Hi all authors of segment routing,
This is the sixth comments on segment routing. It is another issue related with
SR-TE path.
S4---S5
/ \
S1---S2---S3 S8---S9---S10
\ /
S6---S7
For MPLS TE, there are two types of explicit paths: strict and loose. Even for
the loose hop, the ingress node will
calculate all the hops to the loose hop. And the MPLS TE path is independent
from the routing, this means even if
the route change, the MPLS TE path to the loose hop can be kept. For example,
as shown in the above figure, if the
ingress S1 configures the loose hop S8 to the destination S10, it will
calculate the path S1->S2->S3->S4->S5->S8->S9->S10.
If the metric changes for the link between S3 and S4 and make the shortest
route path to S8 from the S3->S4->S5->S8 to
S3->S6->S7->S8. But the exiting MPLS TE path will not change accordingly.
For SR-BE Path, if the loose hop is configured, there will be two possible
choices:
Option 1. Same as the existing MPLS TE.
If the option is adopted, there will be following challenges:
1) This means the loose hop cannot reduce the label stack for the SR-TE path.
It need more labels to be encapsulated after
calculating the whole path and representing each hop with node label or
adjacency label. The deep depth of the label statck
may exceeds the hardware limitation or cause low efficiency payload.
2) More challenges can be proposed from the inter-area bahavior. If PCE is not
adopted, if the ingress node has to only
specify the ABRs as the loose hop to the destination for inter-area, for MPLS
TE, the ingress node can acquire the end-to-end
path info through RRO. But since there is no RRO-like mechanism in IGP, it is
impossible for SR-TE path to get the end-to-end
path info. This means the ingress node cannot get the whole label stack info to
determine the whole SR-TE path.
Option 2. Limited labels for the loose hop. For example, since only loose hop
S8 is specified, then the label stack for the
SR-TE path to the S10 is [S8 S10]. That is, two layers of labels is enough.
This method can avoid the deep label stack issue. But it will introduce other
issues. In fact, for the SR-TE path, it mixes
the route path and the TE path together. There may be following issues:
1) The route change will have effect on the SR-TE path. If the routes to the S8
is changes, the SR-TE path to the S10 will
change accordingly.
2) For the ingress node, from the point of the network maintenance view, it
loses the control on the end-to-end path. That is,
the end-to-end path is unknown to the ingress node.
3) If there is load balance for the route, for example, there are two equal
paths to S8 as the above topology: S3->S4->S5->S8,
S3->S6->S7->S8.If BFD is enabled for the SR-TE path, the BFD packets and the
real traffic may go through different path.
There will be the possibility that BFD will detect the wrong path and may cause
furthe problem such as the wrong traffic switch.
I wonder if the issue has been taken into account and which option you prefer.
Regards,
Robin
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