Peter, Lots of thanks for a prompt and very encouraging response. Do you think that the new Algo specific Adj-SID sub-TLV could be added to the current IS-IS segment Routing Extensions draft, or should be handled in a small dedicated document?
Regards, and lots of thanks in advance, Sasha Office: +972-39266302 Cell: +972-549266302 Email: [email protected] -----Original Message----- From: Peter Psenak <[email protected]> Sent: Tuesday, March 5, 2019 5:28 PM To: Alexander Vainshtein <[email protected]> Cc: [email protected]; [email protected]; [email protected] Subject: Re: [spring] FlexAlgo and Global Adj-SIDs Hi Sasha, On 02/03/2019 18:57 , Alexander Vainshtein wrote: > Peter, > Lots of thanks for a prompt and hivhly informative response. > > It seems that per-FlexAlgo Adj-SIDs can be useful even if they are local. > The relevant use case could be a protected local Adj-SID that is used > in a SR-TE LSP that has been set up with some constraints in mind. > These constraints would be preserved when the protected adjacency in > question is UP, but could be violated when it fails and the shortest > path to the node at the remote end of adjacency is used. yes, this is a possible use case. This would require a new Algo specific Adj-SID sub-TLV to be defined. This could be done. thanks, Peter > > Local protected Adj-SIDs for sure have bern implemented, so this looks > like a more relevant use case than global adjacencies to me. > > What do you think? > > Regards, and lots of thanks in advance, > > Thumb typed by Sasha Vainshtein > > ---------------------------------------------------------------------- > -- > *From:* Peter Psenak <[email protected]> > *Sent:* Thursday, February 28, 2019 9:56:49 PM > *To:* Alexander Vainshtein; > [email protected] > *Cc:* [email protected]; [email protected] > *Subject:* Re: [spring] FlexAlgo and Global Adj-SIDs > > Hi Alexander, > > On 28/02/2019 19:19 , Alexander Vainshtein wrote: >> Dear colleagues, >> >> I have a question regarding global Adj-SIDs in >> draft-ietf-isis-segment-routing-extensions. >> >> >> >> Section 3.4 of RFC 8402 <https://tools.ietf.org/html/rfc8402> >> defines definition and handling of global Adj-SIDs. >> >> The relevant text is given below: >> >> >> >> <quote> >> >> Similarly, when using a global Adj-SID, a packet injected anywhere >> >> within the SR domain with a segment list {SNL}, where SNL is a >> global >> >> Adj-SID attached by node N to its adjacency over link L, will be >> >> forwarded along the shortest path to Nand then be switched by N, >> >> without any IP shortest-path consideration, towards link L. >> >> ... >> >> The use of global Adj-SID allows to reduce the size of the segment >> list >> >> When expressing a path at the cost of additional state (i.e., the >> global >> >> Adj-SID will be inserted by all routers within the area in their >> >> forwarding table). >> >> <end quote> >> >> >> >> The definition of the Adjacency Segment Identifier Sub-TLV in Section >> 2.2.1 of the draft matches the behavior defined in RFC 8402, i.e., it >> allows advertisement of global Adj-SIDs. >> >> These advertisements can be associated with a specific IGP adjacency >> and, in multi-topology scenarios, a specific topology. But it is not >> associated with any specific algorithm since the default algorithm >> for reaching the advertising node is implicitly assumed in full >> alignment with RFC 8402. >> >> >> >> My question is about the situation in which multiple algorithms are >> supported by the routers in the SR domain, so that each node >> advertises a dedicated Node SID for each of these algorithm. >> >> In this scenario, the operator can set up a SR-TE LSP that meets >> specific constraints (incorporated in one of these algorithms, see >> IGP Flexible Algorithm >> <https://tools.ietf.org/html/draft-ietf-lsr-flex-algo-01> draft) by >> using Node SIDs that have been advertised with the corresponding >> algorithm and local Adj-SIDs. However, global Adj-SIDs cannot be used >> (e.g., for reducing the label stack depth) in this situation. >> >> I wonder if the possibility to advertise global Adj-SID (that can be >> considered as replacing a combination of the Node SID of the >> advertising node and one of its local Adj-SIDs) as associated with a >> specific algorithm has ever been considered? > > to be honest, it has been not. > > The usage of the global Adj-SID has not been widely considered due to > the additional forwarding entries it requires on rest of the routers > in the area. Not sure if there are implementations of the global > ADj-SID out there. > > If you believe this is important, we can certainly addressed that in a > small draft. > > thanks, > Peter > > > > >> >> >> >> Regards, and lots of thanks in advance, >> >> Sasha >> >> >> >> Office: +972-39266302 >> >> Cell: +972-549266302 >> >> Email: [email protected] >> >> >> >> >> _____________________________________________________________________ >> ______ >> >> This e-mail message is intended for the recipient only and contains >> information which is CONFIDENTIAL and which may be proprietary to ECI >> Telecom. 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