Hi Authors,
Since PCE Controller has global view of the network. There are
services which spread across multiple AS or IGP domain. SR domain can be
extended across IGP domain to achieve the service requirement as SR domain can
also span across IGP Domain. But by using IGP to distribute Segment labels has
one drawback, Where all the external prefixes imported by an ASBR will be
originated as prefix SID(usually generated by ASBR) in area or AS scope RI LSA
TLV (OSPF terminology). But will not be able to originate adjacent segment
SIDs of other IGP domain due to this intermediate nodes on the path has to
understand LSP information and push required SR labels which is not desired in
segment routing (path has to be decided at the ingress).
In case of SR-TE, PCE can give end to end(across AREA or across AS) SR-TE path
with all the TE requirement satisfied, PCE as a controller should ensure
SR-Node labels remain unique across AS till the scope of SR domain.
I would like to add a section under "5.5.2. PCECC Segment Routing (SR)" as
below :
5.5.2.3. PCECC SR-TE IGP/SR Domain
PCECC will ensure in allocating unique SR Node Lables across SR
Domain. There will be cases where some service spread across multiple
IGP domain yet with in SR Domain. One Such example is Seamless MPLS
which provides an architecture to support a wide variety of different
services on a single MPLS platform fully integrating access,
aggregation and core network. In such cases PCE can help SR ingress
router to have label stack consist of SR node label and SR Adj Labels
from different IGP domain.
The PCECC will send PCLabelUpd to update the SR label information which
might belong to different IGP doamin yet belong to same SR domain to
required nodes which fall on the path to reach till the destination.
PCECC will ensure all TE requirements are addressed while setting up
the explicit path and required SR label stack is updated to ingress
node.
The forwarding behavior will not be different as labels processed
by intermediate nodes. ASBR will seamlessly forward the packet
as the FEC is updated by PCE using PCLabelUpd message.
The Path Setup Type MUST be set for PCECC SR-TE (see Section 7.3).
The rest of the PCEP procedures and mechanism are similar to
[I-D.ietf-pce-segment-routing].
PCE rely on the Adj label cleanup using the same PCLabelUpd message.
p.s. Segment Routing Architecture [draft-ietf-spring-segment-routing-01] says
"IGP-prefix Segment, Prefix-SID: an IGP-Prefix Segment is an IGP segment
attached to an IGP prefix.
An IGP-Prefix Segment is always global within the SR/IGP domain"
Thanks & Regards
Anil S N
"Be liberal in what you accept, and conservative in what you send" - Jon Postel
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