Hi Stefano,
If a BGP route is received that contains a Prefix-SID attribute with an
Originator SRGB TLV, but the prefix field of the NLRI does not contain a
host address, the attribute SHOULD be regarded as malformed. If a
Prefix-SID attribute contains more than one SRGB TLV, it SHOULD be
regarded as malformed. See section 7 for the treatment of a malformed
Prefix-SID attribute.
When a route carrying the Prefix-SID attribute is propagated, the
Originator SRGB TLV (if present) MUST NOT be changed.
why would you need such limitation ? A prefix may have a shorter mask
than 32 (or 128) and still be ok for the Originator SRGB to be there.
The SRGB is a property of a node, not a property of a prefix. To make
use of the "Originator SRGB", you have to know the node whose property
it is. And you have to be able to tunnel packets to that node. In the
text I wrote above, the prefix field in the NLRI identifies the node to
which the "Originator SRGB" belongs, and the prefix-SID field
essentially gives you a node-SID that you can use to tunnel to the node
in question.
The Originator-SRGB may only be inserted by the originator of the
prefix, maybe we should emphasize that, but the masklength is mostly
irrelevant here.
I don't see that the Originator-SRGB TLV is useful without an explicit
identification of the node whose SRGB it describes. Certainly if you
are trying to set up an explicitly routed path (perhaps as a loose
source route) what you need are the node-SIDs are of the hops you want
to specify, and the SRGB of each hop.
When you talk about "the originator of the prefix", I think what you
really mean is "the last node of the BGP prefix segment". But I don't
think that that term necessarily denotes a unique node, as there might
be multiple ECMP paths to the prefix, and the prefix-SID does not
distinguish among them. E.g., the prefix itself might be multi-homed,
or there might be multiple exit points from the SR domain, all of which
are equidistant from the prefix. In cases like that, you have no way of
knowing whether a label computed from the Originator-SRGB is actually
going to be correctly interpreted, because you don't really know the
path a packet will take when it is labeled with the prefix-SID.
Eric
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