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 N and 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?
Regards, and lots of thanks in advance,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: [email protected]
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