This article describes which interface will count packet to/from loopback interface of a Junos router.
When a Junos router receives a packet destined to loopback IP address, the packet is counted on the interface toward the next-hop of the source IP address of the packet.
Example topology:
+---------+ xe- xe- +---------+ | R1 | 0/0/7 0/0/8| R2 | |10.1.1.1 +--------------------+ 10.2.2.2| | | .1 192.0.2/24 .2| | +---------+ +---------+
In the diagram above, router R2 sends ping packets to loopback IP address of R1.
lab@R2> ping 10.1.1.1 rapid count 10
With this command, the ping is sent with source IP address 192.0.2.2, which is interface IP of xe-0/0/8.0 on R2.
On router R1, the next-hop of 192.0.2.2 is identical to itself since R1 has interface xe-0/0/7.0 which takes part in the subnet 192.0.2.0/24. Hence, the kernel binds to xe-0/0/7.0 receiving and sending interface of ping packet.
lab@R1> show route inet.0: 11 destinations, 11 routes (11 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both <..snip..> 10.1.1.1/32 *[Direct/0] 10:21:12 > via lo0.0 192.0.2.0/24 *[Direct/0] 08:13:13 > via xe-0/0/7.0
Lets send ping from R2 to R1 and see if ping packet is really counted on the interface xe-0/0/7.0.
lab@R1> show interfaces xe-0/0/7 extensive | no-more Physical interface: xe-0/0/7, Enabled, Physical link is Up Interface index: 147, SNMP ifIndex: 525, Generation: 150 Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online 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 Hold-times : Up 0 ms, Down 0 ms Damping : half-life: 0 sec, max-suppress: 0 sec, reuse: 0, suppress: 0, state: unsuppressed Current address: 56:68:a3:1e:5d:cd, Hardware address: 56:68:a3:1e:5d:cd Last flapped : 2019-06-06 10:47:34 PDT (1w5d 09:14 ago) Statistics last cleared: 2019-06-18 20:01:54 PDT (00:00:06 ago) Traffic statistics: Input bytes : 0 0 bps Output bytes : 0 0 bps Input packets: 0 0 pps Output packets: 0 0 pps <..snip..> Logical interface xe-0/0/7.0 (Index 334) (SNMP ifIndex 546) (Generation 149) Flags: Up SNMP-Traps 0x4004000 Encapsulation: ENET2 Traffic statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Local statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Transit statistics: Input bytes : 0 0 bps Output bytes : 0 0 bps Input packets: 0 0 pps Output packets: 0 0 pps
lab@R2> ping 10.1.1.1 rapid count 10 PING 10.1.1.1 (10.1.1.1): 100 data bytes !!!!!!!!!! --- 10.1.1.1 ping statistics --- 10 packets transmitted, 10 packets received, 0% packet loss round-trip min/avg/max/stddev = 1.587/1.977/3.868/0.637 ms
lab@R1> show interfaces xe-0/0/7 extensive | no-more Jun 18 21:45:08 Physical interface: xe-0/0/7, Enabled, Physical link is Up Interface index: 147, SNMP ifIndex: 525, Generation: 150 Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online 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 Hold-times : Up 0 ms, Down 0 ms Damping : half-life: 0 sec, max-suppress: 0 sec, reuse: 0, suppress: 0, state: unsuppressed Current address: 56:68:a3:1e:5d:cd, Hardware address: 56:68:a3:1e:5d:cd Last flapped : 2019-06-06 10:47:34 PDT (1w5d 10:57 ago) Statistics last cleared: 2019-06-18 21:44:57 PDT (00:00:11 ago) Traffic statistics: Input bytes : 840 0 bps Output bytes : 840 0 bps Input packets: 10 0 pps Output packets: 10 0 pps <..snip..> Logical interface xe-0/0/7.0 (Index 334) (SNMP ifIndex 546) (Generation 149) Flags: Up SNMP-Traps 0x4004000 Encapsulation: ENET2 Traffic statistics: Input bytes : 840 Output bytes : 840 Input packets: 10 Output packets: 10 Local statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Transit statistics: Input bytes : 840 0 bps Output bytes : 840 0 bps Input packets: 10 0 pps Output packets: 10 0 pps
The interface which counts packet to/from loopback IP address is determined and bound in the same way, even receiving and sending interface is logical, like service interface si.
lab@mx> show route inet.0: 18 destinations, 18 routes (18 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both <..snip..> 203.0.113.0/16 *[Static/5] 06:06:30 > via si-4/0/0.2 via si-4/1/0.2 via si-4/2/0.2 via si-4/3/0.2 Logical interface si-4/0/0.2 (Index 344) (SNMP ifIndex 1195) (Generation 153) Flags: Up Point-To-Point SNMP-Traps Encapsulation: Adaptive-Services Traffic statistics: Input bytes : 0 Output bytes : 640 Input packets: 0 Output packets: 10 IPv6 transit statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Local statistics: Input bytes : 0 Output bytes : 640 Input packets: 0 Output packets: 10