Troubleshooting Junos

Tue, 02 Jun 2026 21:31:13 +0200

Notes from a Juniper Junos troubleshooting course

Packets reaching the irb interface trigger a routing lookup.

When using vMX, typically in a lab, then there should be two VMs: one for the control plane and one for the forwarding plane, named something like vmx1-vcp and vmx1-vfp respectively.

When pinging the IP address of an aggregate Ethernet interface, the ICMP Echo Reply packets are duplicated:

lab@vEX> ping 172.16.200.2 count 3    
PING 172.16.200.2 (172.16.200.2): 56 data bytes
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.034 ms
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.209 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.251 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.295 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.346 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.387 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.418 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=0 ttl=63 time=3.448 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.641 ms
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.688 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.711 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.717 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.871 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.883 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.919 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=1 ttl=63 time=3.923 ms (DUP!)
64 bytes from 172.16.200.2: icmp_seq=2 ttl=63 time=2.626 ms

--- 172.16.200.2 ping statistics ---
3 packets transmitted, 3 packets received, +14 duplicates, 0% packet loss
round-trip min/avg/max/stddev = 2.626/3.492/3.923/0.343 ms

lab@vEX>

The ‘hot swappable’ and ‘hot pluggable’ are chassis capabilities and are not the same.

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Troubleshooting Junos

Notes from a Juniper Junos troubleshooting course