On Fri, Jan 6, 2012 at 12:08 PM, Dave Howorth <dhoworth@mrc-lmb.cam.ac.uk> wrote:
Greg Freemyer wrote:
When they bring up a new machine and do dhcp, it takes about 45 seconds to get the new IP. I'm convinced most of the time is the switches playing games trying to figure out the most efficient way to connect the newly introduced machine to the dhcp server.
Isn't that any easy problem to solve? Whichever switch the new machine is connected to just looks in its tables and asks "how did I send traffic to <the dhcp server> last time somebody asked me?" The answer doesn't depend in any way on there being a new machine.
But I'm no expert, so perhaps I'll learn something!
In theory it could do that, but it doesn't seem to work out in practice. I don't know why in detail. === background for anyone interested At the switch level all communications is actually MAC to MAC, not IP to IP. So when you add a new machine to a LAN segment, the local switch network has to add your MAC to the tree-spanning tables which are maintained by all the switches. But when it does that, it makes a concerted effort to map your MAC to all the other MACs by the shortest path. If you have a class C (/24) then there aren't that many options so it takes no time to build the spanning tree. If you have a full class A (ie. /8 or 10.x.x.x) then you conceivably have 10's of thousands (or more) MACs to potentially keep up with and setup spanning trees for. In that case a haphazard interconnection of all the switches can cause loops which complicate the process of bringing up a new machine. The traditional solution is to introduce smaller lan segments and then have routers between the lan segments. ie. 10.0.0.x, 10.0.1.x, 10.0.2.x are all in use at my office and we have a routers setup that send traffic between those lan segments. You can get a cheap router these days, so it is not expensive to do that if it makes sense. But as I said, I have one client with 80 24-port switches in one huge lan segment. So that would be about 2000 ports / MAC addresses the switches have to map and track with various paths from one port to another. In their case it takes about 45 seconds to add a new machine (MAC) to the lan segment (switch network) and get a IP via DHCP. They are in the middle of new network design now to reduce that time significantly. Greg -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org