The customer had configured the port mirror feature in GRE tunnel interface in the MX router, they confirmed the gr-0/0/0.1 were receiving the testing traffic from the 3rd party device (Arista router).

However, the traffic is NOT being mirrored and forwarded to the analyzer via port-mirroring output interface et-0/0/0.

set chassis fpc 0 port-mirror-instance MIRROR

set firewall family inet filter MIRROR term default-term then count MIRROR-Allowed
set firewall family inet filter MIRROR term default-term then port-mirror
set firewall family inet filter MIRROR term default-term then accept

set forwarding-options port-mirroring instance MIRROR input rate 1
set forwarding-options port-mirroring instance MIRROR input run-length 1
set forwarding-options port-mirroring instance MIRROR family inet output interface et-0/0/0.0 next-hop 10.0.0.2
set forwarding-options port-mirroring instance MIRROR family inet output no-filter-check'

set interfaces et-0/0/0 unit 0 family inet address 10.0.0.1/30 arp 10.0.0.2 mac 01:02:03:04:05:06

set interfaces gr-0/0/0 description "GRE tunnel"
set interfaces gr-0/0/0 enable
set interfaces gr-0/0/0 unit 1 tunnel source 10.11.23.177
set interfaces gr-0/0/0 unit 1 tunnel destination 10.11.23.35
set interfaces gr-0/0/0 unit 1 family inet filter input MIRROR
set interfaces gr-0/0/0 unit 1 family inet address 10.55.66.5/30

user@router> show interfaces gr-0/0/0.1
Logical interface gr-0/0/0.1 (Index 137) (SNMP ifIndex 610)
Flags: Up Point-To-Point SNMP-Traps 0x4000 IP-Header 10.11.23.35:10.11.23.177:47:df:64:0000000000000000 Encapsulation: GRE-NULL
Copy-tos-to-outer-ip-header: Off, Copy-tos-to-outer-ip-header-transit: Off
force-control-packets-on-transit-path: Off
Gre keepalives configured: Off, Gre keepalives adjacency state: down
Input packets : 14620274                                       >>>>>>>>>>>>> gr-0/0/0.1 receiving the testing traffic
Output packets: 0
Protocol inet, MTU: 9462
Max nh cache: 0, New hold nh limit: 0, Curr nh cnt: 0, Curr new hold cnt: 0, NH drop cnt: 0
Flags: Sendbcast-pkt-to-re
Addresses, Flags: Is-Preferred Is-Primary
Destination: 10.55.66.4/30, Local: 10.55.66.5, Broadcast: 10.55.66.7

However, the output pps on interface et-0/0/0 is not incrementing

user@router> show interfaces et-0/0/0
Physical interface: et-0/0/0, Enabled, Physical link is Up
Interface index: 175, SNMP ifIndex: 840
Description: To Lab-a10tps-1 eth52 - TAP
Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None,
Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled
Pad to minimum frame size: Disabled
Device flags : Present Running
Interface flags: SNMP-Traps Internal: 0x4000
Link flags : None
CoS queues : 8 supported, 8 maximum usable queues
Schedulers : 0
Current address: 88:28:fb:30:96:93, Hardware address: 88:28:fb:30:96:93
Last flapped : 2023-11-14 22:37:31 NZDT (19:12:35 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)                                  >>>>>>>>>>>>>>>>  no output traffic
Active alarms : None
Active defects : None
PCS statistics

1) Make sure the 3rd party device generates the packets with correct data format.

In this case, the GRE header 'protocol type' field setting in Arista could be wrong, needs to manually set to 0x0800 to make it work, the customer confirmed "Turns out the mirror GRE tunnel protocol on Arista end, I need to manually set to 0x0800. "

2) Ideally, the decapsulated packets should be routable, otherwise, they will be discarded, but mirror packet will still be generated and sent to the analyzer.

For the GRE sub-interface configured with redirect knob - please check the VRF table.

set interfaces <gr-x/y/z> unit <#> tunnel routing-instance destination <VRF_name>

2023-11-17 : Article Created

2026-01-20: Font formatting has been corrected throughout the document.