duplicate packets on a vlan interface

[Jeff Aitken]

I'm seeing a very strange problem involving a freshly-installed 
5.3-RELEASE system using vlans.

The machine has a single active ethernet interface (em0) with a pair
of vlan pseudo-interfaces created on it:

em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        options=1b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING>
        ether 00:09:6b:71:8c:38
        media: Ethernet 100baseTX <full-duplex>
        status: active
vlan0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        inet A1.B1.C1.50 netmask 0xffffffc0 broadcast A1.B1.C1.63
        ether 00:09:6b:71:8c:38
        media: Ethernet 100baseTX <full-duplex>
        status: active
        vlan: 20 parent interface: em0
vlan1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        inet A2.B2.C2.56 netmask 0xffffff00 broadcast A2.B2.C2.255
        ether 00:09:6b:71:8c:38
        media: Ethernet 100baseTX <full-duplex>
        status: active
        vlan: 842 parent interface: em0

>From a remote machine, if I send a single packet to the IP address
configured on vlan0 (A1.B1.C1.50), I get a single packet in response. 
However, if I send a single packet the address configured on vlan1 
(A2.B2.C2.56), I get a total of 62 replies (the original plus 61
duplicates).  Running a tcpdump on the machine sending the ICMP
requests clearly shows that it's only sending a single request, yet
running tcpdump (tcpdump -p -i vlan1) on the receiving host shows
that it is receiving a total of 62 echo-requests.  

This does not appear to be unique to ICMP.  Sending TCP packets seems
to yield the same packet duplication on the destination box.  Oddly,
a traceroute does NOT generate the same result, so perhaps UDP packets
do not trigger whatever is going on.

A google search did not yield anything useful.  Any ideas?


--Jeff