em(4) VLAN + PROMISC followup question

[Jon Simola]

Referencing:
http://lists.freebsd.org/pipermail/freebsd-net/2004-November/005738.html

I appear to have hit a similar or the same problem (with the exception
that I'm not bridging with the vlan), has anyone (Robert Watson
appears to be the lead) come up with anything?

My config is 5.3-STABLE with em1 the parent for a handful of vlans.
em1: flags=18843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,POLLING> mtu 1500
        options=5b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,POLLING>
        inet6 fe80::230:48ff:fe72:f30b%em1 prefixlen 64 scopeid 0x6
        ether 00:30:48:72:f3:0b
        media: Ethernet autoselect (1000baseTX <full-duplex>)
        status: active
vlan120: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        inet xxx.xx.177.254 netmask 0xffffff00 broadcast 209.53.177.255
        inet6 fe80::205:5dff:fe71:8d20%vlan120 prefixlen 64 scopeid 0xd
        ether 00:30:48:72:f3:0b
        media: Ethernet autoselect (1000baseTX <full-duplex>)
        status: active
        vlan: 120 parent interface: em1

Running a tcpdump on either em1 or one of the vlan interfaces reduces
throughput by a factor of 10 or so. Running tcpdump with the -p option
does not.

This is a steady stream of 10 to 20 Mbps of traffic, routing 11 /24s
over 4 vlans, kernel polling is enabled for the em devices.

Thank you,
Jon Simola
Systems Administrator
ABC Communications