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XTC dynamic SR-TE tunnels

In our lab setup, using all IOS XR routers and an XTC based PCE, we do not understand why all the dynamic SR-TE tunnels calculated by XTC are expressed as SR Adjacency SIDs, never as Node SIDs, hence never taking advantage of all the ECMP paths and resulting in very long SID-lists (a label stack always equal to the number of hops of the tunnel).

Examples found on dcloud labs/demos show PCE calculated path with node-SIDs and/or a combination of Adj-SID and Node-SIDs with the exact same network setup and configuration. Any idea what could be causing this issue?

Just a Note: when configuring explicit SR-TE tunnel with a manually configured SID-list based on Node-SIDs, everything works well.

More info:

The headend, a ASR9001-001 is configured with an TE interface tunnel that request dynmaic SR path calculation to the XTC and the resulting SID-list is a sequence of Adjacency SID in all scenarios (the setup has 10 nodes, 5 of which in the same domain and the other 5 in another domain, but same result). In the cases where the tail-end (egress PE) is for example 7 hops away, the XTC calculated path is 7 Adjacency SIDs

Example of an output:

The tunnel configuration:

RP/0/RSP0/CPU0:ASR9001-001#show run int tunnel-te 100

Wed Nov 15 12:16:57.695 CET

interface tunnel-te100

description min TE metric tunnel

ipv4 unnumbered Loopback0

destination 10.92.224.105

path-selection

  metric te

!

path-option 1 dynamic pce segment-routing

and the PCE calculated path (6 hops, all Adj-SID when our topology has a few ECMP paths that satisfies the object "minimum TE metric"

(same result with IGP metric):

RP/0/RSP0/CPU0:ASR9001-001#show mpls traffic-eng tunnels 100
Wed Nov 15 12:20:19.981 CET


Name: tunnel-te100  Destination: 10.92.224.105  Ifhandle:0x520
  Signalled-Name: ASR9001-001_t100
  Status:
    Admin:    up Oper:   up   Path:  valid   Signalling: connected

    path option 1, (Segment-Routing) type dynamic pce (Basis for Setup, path weight 230)
    G-PID: 0x0800 (derived from egress interface properties)
    Bandwidth Requested: 0 kbps  CT0
    Creation Time: Thu Nov  2 08:49:44 2017 (1w6d ago)
  Config Parameters:
    Bandwidth:        0 kbps (CT0) Priority:  7  7 Affinity: 0x0/0xffff
    Metric Type: TE (interface)
    Path Selection:
      Tiebreaker: Min-fill (default)
      Protection: any (default)
    Hop-limit: disabled
    Cost-limit: disabled
    Path-invalidation timeout: 10000 msec (default), Action: Tear (default)
    AutoRoute: disabled  LockDown: disabled   Policy class: not set
    Forward class: 0 (default)
    Forwarding-Adjacency: disabled
    Autoroute Destinations: 0
    Loadshare:          0 equal loadshares
    Auto-bw: disabled
    Path Protection: Not Enabled
    BFD Fast Detection: Disabled
    Reoptimization after affinity failure: Enabled
    SRLG discovery: Disabled
  History:
    Tunnel has been up for: 00:08:12 (since Wed Nov 15 12:12:08 CET 2017)
    Current LSP:
      Uptime: 00:08:12 (since Wed Nov 15 12:12:08 CET 2017)
    Reopt. LSP:
      Last Failure:
        LSP not signalled, has no S2Ls
        Date/Time: Wed Nov 15 12:09:28 CET 2017 [00:10:52 ago]
    Prior LSP:
      ID: 34 Path Option: 1
      Removal Trigger: tunnel shutdown

  Segment-Routing Path Info (PCE computed path)
    Segment0[Link]: 10.93.11.1 - 10.93.11.2, Label: 24001
    Segment1[Link]: 10.93.2.13 - 10.93.2.14, Label: 24028
    Segment2[Link]: 10.93.3.10 - 10.93.3.9, Label: 24001
    Segment3[Link]: 10.93.2.17 - 10.93.2.18, Label: 24003
    Segment4[Link]: 10.93.4.5 - 10.93.4.6, Label: 24010
    Segment5[Link]: 10.94.2.1 - 10.94.2.2, Label: 24004
Displayed 1 (of 7) heads, 0 (of 0) midpoints, 0 (of 0) tails
Displayed 1 up, 0 down, 0 recovering, 0 recovered heads

Everyone's tags (5)
2 REPLIES
Rising star

Re: XTC dynamic SR-TE tunnels

Mehdi,

Please refer to the following documents for more information:

https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r6-2/segment-routing/configuration/guide/b-segment-…

ASR9000/XR Introduction to Segment Rout... - Cisco Support Community

I also recommend you post this to the Cisco Support Community for more feedback.

Kelli Glass

Moderator for Cisco Customer Communities

Beginner

Re: XTC dynamic SR-TE tunnels

Looks like there is an issue with OSPF in genereal.

I reconfigured the IGP in all the domains with ISIS (instead of OSPF) and the problem was resolved: the PCE is now provided paths with node SIDs using a minimum number of SIDs (instead of Adj-SID for EVERY hop) allowing for ECMP and reducing the label-stack depth.

Is this a known issue with OSPF? 

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