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. If you have
received this
transmission in error, please inform us by e-mail, phone or fax, and
then delete the original
and all copies thereof.
___________________________________________________________________________
_______________________________________________
spring mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/spring
_______________________________________________
Lsr mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/lsr
