Segment Routing policies (static and
BGP) for which the local router is head-end are
processed by the local segment routing database. For
each (color, endpoint) combination, the database must
validate each candidate path and choose one to be the
active path. The steps of this process are outlined in
the Segment Routing policy validation and selection
process.
-
Is the path missing a binding SID in the form
of an MPLS label?
- Yes: This path is invalid and cannot be
used.
- No: Go to next step
-
Does the path have any segment-list
containing a segment type not equal to 1 (an MPLS
label)?
- Yes: This path is invalid and cannot be
used.
- No: Go to next step
-
Are all segment-lists of the path
invalid?
A segment-list is invalid if it is empty, if
the first SID cannot be resolved to a set of one
or more next-hops, or if the weight is 0.
- Yes: This path is invalid and cannot be
used.
- No: Go to next step
At this step the router attempts to resolve
the first segment of each segment-list to a set of
one or more next-hops and outgoing labels. It does
so by looking for a matching SID in the segment
routing module, which must correspond to one of
the following:
- SR-ISIS or SR-OSPF node SID
- SR-IS or SR-OSPF adjacency
SID
- SR-IS or SR-OSPF adjacency-set SID
(parallel or non-parallel set)
Note: The label value
in the first segment of the segment-list is
matched against ILM label values that the local
router has assigned to node-SIDs, adjacency-SIDs,
and adjacency-set SIDs. The matched ILM entry may
not program a swap to the same label value encoded
in the segment routing policy - for example, in
the case of an adjacency SID, or a node-SID
reachable through a next-hop using a different
SRGB base.
-
Is the binding-SID an available label in the
reserved-label-block range?
- Yes: Go to next step.
- No: This path is invalid and cannot be
used.
-
Is there another path that has reached this
step that has a higher preference value?
- Yes: This path loses the tie-break and
cannot be used.
- No: Go to next step.
-
Is there a static path?
- Yes: Select the static path as the active
path because the protocol-origin value associated
with static paths (30) is higher than the
protocol-origin value associated with BGP learned
paths (20).
- No: Go to next step.
-
Is there a BGP path with a lower originator
value?
The originator is a 160-bit numerical value
formed by the concatenation of a 32-bit ASN and a
128-bit peer address (with IPv4 addresses encoded
in the lowest 32 bits.)
- Yes: This path loses the tie-break and
cannot be used.
-
Is there another BGP path with a higher
distinguisher value?
- Yes: Select the BGP path with the highest
distinguisher value.