s-BFD MPLS reverse path binding label not being sent

[markk2316]

Hello !

I have a seamless bfd session running on a SR-TE policy what is configured and is UP. 
segment-routing
traffic-eng
policy SRTE_bla
source-address ipv4 66.110.0.63
color 1 end-point ipv4 33.33.33.33
bfd
minimum-interval 100
multiplier 5
invalidation-action none
!

Right now these are the BFD packets 

Now i wanted to specify the reverse-label-path so that the reflector will be able to return the packets. 

segment-routing
traffic-eng
policy SRTE_bla
bfd
+ reverse-path
+ binding-label 16083
!
!
!
!
!

After the commit passes i noticed in a wireshark capture that the destination address of the s-bfd changed to the 66.110.0.63 (was 127.0.0.1 prior). And the MPLS label i described as the reverse path isn't what i configured 

I wanted to know if this is known behavior and what is the purpose of the reverse-path label if it isn't to specify the reflector which label should it use to get back to the initiator.?
Thanks in advance !

[Harold Ritter]

Hi @markk2316 ,

> destination address of the s-bfd changed to the 66.110.0.63 (was 127.0.0.1 prior)

This is normal given that the BFD mode is changed to "echo mode" when the reverse path is changed from the default ipv4 to mpls, as described in the documentation.

"Note that when MPLS return path is used, BFD uses echo mode packets, which means the tail-end’s BFD reflector does not process BFD packets at all."

> And the MPLS label i described as the reverse path isn't what i configured 

This looks like a bug. What version of IOS-XR do you use?

Regards,

[markk2316]

Hey Harold, Thank you for the reply your explanation helped me understand what's going on. 

I'm using the following version as shown via the show version detail

Cisco IOS XR Software, Version 7.5.1
Copyright (c) 2013-2021 by Cisco Systems, Inc.

Build Information:
Built On : Sun Nov 28 10:39:00 PST 2021
Built Host : iox-ucs-055
Workspace : /auto/srcarchive15/prod/7.5.1/asr9k-x64/ws
Version : 7.5.1
Location : /opt/cisco/XR/packages/
Label : 7.5.1

[Harold Ritter]

Hi @markk2316 ,

Can you please provide the "sh bfd label session detail" output.

Regards,

[markk2316]

Hey Harold, Apologies for the late reply but better late than never

segment-routing
traffic-eng
policy SRTE_bla
source-address ipv4 66.110.0.63
color 50 end-point ipv4 33.33.33.33
autoroute
force-sr-include
!
bfd
minimum-interval 50
multiplier 2
invalidation-action none
reverse-path
binding-label 16063
!
!

The traffic is sent with the label 917504

While the output of the sh bfd label session detail is 

RP/0/RSP0/CPU0:Cisco-9K(config)#do show bfd label session detail 
Wed Jul 12 05:52:21.606 UTC

I/f: srte_c_50_ep (LSP:2), Location: 0/0/CPU0, Label: 38151
State: DOWN for 0d:0h:18m:5s, number of times UP: 0
Session type: SW/LB/V4/SH/TH/SB
Reverse Label: 3208511488
Received parameters:
Version: 0, desired tx interval: 0 ms, required rx interval: 0 ms
Multiplier: 0, diag: None
My discr: 65541, your discr: 0, H/D/F/P/C/A: 0/0/0/0/0/0
Transmitted parameters:
Version: 1, desired tx interval: 2 s, required rx interval: 0 ms
Multiplier: 2, diag: None
My discr: 65541, your discr: 65541, state DOWN, D/F/P/C/A: 1/0/0/1/0
Timer Values:
Local negotiated async tx interval: 2 s
Remote negotiated async tx interval: 0 ms
async detection time: 0 ms
Local Stats:
Intervals between async packets:
Tx: Number of intervals=100, min=1663 ms, max=1994 ms, avg=1835 ms
Last packet transmitted 1704 ms ago
Rx: Number of intervals=0, min=0 s, max=0 s, avg=0 s
Last packet received 0 s ago
MP download state: BFD_MP_DOWNLOAD_ACK
State change time: Jul 12 05:44:51.983
Session owner information:
Desired Adjusted
Client Interval Multiplier Interval Multiplier
-------------------- --------------------- ---------------------
XTC 50 ms 2 2 s 2

[Harold Ritter · Accepted solution]

Hi @markk2316 ,

This output does not look right:

Reverse Label: 3208511488

I would definitely recommend trying with more recent code.

Regards,