IOS XR - Ping works and Traceroute Request time out

[AmyEST]

I'm able to get response to any destination when using ping, but got traceroute timeout on the ASR9K version 6.5.3. I cannot receive probe packets.I just want to understand is whether there is anything on the ASR9K could make traceroute not getting to the destination. Ping works fine, but trace does not. The traceroute output is not so normal?. Would appreciate any help or response on this.

Tracing the route to 10.100.0.8

1 * * *
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *

RP/0/0/CPU0:XRv3#ping 10.100.0.8 sweep
Type escape sequence to abort.
Sending 89945, [36..18024]-byte ICMP Echos to 10.100.0.8, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Harold Ritter]

Hi @AmyEST ,

There is a difference in the way ping and traceroute work under the hood. ping uses icmp echo request and echo reply messages. traceroute uses udp datagrams towards the destination and will receive a combination of icmp ttl expired and icmp port unreachable messages back. If you have any ACL blocking these messages, it might explain why it works with one and not with the other.

Regards,

[AmyEST]

Hi Harold,

Thank you for your detailed reply. That's the point, there is no ACL blocking it. That only happens with any IPv4 addresses off an ASR9K. No issue when tracing IPv6 address of an ASR9K. If we trace IPv4/v6 between different nodes "NON-ASR9K" by having ASR9K transit nodes on the path, the trace works without issue . The packet can reach the final destination even if it traverses thru an ASR9K transit node. The ASR9K only returns the * * * message ouput between IPv4 address of ASR9K devices even though connectivity looks fine in the network. Not sure what it could be because trace is ok with v6 . I know it is not routing issue. We're having ISIS as IGP. Would appreciate any input on this.

non-asr9k < > asr9k < asr9k < > non-asr9k = Ping & Trace works ok with IPv4 & IPv6.

asr9k < > asr9k < asr9k < > asr9k = Ping works ok but not trace for IPv4. Ping & trace work ok with IPv6.

Thank you,

AmyEST.

[smilstea]

There are many reasons for traceroute not to return any data. Is this MPLS? With MPLS a traceroute will go to the P or PE router and then it has to complete the LSP by going to the CE and then back to the source of the traceroute. If its plain IPv4 network then the traceroute will return immediately to the source. Its a good idea to check the routing and debugs to see if the packets are seen and sent back properly.

Sam

[AmyEST · Accepted solution]

I finally found the problem. There were some p2p links having the ipv4 unreachable disable command configured. After removing it, I was able to make an ipv4 traceroute work as smoothly as it does for an ipv6 traceroute. That was the reason why it was not working. Thank you all for the help!.

AmyEst

[Harold Ritter]

Hi @AmyEST ,

Glad you found the issue. Thanks for the update.

Regards,