[wellylug] Multi port NIC

[Ewen McNeill]

In message <20050411030737.72521.qmail at web31709.mail.mud.yahoo.com>, Brent Wood writes:
>A few installations of such things seem to be using Gb copper cards into a Gb
>switch to achieve similar functionality (depending on just what you want to
>achieve).

FWIW, this is a common enterprise/ISP approach: use a VLAN capable
network interface (ideally with > 1500 octet frame support), OS (eg,
Linux :-) ), and switch.  (Eg, with 802.1Q support.)  Then you set up
the port to the router as a trunk port, define a bunch of 802.1Q tagged
virtual interfaces on the router, and assign a bunch of switch ports to
be associated with those virtual interfaces on the router (ie, same VLAN
tag) and plug things into those switch ports.

The major catches are:
- more single points of failure
- traffic will most likely traverse the router/switch link multiple times
- debugging is a bit more difficult because if you're not careful
  you can end up looking at (eg, tcpdump) all the traffic on all the
  interfaces at once; and you don't see the "link down" cable disconnects
  since your layer 1 connection is now on a remote device (the latter
  makes BGP behave somewhat worse, for instance)

But it provides a very cost effective way to get 8-16 interfaces "on" a
router.  I'm not sure whether it'd be cost effective for much less than
4-8 ports though.

The higher traffic load issues is typically avoided by specifying a
higher speed interface for the router/switch link than is expected to
flow through any single link.  Eg, 100Mbps for the router/switch link
for things expecting 10-20Mbps of traffic per "router port", and 1Gbps
for the router/switch link for anything expecting more per link.

I have clients with that sort of thing in production.  My own network
uses a VLANable switch, but the router currently has 4 ports into the
switch (one for each VLAN), because I had a surplus of multiport cards.
(I've tended to buy up the multiport cards I've seen on Trademe, as
they're generally much cheaper than new ones.  But they don't turn up
very often.)

 [Gigabit cards]
>Also note the 32 bit PCI bus speed limits, some such NIC's are 64bit to achieve
>the rated bandwidth.

I wouldn't expect much more than about 200Mbps through a 32-bit
"Gigabit" network interface.  A 64-bit one should be capable of more
like 500-600Mbps sustained.  Often people just want "more than 100mbps"
so this isn't a big deal.

Also many low end switches aren't capable of switching at full gigabit
speed all the time, especially not on more than 1-2 links.

Ewen