This article describes how to troubleshoot the issue when we are unable to ping the loopback IP under ISIS instance from direct router which is CISCO ASR 9K

Customer informed they started facing the issue after upgrading to 23.4R1-S1.5

• Customer is having 2 Cisco routers labroot1 directly connected to Juniper labroot4 router and another Cisco router labroot2 directly connected to labroot3 Juniper router. both cisco routers are directly connected and in the same way both Juniper routers are directly connected
•  All are connected via ae interface
• Customer informed they started facing the issue after upgrading to 23.4R1-S1.5 which they have upgraded to targeted version on August 30 
• Issue is Cisco routers labroot1 and labroot2 and Juniper router labroot3 able to ping the Juniper router labroot4 loopback IP  without sourcing from source loopback
• These routers unable to ping the loopback IP of labroot4 if they use source loopback
• When the interface ae32.6 connecting the labroot4 and labroot1 is disabled , loopback IP of labroot4 can be pinged from any router sourcing from loopback as its taking via another path
• When interface ae32.6 is up unable to ping the loopback Ip of labroot4

labroot4> ping 2400:9800:7008:8c00:: rapid

Sep 23 11:32:45

PING6(56=40+8+8 bytes) 2400:9800:7008:8c01:1000::1 --> 2400:9800:7008:8c00::

!!!!!

--- 2400:9800:7008:8c00:: ping6 statistics ---

5 packets transmitted, 5 packets received, 0% packet loss

round-trip min/avg/max/std-dev = 2.798/2.923/3.022/0.080 ms

labroot4> ping 2400:9800:7008:8c00:: interface lo0

Sep 23 10:37:24

PING6(56=40+8+8 bytes) 2400:9800:7000:3003:: --> 2400:9800:7008:8c00::

^C

--- 2400:9800:7008:8c00:: ping6 statistics ---

3 packets transmitted, 0 packets received, 100% packet loss

labroot4> ping 2400:9800:7000:309e:: interface lo0 rapid

Sep 23 10:52:49

PING6(56=40+8+8 bytes) 2400:9800:7000:3003:: --> 2400:9800:7000:309e::

.....

--- 2400:9800:7000:309e:: ping6 statistics ---

5 packets transmitted, 0 packets received, 100% packet loss

labroot4> ping 2400:9800:7000:309e:: source 2400:9800:7000:3003:: rapid

Sep 23 10:53:46

PING6(56=40+8+8 bytes) 2400:9800:7000:3003:: --> 2400:9800:7000:309e::

.....

--- 2400:9800:7000:309e:: ping6 statistics ---

5 packets transmitted, 0 packets received, 100% packet loss
 

• Facing this issue only with labroot4  router, when checked with labroot3 router  we are able to ping all the Cisco routers with and without loopback
• With labroot3 router we are unable to ping labroot4 when interface ae32.6 is up , when interface is disabled we are able to ping
• ISIS adjacency is up between labroot4 and labroot3

labroot4> show isis adjacency igp-instance PAG-JIKPN1319-02C_ins

Sep 23 11:37:14

Interface System L State Hold (secs) SNPA

ae32.6 PAG-JIKPN1319-02C 2 Up 24

lt-1/0/0.101 AG-SBWLG-02 2 Up 20
 

• We did observe bgp for some of IPV6 loopback of Cisco is not established and they are facing this issue.
• These routers unable to ping the loopback IP of labroot4
•  Issue was on the Cisco side with a bad MAC programming. After rebooting the Cisco device labroot1 the issue was fixed we were able to ping the loopback IP of labroot4
• We also observed the MAC address of the physical interface changed after upgrade, this behavior is expected in Junos version from 23.1R1 and above releases.