PCE with inter-domain SRTE (inaccessible next-hop)

[n_abromov]

Dear All,

I’m looking for some guidance. I have a multidomain ISIS setup with Segment Routing (SR) and Path Computation Element (PCE) on IOS XR. Additionally, I’m using the PCE as a Route Reflector (RR) in a lab environment. The issue arises when routes are sent back to the RR clients; they are not getting installed. The reason seems to be that there is no reachable next-hop in domain 1 for a PE in domain 2.

PE1#show bgp vpnv4 unicast all 55.55.55.0/24
BGP routing table entry for 100:1:55.55.55.0/24, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  Refresh Epoch 1
  100
    11.11.11.11 (inaccessible) (via default) from 16.16.16.16 (16.16.16.16)
      Origin incomplete, metric 0, localpref 100, valid, internal
      Extended Community: RT:100:1
      Originator: 11.11.11.11, Cluster list: 16.16.16.16
      mpls labels in/out nolabel/20
      rx pathid: 0, tx pathid: 0
Updated on Aug 7 2024 18:39:09 UTC

My question is: What are the ways to resolve this issue? I would assume redistributing the loopbacks between the domains contradicts the purpose of having a multidomain setup for SR.

update: Found a workaround on a web page using a static route, which seems to be resolving the problem but I assume there should be better way.

[Harold Ritter · Accepted solution]

Hi @n_abromov ,

In IOS-XR, you can use a feature called BGP on demand next hop (ODN). With this feature, an SRTE LSP would be signaled automatically for prefixes learned via BGP.

I do not think it is available in IOS-XE yet though.

Please refer to this document that discusses BGP ODN Among other things.

https://xrdocs.io/design/blogs/latest-peering-fabric-hld

Regards,

[n_abromov]

Hi @Harold Ritter 

Thank you for your response, I found couple of useful sources of information meanwhile. 

ODN seems to be supported in the XE17, which is what I have in the lab. 

ODN XE:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/seg_routing/configuration/xe-17/segrt-xe-17-book/m_sr-te-on-demand-next-hop.html

XR next-hop issue described:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/217744-deploy-bgp-soft-next-hop-in-cisco-ios-xr.html

I will investigate the ODN feature later today.

Nik