-LS Attribute

            Flags: 0x80, Optional, Non-transitive, Complete

                1... .... = Optional: Set

                .0.. .... = Transitive: Not set

                ..0. .... = Partial: Not set

                ...0 .... = Extended-Length: Not set

                .... 0000 = Unused: 0x0

            Type Code: BGP-LS Attribute (29)

            Length: 12

            Link State

                Prefix SID TLV

                    Type: 1158

                    Length: 8

                    Flags: 0x40, Node-SID (N)

                        0... .... = Re-advertisement (R): Not set

                        .1.. .... = Node-SID (N): Set

                        ..0. .... = No-PHP (P): Not set

                        ...0 .... = Explicit-Null (E): Not set

                        .... 0... = Value (V): Not set

                        .... .0.. = Local (L): Not set

                    Algorithm: 0

                    SID/Index: 3333

Here is confusion, how, let's say, a controller, which receives such BGP-LS update, could  distinguish it from regular Prefix/Node SID?
[KT] Node SID or Anycast SID are sub-sets of the Prefix SID and therefore both are advertised using the Prefix SID TLV. The way to distinguish them would be based on the flags in the Prefix Attribute Flags TLV https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-ls-segment-routing-ext-18#section-2.3.2. It carries the Node and Anycast flags. Note that the Anycast flag was introduced very recently. The indication of Node flag surely eliminates the Prefix being Anycast – this is by definition. Of course, a prefix’s association with the Node or Anycast flag will depend on the accuracy of the router configuration by the operator. In the case of anycast, this can be verified by the controller using the network-wide topology.

Unfortunately draft-ietf-idr-bgp-ls-segment-routing-ext-18 does not clarify this issue because it only describes Prefix SID TLV type 1158. Any suggestions?

2)  draft-ietf-idr-bgp-ls-segment-routing-ext-18 has two types of labels TLV: SID/Label TLV type 1161 and Prefix SID  TLV type 1158.  I checked several vendors implementations of BGP-LS, all of them use only Prefix SID TLV.
Why the draft does not have some conditions or clarifications when to use SID/Label TLV and when Prefix SID TLV?
[KT] The SID/Label TLV (1161) is used as a sub-TLV of SR Capabilities and SRLB TLVs as indicated here https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-ls-segment-routing-ext-18#section-2.1.1. I am not sure why this would be mixed up with the Prefix SID TLV which is very different.

Thanks,
Ketan


Thank you.

SY,
Boris