[j-nsp] MX loopback filter and monitor traffic

[Clarke Morledge]

I have a question about how the "monitor traffic" capability works on the 
loopback interface, particularly with respect to a filter.

If write a filter, such as under a "firewall family inet filter 
re-protect" stanza, and apply it to the loopback address, unit 0:

set interfaces lo0 unit 0 family inet filter input re-protect

I can see traffic hitting the filter, if I have any counters configured in 
the filter.   I can see that the traffic coming into the filter is getting 
to the RE via any IRBs or other layer 3 interfaces that are terminated on 
the MX.   I can do a "monitor traffic"  on any of these layer 3 interfaces 
on the input side and see the relevant traffic (to and/or from the RE).

However, if I do a "monitor traffic" on the loopback interface itself, I 
see nothing:

MX> monitor traffic interface lo0.0 no-resolve
no-domain-names
verbose output suppressed, use <detail> or <extensive> for full protocol
decode
Address resolution is OFF.
Listening on lo0.0, capture size 96 bytes

^C
0 packets received by filter
0 packets dropped by kernel


If all of the traffic that comes into the router to the RE via these 
exposed Layer3 interfaces eventually makes it way to the RE via the 
loopback address, at unit 0, why is that the "monitor traffic" command 
does not show me anything?    Why is the loopback interface so "special"?


Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
Williamsburg VA 23187