Inter-AS SR-MPLS option C with IOS-XE 17.13.01a

[n_abromov]

Hello All,

I have a simple topology with a few XE routers running SR-MPLS in each ISIS domain. PE1 and PE2 are acting as ASBRs. Between PE1 and PE2, I’m running BGP-LU and VPNv4 AF.

The issue I’m facing is that the label assigned by BGP-Labeled Unicast is not from the SRGB block.

Here’s a traceroute from R1 (in AS1) to R5 (in AS2):

R1#traceroute 5.5.5.5
1 10.10.10.2 [MPLS: Labels 10003/17/10005 Exp 0]
2 10.10.23.2 [MPLS: Labels 17/10005 Exp 0]

Looking the MPLS label range, I can see the label falls under the downstream generic label region, but it's not what I expected to see. 

PE2# show mpls label range
Downstream Generic label region: Min/Max label: 16/1048575
Range for Reserved labels:
1. Range ID: 0 Owner: SR-APP
Min/Max label: 10000/13000
Clients Permitted: ISIS OSPF
BGP-Segment-Routing TE Policy
Clients In Use: ISIS BGP-Segment-Routing
Dynamic Label Clients In Use: ISIS BGP-Segment-Routing
Checkpoint Labels Unclaimed: FALSE
2. Range ID: 1 Owner: SR-APP
Min/Max label: 15000/15999
Clients Permitted: ISIS OSPF
BGP-Segment-Routing TE Policy
Clients In Use:
Checkpoint Labels Unclaimed: FALSE

PE2#

Thank you in advanced,

Thank you.

[Harold Ritter · Accepted solution]

Hi @n_abromov ,

It is normal behavior for the BGP LU allocated label is not from the SRGB. The BGP LU label is allocated locally on PE2 from a separate pool and advertised to PE1.

Regards,

[Albertt]

Hi @n_abromov 

This seems like expected behavior. You might be able to influence the path selection by creating an SR-TE policy from R1 to R5 with an explicit path using a Binding SID at P2. This way, you can ensure that the labels used in the traceroute align with the SRGB and that the traffic between AS1 and AS2 follows a consistent and controlled path.

Let me know if this helps!

[n_abromov]

Thank you!