Hello everyone,
there is a problem for networks that use spring on the MPLS forwarding
plane: It seems it would not be feasible to use anycast segments for
traffic engineering since we introduced indexed SIDs.
I would really like to use of some well-defined anycast addresses to
solve a number of traffic engineering use cases. I.e. an anycast address
would stand for a certain property of possible paths and segment routing
would be responsible to apply this property to the traffic which needs
it. In other words I want to use a path with one anycast segment,
followed by the usual node segment to the actual destination (typically
an egress LER).
The anycast segment itself is fine: it can be built in the usual way.
However for the following node segment the spring source node cannot
calculate the label value. The stack PUSHing router would not know at
which node the second segment would actually start, and thus which SRGB
to apply.
If needed I could make a drawing to illustrate this problem.
As I see it, there would be three different options to address this problem:
1) Abandon anycast segments completely.
This would greatly reduce the usefulness of segment routing in my opinion.
2) Use a homogeneous SRGB, so that label values would effectively be the
same for all nodes.
But in this case, why did we introduce indexes at all?
3) Use a context label. E.g. as defined in
draft-raszuk-mpls-domain-wide-labels.
Since I really hate to add more labels to the stack than really needed,
could we think of a way to only use context labels where needed?
As far as I can see this would only be relevant for the segment
immediately following an anycast segment.
Are there any other options?
Best regards, Martin
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